Bilirubin heme catabolism producing

In the cases of dietary heme and nonheme iron, the iron appears in the bloodstream bound to the transport protein transferrin. After its dissociation from dietary proteins by proteases, the heme is absorbed intact by the enterocyte. The heme i.s then degraded by heme oxidase. Heme oxidase catalyzes the Oj-depend-ent degradation of heme to biliverdm. Biliverdin is further degraded to bilirubin, which is excreted from the body in the bile. Heme absorption, as well as heme oxidase activity, is somewhat higher in the duodenum than in the jejunum and ileum, as determined in studies with rats. The heme catabolic pathway is shown in Figure 10,29, Most of the bilirubin in the body is not produced by the catabolism of dietary heme, but by the catabi lism of the heme present in old, or senescent, red blood cells, between 7S and 80% of the bilirubin formed in the body is derived from senescent red blood cells most of the remainder is derived from the normal turnover of the heme proteins in the liver. 

Bilirubin and carbon monoxide are always produced in equimolar proportions under normal conditions of heme catabolism the process is oxidative and requires molecular oxygen. Labelling studies using 02 show that the labelled oxygen ends up in the bilirubin at the terminal lactam positions, and in the carbon monoxide that is liberated (by removal of the C-5 carbon). The overall process is shown in Figure 5.5. 

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