Haem oxygenase: a reappraisal of the stoicheiometry

Abstract

Haem oxygenase (EC 1.14.99.3) in rat liver microsomes was first described as a mixed function oxidase utilising cytochrome P-450 [I]. Immunochemical studies suggest that NADP(H)-cytochrome c reductase activity is an essential component of the enzyme [2]. Indeed, it has been suggested [3] following the use of partially purified enzyme preparations, that all microsomal haem oxygenase activity could potentially be accounted for by the NADP(H)-cytochrome c reductase activity. This enzyme reduces added haem which then reacts directly with oxygen. A microsomal protein factor is required solely to provide specific cleavage at the a-methene bridge. Studies of a reconstituted haem oxygenase system [4] consisting of a partly purified haem oxygenase protein and a purified NADP(H)-cytochrome c reductase, indicated that the initial steps in the haem oxygenase reaction involved the binding of protohaem to haem oxygenase, followed by the reduction of the ferric haem by NADP(H)-cytochrome c reductase. The ferrous haem then reacts readily with molecular oxygen to cleave the a-methene bridge. The mechanism of ring cleavage by haem oxygenase is still unclear however, although hydrogen peroxide, acting either directly, or through the action of hydroxyl-radicals, has been implicated [3]. Other workers have suggested the involvement of the superoxide anion [5,6] in this step. However, several workers have observed a lack of stoicheiometry in the haem oxygenase reaction [ 1,7], with indications that fl-biliverdin may also be formed [8].