Bilirubin metabolism

INTRODUCTION — Bilirubin is the catabolic product of heme metabolism. Within the physiologic range, bilirubin has cytoprotective and beneficial metabolic effects, but at high levels it is potentially toxic. Fortunately, there are elaborate physiologic mechanisms for its detoxification and disposition. Understanding these mechanisms is necessary for interpretation of the clinical significance of high serum bilirubin concentrations. Furthermore, because bilirubin shares its metabolic pathway with various other sparingly water-soluble substances that are excreted in bile, understanding bilirubin metabolism also provides insight into the mechanisms of transport, detoxification, and elimination of many other organic anions [1].

An overview of the major aspects of bilirubin formation and disposition will be reviewed here. The settings in which bilirubin disposition is impaired will also be discussed briefly. Clinical aspects of serum bilirubin determination, the evaluation of patients with hyperbilirubinemia, and the classification of causes of jaundice are presented separately. (See "Diagnostic approach to the adult with jaundice or asymptomatic hyperbilirubinemia" and "Classification and causes of jaundice or asymptomatic hyperbilirubinemia".)

BILIRUBIN SYNTHESIS

Conversion of heme to bilirubin — Bilirubin is formed by breakdown of heme present in hemoglobin, myoglobin, cytochromes, catalase, peroxidase, and tryptophan pyrrolase. Eighty percent of the daily bilirubin production (250 to 400 mg in adults) is derived from hemoglobin [2]; the remaining 20 percent is contributed by other hemoproteins and a rapidly turning-over small pool of free heme. Enhanced bilirubin formation is found in all conditions associated with increased red cell turnover such as intramedullary or intravascular hemolysis (eg, hemolytic, dyserythropoietic, and megaloblastic anemias).

Heme consists of a ring of four pyrroles joined by carbon bridges and a central iron atom (ferroprotoporphyrin IX). Bilirubin is generated by sequential catalytic degradation of heme mediated by two groups of enzymes:

●Heme oxygenase

●Biliverdin reductase

Heme oxygenase initiates the opening of the porphyrin ring of heme by catalyzing the oxidation of the alpha-carbon bridge (figure 1). This leads to the formation of the green pigment, biliverdin, which is then reduced by biliverdin reductase to the orange-yellow pigment bilirubin IX-alpha. Iron is released in this process, and the oxidized alpha-bridge carbon is eliminated as carbon monoxide (CO). Measurement of intrinsic CO production has been used to quantify bilirubin production [3,4].

Heme oxygenase is rate-limiting in bilirubin production [5]. Heme oxygenase is present in high concentrations in reticuloendothelial cells of the spleen, the principal site of red cell breakdown, and in Kupffer cells in the liver [6]. It is induced by heme (eg, in hemolytic states).

Binding of bilirubin to plasma albumin — Bilirubin is very poorly soluble in water at physiologic pH because of internal hydrogen bonding that engages all polar groups and gives the molecule a contorted "ridge-tile" structure [7]. The fully hydrogen-bonded structure of bilirubin is designated bilirubin IX-alpha-ZZ. Binding to albumin and, to a much lesser degree, high-density lipoprotein, keeps bilirubin in solution in plasma; only a small fraction of bilirubin circulates in the unbound state. Binding to high-density lipoprotein may become significant in states of severe hypoalbuminemia. Albumin binding keeps bilirubin in the vascular space, thereby preventing its deposition into extrahepatic tissues, including sensitive tissues such as the brain, and minimizing glomerular filtration. It also transports bilirubin to the sinusoidal surface of the hepatocyte, where the pigment dissociates from albumin and enters the hepatocyte. (See 'Conjugation of bilirubin' below.)

Several ligands bind to albumin at the same site as bilirubin, including sulfonamides, warfarin, anti-inflammatory drugs, and cholecystographic contrast media. These agents can displace bilirubin from albumin, thereby precipitating bilirubin encephalopathy in newborns without an alteration in the total serum bilirubin concentration [8]. Many other compounds, such as fatty acids, bind at a different albumin site but may, in some cases, reduce the binding constant of albumin for bilirubin [9].

