section 29.3
Heme Catabolism
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.
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.
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
-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
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