Detailed reviews of the chemistry and metabolism of bilirubin are found elsewhere (3,4,5,6,7).

Bilirubin is the end product of the catabolism of iron protoporphyrin or heme, of which the major source is circulating hemoglobin (8). The formation of bilirubin from hemoglobin involves removal of the iron and protein moieties, followed by an oxidative process catalyzed by the enzyme microsomal heme oxygenase, an enzyme found in the reticuloendothelial system as well as many other tissues (9). The α-methane bridge of the heme porphyrin ring is opened and carbon monoxide (CO) and biliverdin are formed (Fig. 35-1). One molecule of CO and biliverdin (and, subsequently, bilirubin) is formed for each molecule of heme degraded (4).