Albumin binding of bilirubin is usually reversible. However, irreversible binding can occur in the presence of prolonged conjugated hyperbilirubinemia (eg, during biliary obstruction). The bilirubin fraction irreversibly bound to albumin (delta bilirubin) is not cleared by the liver or the kidney and, because of the long half-life of albumin, lingers in the plasma [10]. This may result in prolonged hyperbilirubinemia after endoscopic or surgical relief of biliary obstruction. Because delta-bilirubin gives a "direct" diazo reaction, this may give a false impression of a persistent blockage of the bile ducts [11]. The presence of delta-bilirubin can be inferred by the absence of bilirubin excretion in the urine despite the apparent presence of direct hyperbilirubinemia and can be identified by high performance liquid chromatography of serum. (See "Overview of liver biochemical tests", section on 'Bilirubin'.)

METABOLISM OF BILIRUBIN — In the plasma, albumin-binding keeps it water-soluble. Subsequent conversion of bilirubin IX-alpha to a water-soluble form, by disruption of the hydrogen bonds, is essential for its elimination by the liver and kidneys. This is achieved by glucuronic acid conjugation of the propionic acid side chains of bilirubin. Bilirubin glucuronides are water-soluble and are readily excreted in bile. (See 'Active transport into the bile' below.)

Hepatic uptake and storage — In the liver sinusoids, the albumin-bilirubin complex dissociates, and the bilirubin is taken up efficiently by the hepatocyte while the albumin remains in circulation. Bilirubin is taken up by hepatocytes by a process of facilitated diffusion, which is not energy-consuming; as a result, transport cannot occur against a concentration gradient, and is bidirectional (figure 2). Defects in the specific transporters that mediate each of the steps in bilirubin transport can lead to hyperbilirubinemia. (See "Inherited disorders associated with conjugated hyperbilirubinemia" and "Gilbert syndrome".)

Sinusoidal bilirubin uptake requires inorganic anions, such as chloride, and is thought to be mediated by carrier proteins that have not been fully characterized [12,13]. Passage of bilirubin across the hepatocyte sinusoidal surface membrane is bidirectional. Normally, canalicular excretion, rather than sinusoidal uptake or glucuronidation in the endoplasmic reticulum, is rate-limiting for bilirubin throughput. Bilirubin entering the hepatic sinusoids is efficiently extracted by hepatocytes close to its point of entry (ie, periportal region). A fraction of the unconjugated and conjugated bilirubin within the hepatocytes is transported back into the sinusoidal blood. This fraction undergoes re-uptake by hepatocytes downstream to the sinusoidal blood flow. The re-uptake is mediated by two proteins, organic anion transporter protein 1B1 and 1B3 (OATP1B1 and OATP1B3), encoded by the genes SLCO1B1 and SLCO1B3. This results in the recruitment of additional hepatocytes in this process, thereby increasing the net bilirubin excretory capacity of the liver [14]. Within the hepatocyte, bilirubin and other organic anions bind to glutathione S-transferases (GSTs). GST-binding reduces the efflux of the internalized bilirubin, thereby increasing net uptake (figure 2).

This process of bilirubin uptake is impaired in certain disease states:

●Bilirubin uptake is inhibited by certain drugs (eg, rifampin, flavaspidic acid, and cholecystographic dyes).

●The re-uptake of conjugated and unconjugated bilirubin is disrupted in Rotor syndrome, which is caused by mutations or deletion of both SLCO1B1 and SLCO1B3, resulting in loss of function of both OATP1B1 and OATP1B3. (See "Inherited disorders associated with conjugated hyperbilirubinemia", section on 'Rotor syndrome'.)

●In patients with cirrhosis, a portion of the bilirubin produced in the spleen may bypass the liver via portosystemic collaterals. Furthermore, the sinusoidal endothelium, which is normally fenestrated, may lose the fenestrae (capillarization), thereby creating a barrier between the plasma and the hepatocytes. As a result, serum unconjugated bilirubin concentrations often increase in this condition.

Conjugation of bilirubin — Glucuronidation of bilirubin, a large variety of endogenous compounds (eg, steroid hormones, thyroid hormones, catecholamines), and a wide array of exogenous substrates (eg, drugs, toxins, carcinogens, and laboratory xenobiotics), is mediated by a family of enzymes, termed uridine-diphosphoglucuronate glucuronosyltransferase (UGT) (figure 2) [15]. Glucuronides are more water soluble and are readily excreted in bile and urine.

