Bilirubin Metabolism

Describe changes to bilirubin metabolism and iron salvage systems that occur when the rate of fragmentation or macrophage-mediated hemolysis increases.

From: Rodak's Hematology (Sixth Edition) , 2020

Biliary Tract Pathophysiology

A. NAKEEB , ... J. TOOULI , in Surgery of the Liver, Biliary Tract and Pancreas (Fourth Edition), 2007

BILIRUBIN METABOLISM

Normal bilirubin metabolism can be summarized as a series of steps, including (1) production, (2) uptake by the hepatocyte, (3) conjugation, (4) excretion into bile ducts, and (5) delivery to the intestine. Jaundice can result from defects in any of these steps of bilirubin metabolism.

In adults, 250 to 350 mg of bilirubin is produced each day. Approximately 80% to 85% of this bilirubin is derived from the destruction of senescent red blood cells by the reticuloendothelial system. The remaining 15% to 20% comes from the breakdown of nonhemoglobin proteins, such as myoglobin and the cytochromes. Fig. 7a.1 shows the metabolism of bilirubin. In reticuloendothelial cells, the microsomal enzyme heme oxygenase cleaves heme into biliverdin. Biliverdin is reduced to bilirubin by the cytosolic enzyme biliverdin reductase before being released into the circulation. In this unconjugated form, bilirubin is water insoluble and is transported to the liver tightly bound to albumin.

The liver removes unconjugated bilirubin and other organic anions bound to albumin from plasma. When the bilirubin-albumin complex enters the sinusoidal circulation of the liver, three distinct metabolic phases are recognized: (1) hepatocyte uptake, (2) conjugation, and (3) excretion into bile. Unconjugated bilirubin is transported across the sinusoidal membrane of the hepatocyte into the cytoplasm. Inside the hepatocyte, unconjugated bilirubin is bound by a cytoplasmic protein, in this case glutathione S-transferase. The microsomal enzyme uridine diphosphate–glucuronyl transferase then conjugates the insoluble unconjugated bilirubin with glucuronic acid to form the water-soluble conjugated forms, bilirubin monoglucuronide (15%) and bilirubin diglucuronide (85%). Conjugated bilirubin is excreted from the hepatocyte into the bile canaliculus by an active transport mechanism.

Excretion into bile is the rate-limiting step in bilirubin metabolism. After excretion, bile flows through the biliary ductal collecting system, may or may not be stored in the gallbladder, and enters the duodenum. In the terminal ileum and colon, bilirubin is converted by bacterial enzymes into urobilinogen. Ten percent to 20% of the urobilinogen is reabsorbed from the intestine into the portal circulation, creating an enterohepatic circulation. This recycled urobilinogen may be re-excreted into the bile by the liver or into urine by the kidney. The remaining urobilinogen in the intestine is converted to fecobilinogen, which gives stool its characteristic brown color.

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Laboratory Measurement of Hepatic Function

Helen S. Te , in Shackelford's Surgery of the Alimentary Tract, 2 Volume Set (Eighth Edition), 2019

Isolated Unconjugated and Conjugated Hyperbilirubinemia

Disorders of bilirubin metabolism may occur at any of the several steps in the pathway. Unconjugated hyperbilirubinemia may result from bilirubin overproduction, reduced hepatic uptake, or defective bilirubin conjugation. Conjugated hyperbilirubinemia results from bile canalicular transporter defects or impairment of bile flow through the intrahepatic and extrahepatic bile ducts. These isolated cases of hyperbilirubinemia are typically accompanied by normal serum AST, ALT, and alkaline phosphatase, as well as normal liver histology.

Unconjugated hyperbilirubinemia may be due to overproduction of bilirubin, as may occur in hemolysis. The serum total bilirubin rarely increases to greater than 5 mg/dL, and the conjugated value is less than 15% of the total value. The serum conjugated bilirubin also increases because of saturation of the excretory mechanisms with regurgitation of conjugated bilirubin into plasma. Unconjugated hyperbilirubinemia may also result from decreased bilirubin uptake into the hepatocyte due to competition for a carrier-mediated binding site on the hepatocyte plasma membrane by drugs, such as rifampicin and oral cholecystographic drugs.

