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

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.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.verdoheme pi-radical complex, reacted with another equimolar amount of dioxygen to give Fe(III).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 O2----CO----verdohemochrome IX alpha pi-radical O2----Fe(III).biliverdin IX alpha. A similar mechanism may prevail in vivo. The hexa- and pentacoordinated 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).biliverdin IX alpha occurred only in the pentacoordinated apomyoglobin.verdoheme Fe(II) complex.