Enzyme-catalyzed glucuronidation is one of the most important detoxification mechanisms of the body. Of the various isoforms of the UGT family of enzymes, only one isoform, UGT1A1 is physiologically important in bilirubin glucuronidation (figure 3) [16]. UGT1A1 is an intrinsic protein of the endoplasmic reticulum (ER), and its catalytic site is located within the ER lumen. Therefore, the sugar donor substrate uridine diphosphate glucuronic acid (UDPGA) must enter the ER luminal space for conjugation. UDP-N-acetylglucosamine is the natural stimulator of the transport of the polar substrate UDPGA into the ER lumen. It has been suggested that four nucleotide sugar transporter proteins mediate UDP-N-acetylglucosamine stimulation of UDPGA transport [17].

Bilirubin diglucuronide is the predominant pigment in normal adult human bile, representing over 80 percent of the pigment. However, in subjects with reduced bilirubin-UGT activity, the proportion of bilirubin diglucuronide decreases, and bilirubin monoglucuronide may constitute more than 30 percent of the conjugates excreted in bile. Reduction of conjugating enzyme activity to approximately 30 percent of normal results in a mild but discernible increase in serum bilirubin concentrations.

Inhibitory factor(s) for hepatic UGT1A1 is secreted in the milk of some mothers (breast milk jaundice). In other cases, an inhibitory factor present in maternal plasma may be transplacentally transferred to the fetus (the Lucey-Driscoll syndrome). UGT1A1 deficiency also may be seen in neonates, chronic hepatitis, and in certain inherited disorders (Gilbert syndrome and Crigler-Najjar syndrome I and II). (See "Gilbert syndrome" and "Crigler-Najjar syndrome".)

EXCRETION OF CONJUGATED BILIRUBIN

Active transport into the bile — Bilirubin is primarily excreted in normal human bile as the diglucuronide; unconjugated bilirubin accounts for only 1 to 4 percent of pigments in normal bile. Conjugated bilirubin and other substances destined to be excreted in bile are actively transported across the bile canalicular membrane of the hepatocyte against a concentration gradient [18]. Among the four types of canalicular transporters, the multispecific organic anion transporter, also termed multidrug resistance protein 2 or ATP-binding cassette (ABC) C2 (cMOAT/MRP2/ABCC2), appears to be the most important for the canalicular secretion of bilirubin and many other organic anions, with the exception of bile acids (figure 2) [19]. Interestingly, a portion of the conjugated bilirubin is secreted back into the sinusoidal blood via the ATP hydrolysis-coupled pump ABCC3. Reuptake of the conjugated bilirubin by hepatocytes downstream to the sinusoidal blood flow is mediated by the sinusoidal surface organic anion transporters OATP1B1 and OATP1B3. The recruitment of additional hepatocytes increases the hepatic excretory capacity for conjugated bilirubin, which is rate-limiting in bilirubin throughput [14].

Enhanced bile flow (eg, by infusion of bile salts) or phenobarbital treatment increases the maximal bilirubin excretory capacity. On the other hand, the excretion of conjugated bilirubin is impaired in a number of acquired conditions (eg, alcoholic or viral hepatitis, cholestasis of pregnancy) and inherited disorders (eg, Dubin-Johnson syndrome, Rotor syndrome, benign recurrent intrahepatic cholestasis). It can also be caused by a variety of drugs (eg, alkylated steroids, chlorpromazine). (See "Inherited disorders associated with conjugated hyperbilirubinemia" and "Classification and causes of jaundice or asymptomatic hyperbilirubinemia".)

Degradation of bilirubin in the digestive tract — Bile pigment appearing in bile is mostly (more than 98 percent) conjugated. Conjugated bilirubin is water soluble and is not absorbed across the lipid membrane of the small intestinal epithelium; in comparison, the unconjugated bilirubin fraction is partially reabsorbed and undergoes enterohepatic circulation (figure 4) [20].