Unconjugated hyperbilirubinemia due to inadequate intrahepatic bilirubin binding and conjugation is seen in Gilbert syndrome. Gilbert syndrome results from decreased uridine-diphosphoryl-glucuronosyltransferase (UDP-GT) activity that afflicts up to 7% of the general population. Serum unconjugated bilirubin levels tend to increase during fasting, stress, or illness but rarely increase to greater than 5 mg/dL. This is a benign condition that is inherited via a variable pattern and does not affect long-term survival, although it may affect the risk for certain drug toxicity when UDP-GT is involved in the drug detoxification process. 52 A rare autosomal recessive disease, Crigler-Najjar syndrome, is caused by a deficiency in UDP-GT activity. In type I Crigler-Najjar syndrome, there is complete absence of enzyme activity, resulting in severe hyperbilirubinemia (serum total bilirubin >30 mg/dL) and usually, death. In type II, there is partial transferase activity, and this may present with mild to moderate hyperbilirubinemia (5 to 30 mg/dL). This condition may be compatible with life, and medical treatment with phenobarbital can induce expression of UDP-GT to lower the serum unconjugated bilirubin levels. 52

Conjugated hyperbilirubinemia results from inherited disorders in canalicular transport of conjugated bilirubin, such as Dubin-Johnson syndrome and Rotor syndrome. Dubin-Johnson syndrome is an autosomal recessive disorder characterized by intermittent mild jaundice, caused predominantly by a conjugated hyperbilirubinemia (bilirubin 2 to 5 mg/dL). This is thought to be mediated by an inherited defect in the canalicular transport of conjugated bilirubin, due to a mutation of the MRP2 gene that results in defective expression of the MRP2 transporter. Dark pigment accumulates in the hepatic lysosomes. Rotor syndrome is similar to the Dubin-Johnson syndrome, but there is no dark pigment accumulation in the lysosomes. 52 However, the exact pathogenesis is not clear. Most patients are asymptomatic and can lead normal lives.

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The First Month of Life

Patricia Fontaine MD, MS , ... Sherri Fong BS , in Family Medicine Obstetrics (Third Edition), 2008

A. Physiologic Jaundice

Several aspects of bilirubin metabolism contribute to physiologic neonatal jaundice.

1.

Increased bilirubin production

Newborns have high levels of circulating erythrocytes that are broken down into heme, then into iron, carbon monoxide, biliverdin, and eventually bilirubin. 5

2.

Decreased bilirubin uptake and conjugation

Serum albumin binds bilirubin and carries it to the liver, where the newborn's transient deficiency of the enzyme glucuronyl transferase leads to reduced bilirubin conjugation. In addition, newborns have a reduced amount of ligandin (a bilirubin binding protein), which assists with uptake of bilirubin into the liver cell. 5

3.

Increased enterohepatic circulation

Conjugated bilirubin is excreted through the bile into the intestine, where it is deconjugated by a mucosal enzyme, β-glucuronidase, and reabsorbed into the enterohepatic circulation before it can be excreted with the stool. Newborns have slow intestinal motility because of a paucity of gut flora and relative caloric deprivation in the first days of life, both of which promote physiologic hyperbilirubinemia through increased enterohepatic circulation. 5

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The Hepatobiliary System

Ivan Damjanov MD, PhD , in Pathology Secrets (Third Edition), 2009

1 What are the main diseases of the liver and the biliary system?

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Disorders of bilirubin metabolism and excretion resulting in jaundice

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Acute and chronic hepatitis caused by viruses, bacteria, and other pathogens

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Diseases caused by drugs, toxins, and alcohol

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Metabolic and genetic diseases

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Autoimmune diseases

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Cholelithiasis

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Tumors

2 What are the most common morphologic signs of liver injury?

Hepatocytes are the main target of liver injury. Morphologically, liver injury can manifest in several forms:

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Vacuolar change: Affected hepatocytes are enlarged and have a swollen, clear cytoplasm. This hydropic swelling of hepatocytes is a common response to injury. It is reversible, but if intensified or prolonged, it may lead to liver cell necrosis.

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Apoptosis: The hepatocytes undergoing apoptosis appear rounded and detach from the other liver cells. They have a pyknotic nucleus, which ultimately disappears, leaving the remaining cytoplasm as a "round red body," also known as acidophilic body. Apoptosis of single hepatocytes is a typical feature of viral hepatitis, but it may be induced by other liver diseases as well.

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Necrosis: Irreversible injury of hepatocytes induced by ischemia, toxins, and viruses typically involves portions of the liver acinus. It is customary to describe necrosis as either focal (random) or zonal, if limited to one of the three zones of the liver acinus. Zone 3 necrosis (also known as centrolobular necrosis) is the most common form of necrosis and is found in ischemic liver injury (e.g., in hypotensive shock) but also in many forms of toxic liver injury.

