section 29.3

Heme Catabolism

691

Bilirubin

F IG U R E 2 9 -1 1

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.

V

F IG U R E 2 9 -1 2

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.

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 possi-

ble damage by precipitated hemoglobin. Haptoglobin-

hemoglobin and hemopexin-heme complexes are pro-

cessed 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

0

'

2

-globulin and an acute-phase reactant (i.e., its syn-

thesis and release into the circulation are augmented

during an acute insult to the body); the latter is a

/5]-globulin but not an acute-phase protein (see also

Appendix VI).

Circulatory Transport of Bilirubin

Bilirubin formed in extrahepatic tissues is transported to

the liver for excretion in bile. Since bilirubin is virtually

insoluble in aqueous media, it is transported to the liver

bound noncovalently to serum albumin. The bilirubin-

albumin complex increases the amount of bilirubin carried