On the mechanism of the chemical and enzymic oxygenations of alpha-oxyprotohemin IX to Fe.biliverdin IX alpha.

Abstract

.alpha.-Oxyprotohemin IX, an early intermediate in heme catabolism, was synthesized and its autoxidation to biliverdin IX.alpha. was studied. In anaerobic aqueous pyridine, .alpha.-oxyprotohemin (hexacoordinated) underwent autoreduction to yield an Fe(II) .alpha.-oxyprotoporphyrin .pi.-neutral radical bis(pyridine) complex, which reacted with an equimolar amount of dioxygen to give pyridine.cntdot.verdohemochrome IX.alpha. and CO in 75-80% yield via an intermediate with an absorption maximum at 893 nm. Verdohemochrome IX.alpha. did not react with further dioxygen. Reconstituted apomyoglobin-.alpha.-oxyprotohemin IX complex (pentacoordinated) reacted with an equimolar amount of dioxygen to form an Fe(II) oxyporphyrin .pi.-neutral radical intermediate, which rearranged to a green compound (.lambda.max 660 and 704 nm) with elision of CO. The green product, which is probably an apomyoglobin.cntdot.verdoheme .pi.-radical complex, reacted with another equimolar amount of dioxygen to give Fe(III).cntdot.biliverdin IX.alpha.. Demetallation of this gave biliverdin IX.alpha. in overall yield of 70-75%. These results indicate that the sequence of oxyheme autoxidation in the presence of apomyoglobin is .alpha.-oxyprotoheme IX .**GRAPHIC**. verdohemochrome IX.alpha. .pi.-radical .**GRAPHIC**. Fe(III).cntdot.biliverdin IX.alpha.. A similar mechanism may prevail in vivo. The hexa- and penta-coordinated Fe(II) .pi.-radical form of the oxyporphyrin is crucial in triggering the autoxidation of the complex to verdohemochrome IX.alpha.. Further oxygenation of verdohemochrome IX.alpha. to Fe(III).cntdot.biliverdin IX.alpha. occurred only in the pentacoordinated apomyoglobin.cntdot.verdoheme Fe(II) complex.