Key Points: The Hepatobiliary System

1

The liver is a central metabolic organ involved in all metabolic pathways; thus it is affected by most systemic diseases and is a common site of adverse drug reactions.

2

The liver is the source of bile and thus a cause of biliary diseases and jaundice.

3

End-stage liver disease, called cirrhosis, is a fatal disease that can be treated only by liver transplantation.

3 How does the liver respond to injury?

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Inflammation: Injured liver cells must be removed. To this end, the body sends in inflammatory cells that react with injured hepatocytes. For example, hepatocytes infected with hepatitis virus elicit a T-cell response. These lymphocytes enter the liver and kill the infected hepatocytes, which are thereafter removed by macrophages.

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Regeneration: Liver cells are stable facultative mitotic cells that can enter into the mitotic cycle on demand and by dividing replace the damaged cells. Regeneration of the liver is a major response of the liver to injury. In experimental animals, it has been shown that the liver can regenerate even after two thirds of it has been removed. Massive necrosis induced by toxins and viruses typically elicits a major regeneration wave, but in many instances, this is not sufficient to save the patient's life. However, focal necrosis and apoptosis are easily repaired.

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Fibrosis: Extensive necrosis, especially if accompanied by persistent inflammation or toxic influences, cannot be repaired by regeneration. Portions of the liver are replaced by connective tissue scars. Fibrosis associated with nodular regeneration of the remaining hepatocytes is typical of end-stage liver disease, known as cirrhosis.

4 How is liver function evaluated clinically?

Liver function is evaluated with laboratory tests colloquially known as liver function tests (LFTs). These tests were designed to monitor the following:

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Liver cell integrity (known as necroinflammatory indices)

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Hepatic secretory function

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Biliary excretory function

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Hepatic catabolic function

5 Discuss the necroinflammatory indices, that is, the laboratory tests used to monitor the integrity of liver cells

The most widely used tests are those measuring the blood concentration of aspartate aminotransferase (AST) and alanine aminotransferase (ALT). In massive liver necrosis (e.g., after acetaminophen intoxication), blood levels of AST and ALT increase 50 times over the normal values. In viral hepatitis, levels of AST and ALT are 4 to 6 times above the normal values.

6 Which tests are used to measure hepatic secretory function?

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Albumin: Normally, the blood contains 3.5 to 5.0 g/dL (35–50 g/L) albumin, the most copious plasma protein. Chronic liver injury will reduce blood concentration of albumin to less than 3 g/dL.

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Coagulation proteins: The easiest way to estimate the concentration of the coagulation factors is by measuring the prothrombin time (PT), which is normally 10 to 13 seconds. Prolonged PT is a sensitive index of liver function loss.

7 Discuss the tests used to measure biliary excretion

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Bilirubin: Bilirubin that has been conjugated in the liver to be excreted into the intestine may accumulate in the blood of patients who have bile duct obstruction.

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Alkaline phosphatase: This enzyme is found along the liver cell membrane lining the intercellular canaliculi. Obstructive jaundice interferes with the normal biliary elimination of this enzyme, which then appears in the blood. Elevated levels of alkaline phosphatase in blood are typical of obstructive jaundice.

▪

Gamma-glutamyltransferase (GGT): In contrast to alkaline phosphatase, which is found in many other organs, GGT is primarily a hepatic enzyme. Elevation of GGT is a reliable sign of biliary obstruction. However, GGT is also induced in liver cells by alcohol or phenobarbital and some other drugs that stimulate the P450 system. GGT is thus a marker of liver cell injury (especially alcohol-induced injury).

8 Which tests are used for estimating liver catabolic functions?

Catabolic functions of the liver include the detoxification of many metabolites. However, it is impractical to measure these functions and, in practice, the only function that is monitored is the capacity of the liver to remove ammonia. Elevation of blood ammonia is a good marker of severe liver injury.

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Liver Diseases

Anahat Dhillon MD , Randolph H. Steadman MD , in Anesthesia and Uncommon Diseases (Sixth Edition), 2012

Bilirubin

As discussed with normal metabolism, bilirubin is a product of heme breakdown. It exists in conjugated (water soluble) and unconjugated (lipid soluble) forms, which are reported imprecisely as the direct and indirect fractions, respectively. Serum bilirubin is usually less than 1   mg/dL and primarily unconjugated. Elevated serum levels occur in most significant liver diseases; degree of elevation correlates with prognosis in primary biliary cirrhosis, alcoholic hepatitis, and fulminant liver failure. The appearance of conjugated bilirubin in the blood is thought to be caused by reflux from the hepatocyte, but this does not discriminate between obstructive and parenchymal causes. Other causes of elevated bilirubin include Gilbert's syndrome, increased production (e.g., hemolysis, ineffective erythropoiesis, hematoma resorption), and inherited disorders of bilirubin transport.

