Theoretical prediction and experimental measurement of the bile-pigment isomer pattern obtained from degradation of catalase haem.

Degradation in vitro of the haem in catalase by a 'coupled oxidation' reaction yields products in which approx. 45% of the haem groups have been cleaved at the alpha-methene bridge, 55% at the beta-bridge and a trace at the delta-bridge. Molecular-mechanics calculations with the three-dimensional structural co-ordinates of catalase shows that these proportions of products can be accounted for by the relative accessibility of the four methene bridges to a haem-linked oxygen molecule, thus further confirming Brown's [(1976) Biochem. J. 159, 23-27] hypothesis that the first stage of haem catabolism in vivo is selective attack by haem-bound oxygen, with selectivity conferred by the surrounding protein moiety.