Biliverdin is a degradation product of heme with a linear chain of four tetrapyrrole rings. Biliverdin is bound inside the up-and-down beta-barrels of insect biliverdin-bindind protein. The polar ends of biliverdin are at the open end of the BBP barrel, similar as that the polar end of retinol is at the open end of the RBP barrel. Insects such as cabbage white butterflies and tobacco horn worms use BBP as one component in camouflage which is an essential ingredient in their survival strategy. Two pigments-one yellow from carotenoids and one blue from biliverdins-combine to be responsible for the matching of the insects' color to their natural residence background. The pigments exist as complexes with specific pigmet-binding proteins in vivo. In human beings, biliverdin and its degradation derivative bilirubin are responsible for the changing color of a bruise(Stryer,1995).

Figure: Degradation pathway of heme to biliverdin and then to bilirubin.

The normal human erythrocyte with a life span of about 120 days must be removed from the circulation and degraded by the spleen. Eventually the apoprotein of hemoglobin is hydrolyzed to its constituent amino acids (Stocker et al.,1987;1990). Biliverdin and bilirubin are intermediates in this breakdown of heme as shown in the figure. The first step in the heme degradation is the cleavage of its alpha-methene bridge to form the linear tetrapyrrole biliverdin, catalyzed by heme oxygenase. Oxygen and NADPH are required for the first cleavage step. The cleaved methene-bridge carbon is released as carbon monoxide. The endogenous product CO is a competitive inhibitor for oxygen carrier substrates. Biliverdin then is reduced to bilirubin by the enzyme biliverdin reductase and the reductant NADPH. Unsoluble bilirubin complexed to serum albumin is transported to the liver, where its propionate side chains are attached to the solubilizing sugar glucuronate to form soluble bilirubin diglucuronide to be excreted. In reptiles and birds, the end product of heme catabolism is biliverdin rather than bilirubin. Bilirubin is much less soluble in aqueous media than is biliverdin. But mammals select bilirubin as the end product for the benefit that bilirubin is an especially potent antioxidant in membranes, where it rivals vitamin E. Vitamin E protects unsaturated membrane lipids from oxidation. A deficiency in vitamin E can cause infertility in experiment rats( Jukes 1992).