Bilirubin is reduced by bacterial enzymes in the colon to a series of molecules, termed urobilinogens [21]. It is partly absorbed in the bowel and undergoes hepatobiliary recirculation. The fraction that is not cleared by the liver enters the general circulation and is partly excreted in urine as urinary urobilinogen (figure 4).

The two major urobilinoids found in stool, urobilinogen and stercobilinogen, are colorless and turn orange-yellow only after oxidation to urobilins in the gut. In complete biliary obstruction or severe intrahepatic cholestasis (eg, in the early phase of acute viral hepatitis), feces may take the appearance of clay. Thus, the absence of urobilinogen in stool and urine in a jaundiced patient indicates complete biliary obstruction. Urobilinogens and their derivatives are partly absorbed from both the large and small intestine, undergo enterohepatic recycling, and are eventually excreted in urine and feces  (figure 4).

Urinary urobilinogen excretion may be increased in the following situations:

●Excessive bilirubin production (eg, in cases of hemolysis or absorption of hematoma)

●Inefficient hepatic clearance of the reabsorbed urobilinogen (eg, in patients with cirrhosis or at some stages of hepatitis)

●Excessive exposure of bilirubin to intestinal bacteria (eg, constipation or bacterial overgrowth)

Standard clinical tests for urobilinogen do not distinguish between normal and low urinary urobilinogen levels. Furthermore, urobilinogen excretion can be reduced, normal, or elevated in patients with hepatitis as noted above. Tubular reabsorption and instability of the pigment in acid urine can also influence results. Because of these complexities, tests for urinary urobilinogen are usually not useful in the differential diagnosis of liver diseases.

Alternative pathways of bilirubin elimination — Alternative pathways of bilirubin elimination may be important in certain clinical settings. One such pathway is oxidation of bilirubin by mixed function oxidases in liver and other organs. In addition, induction of cytochrome P-450c by chlorpromazine reduced the serum bilirubin concentration in one patient with Crigler-Najjar syndrome type I (enzyme catalyzed oxidation) [22].

In conditions associated with elevated conjugated plasma bilirubin concentrations (eg, intrahepatic or extrahepatic cholestasis), the kidney is responsible for 50 to 90 percent of conjugated bilirubin excretion [23,24]. However, bile remains the main excretion route for unconjugated hyperbilirubinemia.

HYPERBILIRUBINEMIA

Etiology — In normal plasma, about 4 percent of bilirubin is conjugated. However, the proportion of conjugated and unconjugated bilirubin in plasma may vary in disease states. As examples:

●In inherited disorders of bilirubin conjugation (eg, Gilbert syndrome, Crigler-Najjar), the proportion of conjugated bilirubin is reduced. (See "Gilbert syndrome".)

●In Rotor syndrome (defect in re-uptake of conjugated and unconjugated bilirubin) and in Dubin-Johnson syndrome (defect in canalicular excretion of bilirubin), both conjugated and unconjugated bilirubin accumulate in plasma [14]. (See "Inherited disorders associated with conjugated hyperbilirubinemia".)

●In biliary obstruction or hepatocellular diseases, both conjugated and unconjugated bilirubin accumulate in plasma [10].

●In hemolytic jaundice, total plasma bilirubin increases, but the proportion of the unconjugated and conjugated fractions remains unchanged.

Other causes of hyperbilirubinemia and the evaluation and management of hyperbilirubinemia are discussed in detail separately. (See "Classification and causes of jaundice or asymptomatic hyperbilirubinemia" and "Diagnostic approach to the adult with jaundice or asymptomatic hyperbilirubinemia".)

Toxicity of unconjugated bilirubin — Bilirubin binds to the elastic tissue of skin and sclera and is also found in all tissue fluids with a high albumin content. The usual tight but reversible binding to albumin precludes glomerular filtration of unconjugated bilirubin; similar principles apply to irreversible covalently bound bilirubin (delta-bilirubin). In contrast, conjugated bilirubin is less strongly bound to albumin and can be excreted in the urine. Thus, the finding of bilirubin in the urine, in the absence of albuminuria, indicates the presence of an increased amount of conjugated bilirubin in the plasma.