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Hepatic and gastrointestinal disorders

Sakil Kulkarni , ... Janis M. Stoll , in Biochemical and Molecular Basis of Pediatric Disease (Fifth Edition), 2021

Inherited disorders of bilirubin metabolism

Four inherited disorders of bilirubin metabolism have been described and apart from Gilbert syndrome, are extremely rare. Gilbert and Crigler–Najjar syndromes are due to defects in the hepatic conjugating enzyme and cause unconjugated hyperbilirubinemia. Rotor and Dubin–Johnson syndromes both result in a mixed hyperbilirubinemia. Gilbert syndrome causes chronic, mild, fluctuating unconjugated hyperbilirubinemia due to decreased UDP-glucuronosyltransferase activity to less than 50% of normal . UGT1A1 encodes this enzyme, and Gilbert syndrome is associated with homozygosity for an additional TA repeat in the promoter region (UGT1A1*28). It occurs in approximately 9% of Caucasians and is inherited in an autosomal-recessive fashion. The condition is benign and rarely becomes apparent before puberty. Bilirubin concentrations seldom exceed 100 µmol/L (6 mg/dL) and are usually less than 50 µmol/L. The bilirubin concentration may be increased by fasting or intercurrent illness and reduced by phenobarbital administration, which induces the enzyme activity. The diagnosis of Gilbert syndrome is usually one of exclusion once hemolysis and liver disease have been ruled out.

Crigler–Najjar syndrome occurs in two forms, is caused by mutations in UGT1A1, displays autosomal-recessive inheritance. The mutations in type 1 result in complete absence of functional enzyme and lead to severe unconjugated hyperbilirubinemia [bilirubin >340 µmol/L (20 mg/dL)] in the first few days of life, with no evidence of hemolysis or other LFT abnormalities. Death from the neurologic sequelae of kernicterus invariably occurs by 18 months without intensive treatment such as liver transplantation. Type 2, which is caused by a partial enzyme deficiency, leads to less severe unconjugated hyperbilirubinemia [bilirubin 100–340 µmol/L (6–20 mg/dL)], which is usually recognized during the first year of life. It does not usually cause clinical problems unless it goes unrecognized for many years, when some neurological signs may occur. Measurement of UDP-glucuronosyltransferase activity in a liver biopsy or mutation analysis is required to confirm the diagnosis. Dubin–Johnson syndrome (DJS) is inherited autosomal recessively and is due to mutations in ABCC2 that result in defective excretion of conjugated bilirubin across the canalicular membrane. It causes mixed hyperbilirubinemia with the conjugated fraction accounting for ∼60% of the total. It tends to present after puberty with jaundice, normal LFTs, and no evidence of hemolysis. The total bilirubin concentration is usually <90 µmol/L (5 mg/dL) but can increase to up to 400 µmol/L (23 mg/dL), particularly at times of intercurrent illness. Patients with DJS excrete normal amounts of coproporphyrinogen, but the proportions of the isomers I and III are altered such that >80% is coproporphyrinogen I. Rotor syndrome and DJS have many similar features, including mode of inheritance, age at recognition, and the degree of benign mixed hyperbilirubinemia. The molecular defect, however, is not in ABCC2 and is currently unknown. The disorder may be due to deficiency in the intracellular storage capacity of the liver. Patients with Rotor syndrome show increased excretion of coproporphyrinogen, with typically <80% as the coproporphyrinogen isomer I.

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Pediatric liver disease

Bernadette Vitola MD , William F. Balistreri MD , in Handbook of Liver Disease (Third Edition), 2012

Hyperbilirubinemia

Pathophysiologic Mechanisms

Alterations in any step of bilirubin metabolism may cause jaundice in excess of physiologic jaundice ( Fig. 23.1; the numbers in the figure correspond to the numbers (1–7) in the following outline):

1.

Increased bilirubin production: This can result from an increase in the release of heme from red blood cells, for the following reasons:

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Hemolysis due to Rh incompatibility, ABO incompatibility, or other minor blood group incompatibilities

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Increased fragility of erythrocytes in congenital spherocytosis, hereditary elliptocytosis, polycythemia, or red blood cell enzyme defects (glucose-6-phosphate dehydrogenase [G6PD]; pyruvate kinase [PK]; hexokinase [HK])

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Enclosed hematoma

2.

