Malate-vitamin K Reductase, a Phospholipid-requiring Enzyme.

During the course of studies on the effect of vitamin K on several respiratory chain enzymes from 1VIycobacterium phlei, it was found that the soluble fraction contained a unique enzyme which required vitamin K suspended in phospholipid for the reduction of thiazolyl blue tetrazolium by malate. Malate oxidation in most tissues is catalyzed by the classical pyridine nucleotide-linked malic dehydrogenase (1); however, certain bacteria oxidize this substrate by a pyridine nucleotide-independent pathway (2-5). Although flavin adenine dinucleot’ide was shown to serve as a cofactor for the latter enzyme (4), the natural electron acceptor was not known. The demonstration of a requirement for vitamin K and phospholipid for activity with the flavin adenine dinucleotide-linked enzyme (6, 7) provided insight as to the nature of respirat,ory chain of the nicotinamide adenine dinucleotide-independent malate pathway. A number of st,udies have recently appeared on the role of phospholipid for mitochondrial and microsomal bound enzymes or isolated enzyme systems (S-17). P-Hydroxybutyric dehydrogenase solubilized from mitochondria was shown specifically to require lecithin for activity (18, 19). Recently, a requirement of phospholipid for malate dehydrogenase from Il/lycobacterium a&urn has been reported (20). A requirement for natural quinones for enzymes other than those of mitochondrial origin has been reported (21-24), but none of these enzymes have been shown to require phospholipid. The present report concerns the properties and nature of the malate-vitamin K reductase from M. p&i. A preliminary account of this work has been published (6).