Stereospecific haem cleavage. A model for the formation of bile-pigment isomers in vivo and in vitro.

A new approach is suggested for an explanation of sterospecific haem degradation to biliverdin and bilirubin. A model is proposed in which an oxygen molecule, bound to the haem iron atom, attacks a methene-bridge carbon atom in an intramolecular reaction. Specificity of macrocyclic ring cleavage is explained on the basis of the different accessibilities of the bound oxygen molecule to the four methene bridges. The consequences of these ideas are assessed in relation to coupled oxidation in model systems and to haem catabolism.