Decreased bilirubin uptake into the hepatocyte:

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This may be caused by hypothyroidism or gestational hormones that may inhibit the uptake of bilirubin across the hepatocyte membrane. Thyroxine is important for liver plasma membrane function.

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A decrease in the amount of bilirubin bound to serum proteins also results in decreased uptake by hepatocytes. The reduction in bilirubin binding may be the result of hypoalbuminemia, generalized hypoproteinemia, or displacement of bilirubin from these proteins by drugs, including sulfonamides, salicylates, heparin, and caffeine.

3.

Abnormalities of intracellular binding or storage of bilirubin within hepatocytes: These are rare disorders and include deficiencies or alteration in GST, the primary intracellular binding protein for bilirubin.

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Treatment is not indicated, because no associated morbidity or mortality exists.

4.

Inefficient conjugation of bilirubin within the hepatocyte: Within the hepatocyte, bilirubin is conjugated with glucuronic acid by bilirubin uridine diphosphate (UDP)-glucuronyl transferase to form bilirubin monoglucuronide or diglucuronide.

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A decrease in UDP-glucuronyl transferase activity is seen in Gilbert's syndrome, resulting in benign elevations in serum unconjugated bilirubin levels, especially during stresses such as viral illnesses.

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Crigler–Najjar syndrome, which localizes to chromosome 2q37, is characterized by the absence of bilirubin UDP-glucuronyl transferase that leads to severe hyperbilirubinemia with associated neurologic effects secondary to kernicterus.

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Hepatic parenchymal disease can cause hyperbilirubinemia in the neonatal period, presumably secondary to hepatocyte damage (see later).

5.

Alterations in the excretion of bilirubin through the canalicular membrane into the biliary tract: Bilirubin diglucuronide is excreted into the canaliculus by a carrier protein.

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Alterations in this carrier protein are thought to be the cause of Dubin–Johnson syndrome. The genetic defect is on chromosome 10q24. Although this syndrome has no associated morbidity or mortality, it is characterized by elevated serum levels of conjugated and unconjugated serum bilirubin.

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Hyperbilirubinemia is accentuated during pregnancy or with use of oral contraceptives.

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The syndrome is likely an autosomal recessive disorder.

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Affected persons have an increase in urinary coproporphyrin I levels.

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Liver biopsy specimens show a characteristic melanin-like pigment deposited in hepatocytes but are otherwise histologically normal.

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Because of the benign nature of this syndrome, no treatment is required.

6.

Structural abnormalities of the biliary tract can prevent drainage of bile from the canaliculus into the intestine and can cause accumulation of bile and reflux of bilirubin into the systemic circulation:

a.

Biliary atresia (BA) is a progressive disease characterized by inflammation and fibrosis of the extrahepatic biliary tract resulting in partial or complete obliteration of the extrahepatic bile ducts.

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BA typically manifests between 2 and 6 weeks of age as cholestasis (conjugated hyperbilirubinemia).

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Liver biopsy specimens show fibrosis and bile duct proliferation.

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BA may be syndromic (in less than 15% of cases) and associated with cardiac anomalies, polysplenism, and malrotation or situs inversus, or nonsyndromic (majority of cases), origin unknown.

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This anomaly is treated initially with a Kasai portoenterostomy, which allows drainage of bile directly from the liver to the intestine. Although the procedure is not curative, it often delays the progression of disease.

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End-stage liver disease secondary to BA is the most common reason for liver transplantation in children.

b.

Intrahepatic cholestasis is often associated with the histologic finding of bile duct paucity, defined as a reduced ratio of interlobular bile ducts to portal tracts (normal is 0.9 to 1.8; paucity is less than 0.5). Paucity of bile ducts may be syndromic (Alagille's syndrome, which is associated with peripheral pulmonic stenosis, butterfly vertebrae, and characteristic facies). Many forms of intrahepatic cholestasis do not exhibit bile duct paucity. Treatment for all forms is symptomatic, with special consideration given to management of malnutrition and pruritus. Liver transplantation may be required in some cases.

