Biochemistry of Man, Volume 1 - Murray R. 1993

Protein and Amino Acid Metabolism
Porphyrins and Bile Pigments
Catabolism of Heme: Formation of Bile Pigments

Under physiological conditions, 1—2∙10⁸ erythrocytes are destroyed per hour in the body of an adult human. Thus, approximately 6 g of Hemoglobin is turned over daily in a 70 kg human. Upon hemoglobin breakdown, its protein moiety (globin) can be utilized as such or following Hydrolysis into its constituent Amino Acids; the heme iron enters the general iron pool and is likewise reused. Meanwhile, the iron-free porphyrin portion of heme is obligatorily degraded, a process that occurs primarily within the reticuloendothelial Cells of the Liver, Spleen, and Bone Marrow.

The Catabolism of heme liberated from any hemoprotein is carried out in the microsomal fraction of reticuloendothelial cells by a complex enzyme system known as heme oxygenase. By the time heme is delivered from Hemoproteins to the heme oxygenase system, the iron is typically oxidized to the ferric state (converting heme to hemin); hemin can readily bind to albumin to form methemalbumin. The heme oxygenase system is substrate-inducible and localized near the microsomal electron transport system. As illustrated in Fig. 33.12, hemin is reduced to the ferrous state by NADPH; subsequently, in the presence of NADPH, oxygen is added to the a-methenyl bridge between pyrrole rings I and II. The ferrous iron is reoxidized to the ferric state. Further oxygen addition results in the release of the ferric ion, the liberation of a carbon monoxide molecule, and the opening of the tetrapyrrole ring to yield an equimolar amount of biliverdin IX-a. In this reaction, heme itself acts catalytically.

Class="center">

Fig. 33.12. Schematic diagram of the microsomal heme oxygenase system. (Reproduced with modifications from the review by Schmid R., McDonough A. E. The Porphyrins. Dolphin D. (ed.). Academic Press, 1978.)

In birds and amphibians, the green pigment biliverdin IX-a is excreted from the Organism; in mammals, the soluble enzyme biliverdin reductase catalyzes the reduction of the methenyl bridge between pyrroles III and IV into a methylene group, yielding the yellow pigment bilirubin IX-a (Fig. 33.12).

Calculations indicate that 35 mg of bilirubin is produced from 1 g of hemoglobin. The daily production of bilirubin in an adult human is approximately 250—350 mg.

The chemical conversion of heme to bilirubin by reticuloendothelial cells can be observed in vivo: in a hematoma, the purple color caused by heme slowly transitions into the yellow color of bilirubin.

Further METABOLISM of bilirubin occurs predominantly in The Liver and comprises three processes: 1) uptake of bilirubin by hepatic parenchymal cells; 2) conjugation of bilirubin within the smooth Endoplasmic reticulum; and 3) secretion of bilirubin from The endoplasmic reticulum into the Bile. Let us examine each of these processes individually.

Hepatic Uptake of Bilirubin

Bilirubin is sparingly soluble in plasma and Water; in plasma, it is specifically bound to albumin. Each albumin molecule apparently possesses two bilirubin-binding sites—a high-affinity and a low-affinity site. 100 ml of plasma can contain 25 mg of bilirubin tightly bound to albumin at its high-affinity site. "Excess" bilirubin is bound to albumin less firmly and readily dissociates from albumin, diffusing into Tissues. Certain compounds—Antibiotics and several other therapeutic agents—compete with bilirubin for the high-affinity site on albumin. These compounds can displace bilirubin from its complex with albumin and manifest significant clinical effects.

In the liver, bilirubin is transferred from albumin to the sinusoidal surface of hepatocytes via a saturable transport system involving a specific carrier. This facilitated transport system has a very high capacity and does not rate-limit bilirubin metabolism even under pathological conditions.

Because facilitated transport establishes equilibrium for bilirubin on both sides of the sinusoidal hepatocyte membrane, bilirubin uptake depends on its subsequent metabolic consumption.

Conjugation of Bilirubin

In the liver, polar groups are attached to bilirubin, converting it into a water-soluble form that is secreted into the bile. The process that enhances the water solubility (i.e., increases the polarity) of bilirubin is termed conjugation. This process, at least in its initial stages, takes place in the smooth Endoplasmic reticulum and is mediated by a specialized set of Enzymes. In mammals, bilirubin is secreted into the bile predominantly as bilirubin diglucuronide (Fig. 33.13). The process begins with The formation of bilirubin monoglucuronide, catalyzed by UDP-glucuronosyltransferase, an enzyme localized in the smooth endoplasmic reticulum and likely composed of multiple components. The catalyzed reaction is depicted in Fig. 33.14. It occurs mainly in the liver, as well as in the Kidneys and intestinal mucosa. When bilirubin metabolism is impaired, its conjugates are found in serum primarily as monoglucuronides.

The formation of bilirubin diglucuronide may occur in the canalicular membrane of hepatocytes through the action of a UDP-glucuronosyltransferase similar to the one discussed above (Fig. 33.14) or another dismutase enzyme that catalyzes The conversion of two molecules of bilirubin monoglucuronide into one molecule of bilirubin diglucuronide and one molecule of free bilirubin (Fig. 33.14). The bilirubin conjugation system will be discussed further below in connection with its inherited disorders.

The activity of UDP-glucuronosyltransferase can be induced by A number of clinically used drugs, notably phenobarbital.

Secretion of Bilirubin into Bile

The secretion of conjugated bilirubin into bile proceeds against a steep concentration gradient and must therefore be accomplished via an Active Transport mechanism. Active transport is presumably the rate-limiting step in the overall hepatic metabolism of bilirubin. The transport of conjugated bilirubin from the liver into bile is induced by the same drugs capable of inducing bilirubin conjugation. Thus, the systems for bilirubin conjugation and its efflux from hepatocytes operate as a single functionally coordinated mechanism.

Fig. 33.13. Structure of bilirubin diglucuronide (conjugated "direct" bilirubin). Glucuronic acid is attached via an ester linkage to two propionic acid groups to form an acyl glucuronide.

Fig. 33.14. Conjugation of bilirubin with glucuronic acid. The glucuronate donor is UDP-glucuronic acid, which is synthesized from UDP-glucose.

Under physiological conditions, virtually all bilirubin secreted into the bile (over 97%) is in the conjugated form. Only following phototherapy can appreciable amounts of unconjugated bilirubin be detected in bile.

The liver possesses numerous systems for the biliary secretion of natural and synthetic pharmacological compounds following their metabolism. Some of these systems are also utilized by bilirubin diglucuronides.