Studies on ferrochelatase. 1. Assay and properties of ferrochelatase from a pig-liver mitochondrial extract

Abstract

The name "ferrochelatase" has been given to the enzyme system that catalyses the insertion of Fe2+ ions into protoporphyrin to form protohaem (Rimington, 1958). The state of reduction of the porphyrin substrate of ferrochelatase is uncertain. Although haem was formed in the presence of protoporphyrinogen (Nishida and Labbe, 1959), prior oxidation to protoporphyrin may have occurred. Nishida and Labbe also demonstrated an initial lag phase in ferrochelatase activity which was abolished by preincubation of protoporphyrin with their enzyme preparation in the absence of Fe2+ ions. This suggested that protoporphyrin might be converted into an active intermediate, perhaps a reduced form. Such a suggestion was attractive because of the established importance of porphyrinogens in haem biosynthesis. That meatal ions can be incorporated into reduced porphyrins has been demonstrated by non-enzymic studies by Orlando (1958) and Heikel, Lockwood and Rimington (1958), and molecular configurations that facilitate such an incorporationof metal ions into reduced porphyrins have been proposed (Orlando, 1958; Lemberg, 1961; Phillips, 1961). In the present paper we descibe a rapid assay for ferrochelatase activity depending on the formation of pyrdine haemochromogen from the synthesized haem. A soluble ferrochelatase system, extracted from pig-liver mitochondriahas been studied and the reduction state of the porphyrin substrate investigated.