BIOCHEMISTRY: A TEXTBOOK FOR MEDICAL UNIVERSITIES - E. S. Severin - 2004

SECTION 13. HEME METABOLISM AND IRON HOMEOSTASIS

III. Catabolism of Hemoglobin

Erythrocytes have a short lifespan (approximately 120 days). Under physiological conditions, about 1 — 2 x 10¹¹ erythrocytes are destroyed daily in the adult human body. Their Catabolism occurs mainly in the reticuloendothelial Cells of the Spleen, Lymph Nodes, Bone Marrow, and Liver. As erythrocytes age, the sialic acid content in Cell/30.html">The Plasma Membrane Glycoproteins decreases. The altered carbohydrate Components of the membrane glycoproteins are recognized by receptors on RES cells, and the erythrocytes are engulfed via endocytosis. The breakdown of erythrocytes in these cells begins with the Cleavage of Hemoglobin into heme and globin, followed by the lysosomal Enzymatic Hydrolysis of the protein moiety of hemoglobin.

A. Heme Catabolism

The first reaction of heme catabolism is mediated by the NADPH-dependent enzymatic complex, heme oxygenase. This enzyme system is localized in The Endoplasmic reticulum membrane, specifically within the Electron Transport Chains of microsomal oxidation. The enzyme catalyzes the Cleavage of the bond between two pyrrole rings containing vinyl residues, thereby opening the ring Structure (Fig. 13-11). This reaction yields a linear tetrapyrrole, biliverdin (a yellow pigment), and carbon monoxide (CO), which is derived from the carbon of the methenyl group. Heme induces the METABOLISM/31.html">Transcription of the heme oxygenase Gene, which is absolutely specific for heme.

Class="center">Fig. 13-11. Heme degradation. M — (-СН₃) — methyl group; V — (-СН = СН₂) — vinyl group; P — (-СН₂-СН₂-СООН) — propionic acid residue. During the reaction, one methyl group is converted into carbon monoxide, thereby opening the ring structure. The resulting biliverdin is converted into bilirubin by the action of biliverdin reductase.

Iron ions released during heme degradation can be reutilized for the synthesis of new hemoglobin molecules or other iron-containing Proteins. Biliverdin is reduced to bilirubin by the NADPH-dependent enzyme biliverdin reductase. Bilirubin is produced not only from hemoglobin breakdown but also during the catabolism of other heme proteins, such as Cytochromes and Myoglobin. The breakdown of 1 g of hemoglobin yields 35 mg of bilirubin, and an adult human produces approximately 250 — 350 mg of bilirubin daily. Further metabolism of bilirubin takes place in the liver.

B. Bilirubin Metabolism

Bilirubin produced in RES cells (spleen and bone marrow) is poorly soluble in Water and is transported through the bloodstream complexed with the plasma protein albumin. This form of bilirubin is referred to as unconjugated bilirubin. Each albumin molecule binds 2 (or even 3) bilirubin molecules, one of which is bound more tightly (with higher affinity) than the others. When Blood pH shifts to the acidic side (due to elevated concentrations of Ketone Bodies or lactate), the charge and conformation of albumin change, resulting in lowered affinity for bilirubin. Consequently, loosely bound bilirubin can be displaced from its binding sites and form complexes with Extracellular matrix Collagen and Membrane Lipids. Several pharmacological agents compete with bilirubin for the high-affinity binding site on albumin.

Uptake of Bilirubin by Parenchymal Liver Cells

The albumin-bilirubin complex, delivered to the liver via the bloodstream, dissociates at The surface of the hepatocyte plasma membrane. The released bilirubin forms a temporary complex with the lipids of the plasma membrane. Facilitated Diffusion of bilirubin into hepatocytes is mediated by Two Types of carrier proteins: ligandin (which transports the bulk of bilirubin) and protein Z. The rate of bilirubin uptake by hepatocytes depends on the rate of its intracellular metabolism.

Ligandin and protein Z are also found in Kidney and intestinal cells, enabling them to compensate for impaired detoxification processes in the liver during hepatic insufficiency.

Conjugation of Bilirubin in the Smooth Endoplasmic Reticulum

In the smooth endoplasmic reticulum of hepatocytes, polar groups—primarily from glucuronic acid—are attached to bilirubin (conjugation reaction). Because bilirubin contains 2 carboxyl groups, it can bind to 2 molecules of glucuronic acid, forming a water-soluble conjugate known as bilirubin diglucuronide (conjugated, or direct, bilirubin) (Fig. 13-12).

Fig. 13-12. Structure of bilirubin diglucuronide (conjugated, "direct" bilirubin). Glucuronic acid is attached via an ester bond to two propionic acid residues, forming an acyl glucuronide.

UDP-glucuronate serves as the donor of glucuronic acid. Specific Enzymes, UDP-glucuronosyltransferases (uridine diphosphate glucuronosyltransferases), catalyze The formation of bilirubin mono- and diglucuronides (Fig. 13-13). Certain drugs, such as phenobarbital (see Section 12), act as Inducers of UDP-glucuronosyltransferase synthesis.

Fig. 13-13. Formation of bilirubin diglucuronide.

Secretion of Bilirubin into Bile

The secretion of conjugated bilirubin into bile occurs via Active Transport, i.e., against a concentration gradient. Active transport is likely the rate-limiting step in the overall pathway of bilirubin metabolism in the liver. Under normal conditions, bilirubin diglucuronide is the primary form of bilirubin excreted into bile, although small amounts of monoglucuronide may also be present. The transport of conjugated bilirubin from the liver into bile is stimulated by the same drugs that induce bilirubin conjugation. Thus, the rate of bilirubin conjugation and The active transport of bilirubin glucuronide from hepatocytes into bile are strictly interdependent (Fig. 13-14).

Fig. 13-14. The bilirubin-urobilinogen cycle in the liver. 1 — Hb catabolism in reticuloendothelial cells of the bone marrow, spleen, and lymph nodes; 2 — Formation of the transport bilirubin-albumin complex; 3 — uptake of bilirubin by the liver; 4 — formation of bilirubin glucuronides; 5 — secretion of bilirubin as part of bile into the intestine; 6 — catabolism of bilirubin by intestinal Bacteria; 7 — excretion of urobilinogens with feces; 8 — reabsorption of urobilinogens into the blood; 9 — hepatic uptake of urobilinogens; 10 — entry of a fraction of urobilinogens into the blood and renal excretion in urine; 11 — secretion of a small fraction of urobilinogens into bile.

B. Catabolism of bilirubin diglucuronide

In the intestine, the delivered bilirubin glucuronides are hydrolyzed by specific bacterial enzymes, $eta$-glucuronidases, which cleave the bond between bilirubin and the glucuronic acid residue. The bilirubin released during this reaction is reduced by the intestinal microflora to form a group of colorless tetrapyrrole compounds—urobilinogens (Fig. 13-15).

Fig. 13-15. Structure of certain bile pigments. Mesobilinogen is an intermediate product of bilirubin catabolism in the intestine.

In the ileum and colon, a small fraction of urobilinogens is reabsorbed and carried via the portal vein blood to the liver. The major portion of urobilinogen from the liver is excreted into the intestine as part of bile and eliminated from the body with feces; a fraction of urobilinogen from the liver enters the bloodstream and is excreted in the urine as urobilin (Fig. 13-14). Normally, most of the colorless urobilinogens produced in the Large Intestine are oxidized in the rectum to the brown pigment urobilin by the action of intestinal microflora and eliminated with feces. The color of feces is due to the presence of urobilin.