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Progressive familial intrahepatic cholestasis, type I (PFIC-1), also known as Byler's disease, is caused by defects in the FIC1 gene on chromosome 18q21–22. This P-type adenosine triphosphatase (ATPase) functions in transport of aminophospholipids across the hepatocyte canalicular plasma membrane. Patients characteristically have a low gamma glutamyltranspeptidase (GGTP) level. The average age of onset is 3 months, and progression to cirrhosis is variable. Watery diarrhea may occur, presumably secondary to intestinal absence of FIC1. Treatment is aimed at pruritus and poor growth. Liver transplantation is curative of the liver disease and typically is required in the first 2 decades of life. Benign recurrent intrahepatic cholestasis (BRIC) maps to the FIC1 locus, but the disease does not progress.

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Progressive familial intrahepatic cholestasis, type II (PFIC-2), is clinically similar to PFIC-1; both are associated with low serum GGTP levels. Watery diarrhea is typically not a component, and progression to cirrhosis may be more rapid than in PFIC-1. The syndrome results from defects in the hepatocyte bile salt export pump (BSEP) on chromosome 2q24.

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Progressive familial intrahepatic cholestasis, type III (PFIC-3) has a rapid progression to cirrhosis and liver failure. Elevated serum levels of GGTP characterize this syndrome. It is caused by mutations in MDR3, which encodes an active export pump involved with the translocation of phosphatidylcholine across the canalicular hepatocyte membrane.

c.

Choledochal cyst, a cystic dilatation of the biliary tree, may be exclusively extrahepatic or include dilatations of the intrahepatic biliary tree.

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It appears to be most common in East Asia; more than 50% of the reported cases come from Japan.

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Most patients present in infancy with abdominal pain and jaundice, with or without a palpable abdominal mass.

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The diagnosis can be made by ultrasonography, computed tomography (CT), or endoscopic retrograde or magnetic cholangiopancreatography (ERCP; MRCP).

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Treatment is with surgical excision of the dilated segment, rather than bypass or drainage, because of the increased incidence of malignancy in the epithelium of the cyst.

7.

Alterations in the enterohepatic circulation can produce an increase in reabsorption of bilirubin from the intestine. The cause may be intestinal obstruction, as in intestinal atresia or Hirschsprung's disease, or alterations in the bacterial flora by the use of antibiotics.

Complications

1.

Unconjugated hyperbilirubinemia

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Kernicterus (bilirubin encephalopathy) may result from elevated levels of unconjugated bilirubin. Populations at risk include neonates and individuals with Crigler–Najjar syndrome, type I.

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Unconjugated bilirubin levels higher than 30 mg/dL are associated with development of encephalopathy.

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Factors that increase the risk of kernicterus include hypoalbuminemia and bilirubin displacement from albumin by drugs or organic anions. Both conditions increase the serum concentration of unbound bilirubin able to diffuse into cells.

2.

Cholestasis

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Malnutrition secondary to fat malabsorption can lead to failure to thrive and fat-soluble vitamin deficiencies

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Intractable pruritus

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Xanthomatosis secondary to alterations in cholesterol metabolism

Treatment

a.

Unconjugated hyperbilirubinemia

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Double-volume exchange transfusion lowers the risk of kernicterus in the newborn by rapidly reducing the serum bilirubin concentration.

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Phototherapy: Photoisomerization of bilirubin to a more polar compound allows excretion of bilirubin in the urine.

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Faster bilirubin metabolism can be accomplished by administration of phenobarbital, which induces microsomal enzymes that facilitate bilirubin metabolism.

b.

Cholestasis

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Surgical correction of anatomic lesions may be effective.

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Ursodeoxycholic acid, a choleretic bile acid, can be used to augment bile flow during cholestasis (15 mg/kg per day in divided doses).

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Supplementation with fat-soluble vitamins is also necessary because absorption is poor without normal bile flow.

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Liver transplantation may be necessary.

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TRANSFUSION MEDICINE

Jeannie Callum , Jon Barrett , in Blood Banking and Transfusion Medicine (Second Edition), 2007

Phenobarbital Administration

The rate-limiting enzyme in bilirubin metabolism is bilirubin-UDP-glucuronosyltransferase (UGT1A1). By increasing the expression of UGT1A1, phenobarbital enhances the capacity of the neonatal liver to conjugate and eliminate bilirubin. Phenobarbital binds to a 290–base pair enhancer sequence on the gene for UGT1A1, leading to an increase in the production of this enzyme. A retrospective case-control study found prenatal administration to be effective in decreasing the need for exchange transfusion. 135 Intrauterine transfusions were continued until 35 weeks' gestation. After the last intrauterine transfusion, women were offered phenobarbital 30 mg orally three times a day for 10 days. Delivery was planned after the 10-day treatment. Of 71 women identified with HDFN, 33 were administered phenobarbital. Both the exchange transfusion rate (9% vs 52%, p < 0.01) and neonatal mortality rate (0% vs 24%) were reduced by the administration of phenobarbital. After adjusting for gestational age at delivery and neonatal peak total bilirubin, administration of phenobarbital decreased the relative risk of exchange transfusion (risk ratio, 0.23; 95% CI, 0.06–0.76). Halpin and colleagues also found promising results in an earlier report on phenobarbital. 136 These preliminary studies suggest that predelivery phenobarbital is an effective method to prevent exchange transfusion in documented HDFN.