Unconjugated bilirubin is toxic to many cells and organelles. Markedly elevated serum unconjugated bilirubin concentrations (usually over 20 mg/dL) in the newborn may result in clinical evidence of brain damage, ranging from subtle neurologic abnormalities to severe encephalopathy or permanent bilirubin-induced neurologic damage (BIND; commonly known as kernicterus) to death [25]. The serum concentration at which clinical signs of neurotoxicity occur is variable and is influenced by many factors such as protein-binding of bilirubin, putative bilirubin transporters in the brain, and enzymes in the central nervous system that oxidize bilirubin [26]. Physiologic mechanisms that protect against bilirubin toxicity include binding to plasma albumin, and rapid uptake, conjugation, and clearance by the liver. Because of these efficient protective mechanisms, the harmful effect of unconjugated bilirubin is limited to neonates with a high degree of unconjugated hyperbilirubinemia and subjects with Crigler-Najjar syndrome.

Within a near-physiological range of serum bilirubin concentrations, the antioxidative action of bilirubin may provide beneficial effects [27-32]. (See "Unconjugated hyperbilirubinemia in neonates: Risk factors, clinical manifestations, and neurologic complications", section on 'Chronic bilirubin encephalopathy (kernicterus)'.)

SUMMARY AND RECOMMENDATIONS

●Bilirubin synthesis – Bilirubin is formed by the breakdown of heme present in hemoglobin, myoglobin, cytochromes, catalase, peroxidase, and tryptophan pyrrolase. Eighty percent of the daily bilirubin production (250 to 400 mg in adults) is derived from hemoglobin. (See 'Bilirubin synthesis' above.)

●Transport in plasma – Bilirubin is very poorly soluble in water at physiologic pH because of internal hydrogen bonding that engages all polar groups and gives the molecule a contorted "ridge-tile" structure. Albumin binding keeps bilirubin in the vascular space, thereby preventing its deposition into extrahepatic tissues, including sensitive tissues such as the brain, and minimizing glomerular filtration. It also transports bilirubin to the sinusoidal surface of the hepatocyte, where the pigment dissociates from albumin and enters the hepatocyte. (See 'Binding of bilirubin to plasma albumin' above.)

●Hepatic uptake and conjugation – In the liver sinusoids, the albumin-bilirubin complex dissociates, and the bilirubin is taken up efficiently by the hepatocyte while the albumin remains in circulation (figure 2). Bilirubin is taken up by hepatocytes by a process of facilitated diffusion. Glucuronidation of bilirubin is mediated by a family of enzymes, termed uridine-diphosphoglucuronate glucuronosyltransferase (UGT) (figure 3). (See 'Metabolism of bilirubin' above.)

●Bilirubin excretion – Bilirubin is primarily excreted in normal human bile as the diglucuronide; unconjugated bilirubin accounts for only 1 to 4 percent of pigments in normal bile.

Conjugated bilirubin is water soluble and is not absorbed across the lipid membrane of the small intestinal epithelium; in comparison, the unconjugated bilirubin fraction is partially reabsorbed and undergoes enterohepatic circulation (figure 4). Bilirubin is reduced by bacterial enzymes in the colon to a series of molecules, termed urobilinogens. It is partly absorbed in the bowel and undergoes hepatobiliary recirculation. The fraction that is not cleared by the liver enters the general circulation and is partly excreted in urine as urinary urobilinogen (figure 4). (See 'Excretion of conjugated bilirubin' above.)

●Hyperbilirubinemia and bilirubin toxicity in selected individuals – In normal plasma, about 4 percent of bilirubin is conjugated. This relationship may vary in disease states. Markedly elevated serum unconjugated bilirubin concentrations (usually over 20 mg/dL) in the newborn may result in clinical evidence of brain damage, ranging from subtle neurologic abnormalities to severe encephalopathy or permanent bilirubin-induced neurologic damage (BIND; commonly known as kernicterus) to death. Physiologic mechanisms that protect against bilirubin toxicity include binding to plasma albumin, and rapid uptake, conjugation, and clearance by the liver. Because of these efficient protective mechanisms, the harmful effect of unconjugated bilirubin is limited to neonates with a high degree of unconjugated hyperbilirubinemia and subjects with inherited disorders of bilirubin conjugation. (See "Classification and causes of jaundice or asymptomatic hyperbilirubinemia" and 'Hyperbilirubinemia' above.)