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Metabolism of Iron and Heme

N.V. Bhagavan , Chung-Eun Ha , in Essentials of Medical Biochemistry (Second Edition), 2015

Formation of Bilirubin

A summary of the pathway for bilirubin metabolism and excretion is shown in Figure 27.10. Release of heme from heme proteins and its conversion to bilirubin occur predominantly in the mononuclear phagocytes of liver, spleen, and bone marrow (previously known as the reticuloendothelial system), sites where sequestration of aging red cells occurs. Renal tubular epithelial cells, hepatocytes, and macrophages may also contribute to bilirubin formation under some conditions. Structures of the intermediates in the conversion of heme to bilirubin are shown in Figure 27.11. The initial step after the release of heme is its binding to heme oxygenase, a microsomal enzyme distinct from the microsomal P-450 oxygenases. Heme oxygenase catalyzes what appears to be the rate-limiting step in catabolism of heme. It is induced by heme and requires O2 and NADPH for activity. The activity of the inducible isoenzyme form of heme oxygenase is highest in the spleen, which is involved in the sequestration of senescent erythrocytes. The constitutive form of heme oxygenase is mainly localized in the liver and brain. After binding, the α-methene carbon of heme is oxidized (hydroxylated) to α-hydroxyhemin, which undergoes autoxidation to biliverdin (a blue-green pigment) with consumption of O2 and release of iron and carbon monoxide (derived from oxidation of the α-methene bridge). Since CO production in mammals occurs primarily by this pathway, measurement of expired CO has been used to estimate heme turnover. A potent competitive synthetic inhibitor of heme oxygenase is tin (Sn) protoporphyrin, which has potential therapeutic use in the treatment of neonatal jaundice (see later).

Figure 27.10. Catabolic pathway for the heme group from hemoproteins (predominantly hemoglobin).

Figure 27.11. Conversion of heme to bilirubin in the monocytic phagocytic cells. Carbon monoxide and bilirubin are generated. The Fe3+ released is conserved and reutilized. Biliverdin and bilirubin are lactams. P=propionic acid; M=methyl; V=vinyl.

Biliverdin is reduced to bilirubin by NAD(P)H-dependent biliverdin reductase, a cytosolic enzyme that acts at the central methene bridge. Although both molecules have two propionic acid groups, the polarity of biliverdin is greater than that of bilirubin. Bilirubin can form six internal hydrogen bonds between the carboxylic groups, the two lactam carbonyl oxygens, and four pyrrolenone ring nitrogens, and thus prevents these groups from hydrogen-bonding with water (Figure 27.12). Esterification of the propionyl side chains of bilirubin with glucuronic acid disrupts the hydrogen bonds and increases its solubility. Phototherapy for neonatal jaundice also acts by disrupting the hydrogen-bonded structure of unconjugated bilirubin.

Figure 27.12. Conformation of bilirubin showing involuted hydrogen-bonded structure between NH/O and OH/O groups. Despite the presence of polar carboxyl groups, bilirubin is nonpolar and lipophilic. Glucuronidation disrupts hydrogen bonds and provides polar groups to yield water-soluble pigments. (For key to letters see Figure 27.11.)

Hemoglobin and heme released from intravascular hemolysis or blood extravasations (e.g., subcutaneous hematomas) are bound, respectively, by haptoglobin and hemopexin to form complexes that cannot be filtered by the kidney. This action prevents renal loss of the heme iron and protects the renal tubules from possible damage by precipitated hemoglobin. Haptoglobin–hemoglobin and hemopexin–heme complexes are processed in mononuclear phagocytic cells in a way similar to that for hemoglobin. Haptoglobin and hemopexin are glycoproteins synthesized in the liver. The former is an α2-globulin and an acute-phase reactant [10].

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Metabolism of Iron and Heme

N.V. BHAGAVAN , in Medical Biochemistry (Fourth Edition), 2002

Formation of Bilirubin

A summary of the pathway for bilirubin metabolism and excretion is shown in Figure 29-10. Release of heme from heme proteins and its conversion to bilirubin occur predominantly in the mononuclear phagocytes of liver, spleen, and bone marrow (previously known as the reticuloendothelial system), sites where sequestration of aging red cells occurs. Renal tubular epithelial cells, hepatocytes, and macrophages may also contribute to bilirubin formation under some conditions. Structures of the intermediates in the conversion of heme to bilirubin are shown in Figure 29-11. The initial step after the release of heme is its binding to heme oxygenase, a microsomal enzyme distinct from the microsomal P-450 oxygenases. Heme oxygenase catalyzes what appears to be the rate-limiting step in catabolism of heme. It is induced by heme and requires O2 and NADPH for activity. The activity of the inducible isoenzyme form of heme oxygenase is highest in the spleen, which is involved in the sequestration of senescent erythrocytes. The constitutive form of heme oxygenase is mainly localized in the liver and brain. After binding, the α-methene carbon of heme is oxidized (hydroxylated) to α-hydroxyhemin, which undergoes autoxidation to biliverdin (a blue-green pigment) with consumption of O2 and release of iron and carbon monoxide (derived from oxidation of the α-methene bridge). Since CO production in mammals occurs primarily by this pathway, measurement of expired CO has been used to estimate heme turnover. Values obtained exceed those derived from plasma bilirubin measurements by about 15%, probably because of bilirubin produced in the liver and excreted into the bile without entering the circulation. A potent competitive synthetic inhibitor of heme oxygenase is tin (Sn) protoporphyrin, which has a potential therapeutic use in treatment of neonatal jaundice (see below).

FIGURE 29-10. Catabolic pathway for the heme group from hemoproteins

FIGURE 29-11. Conversion of heme to bilirubin in the monocytic phagocytic cells. Carbon monoxide and bilirubin are generated. Fe3+ released is conserved and reutilized. Biliverdin and bilirubin are lactams. P, Propionic acid; M, methyl; V, vinyl.

In nonmammalian vertebrates, biliverdin is the final metabolite in heme catabolism. Transport of biliverdin is much easier than that of bilirubin because biliverdin is water-soluble. Conversion of biliverdin to bilirubin may have evolved in mammals because, unlike biliverdin, bilirubin readily crosses the placenta. In this way, the fetus can eliminate heme catabolites via the mother's circulation. However, this explanation may not be complete, since the rabbit (a placental mammal) excretes biliverdin as the major bile pigment.

Biliverdin is reduced to bilirubin by NAD(P)H-dependent biliverdin reductase, a cytosolic enzyme that acts at the central methene bridge. Although both molecules have two propionic acid groups, the polarity of biliverdin is greater than that of bilirubin. Bilirubin can form six internal hydrogen bonds between the carboxylic groups, the two lactam carbonyl oxygens, and four pyrrolenone ring nitrogens, and thus prevents these groups from hydrogen-bonding with water (Figure 29-12). Biliverdin cannot form these hydrogen bonds because of the lack of free rotation imposed by the double bond at the central methene bridge. Esterification of the propionyl side chains of bilirubin with glucuronic acid disrupts the hydrogen bonds and increases its solubility and lability. "Activators," such as ethanol and methanol, used in the van den Bergh test to measure "indirect bilirubin," and phototherapy for neonatal jaundice also act by disrupting the hydrogen-bonded structure of unconjugated bilirubin.

FIGURE 29-12. Conformation of bilirubin showing involuted hydrogen bonded-structure between NH/O and OH/O groups. Despite the presence of polar carboxyl groups, bilirubin is nonpolar and lipophilic. Disruption of hydrogen bonds by glucuronidation or by conversion of bilirubin to configurational or structural isomers yields water-soluble pigments.

Hemoglobin and heme released from intravascular hemolysis or blood extravasations (e.g., subcutaneoushematomas) are bound, respectively, by haptoglobin and hemopexin to form complexes that cannot be filtered by the kidney. This action prevents renal loss of the heme iron and protects the renal tubules from possible damage by precipitated hemoglobin. Haptoglobin-hemoglobin and hemopexin-heme complexes are processed in mononuclear phagocytic cells in a way similar to that for hemoglobin. Haptoglobin and hemopexin are glycoproteins synthesized in the liver. The former is an α2-globulin and an acute-phase reactant (i.e., its synthesis and release into the circulation are augmented during an acute insult to the body); the latter is a β1-globulin but not an acute-phase protein (see also Appendix VI).

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