The Electron Transport System of Hydrogenomonas eutropha

Wosilait and Nason (1) demonstrated that quinones could serve as electron acceptors for the enzymatic oxidation of reduced pyridine nucleotides, and their survey of plant, animal, and microorganism extracts (2) revealed the general occurrence of reduced pyridine nucleotide-quinone reductase activity. This finding and Dam’s (3) earlier report, which indicated that vitamin K was a common constituent of plants and microorganisms, suggested that vitamin K may be the physiological acceptor for quinone reductase. The insolubility of vitamin K in aqueous media caused Wosilait and Nason (4) to select the parent compound, menadione, for their pyridine nucleotide-quinone reductase studies in Escherichia coli. Menadione reductase activity subsequently was demonstrated in several purified mammalian flavoprotein enzymes (5) in extracts of Achromobatter fischeri (6), Streptococcus faecalis (7), and Azotobacter vinelandii (8), and, recently, in yeast extracts (from which menadione reductase has been crystallized (9)). Weber, Brodie, and Merselis (10) used vitamin Ki dissolved in Tween 80 and demonstrated its participation in the electron transport systems of Xycobackrium phlei and Corynebacterium creatinovorans; a vitamin K requirement for mitochondrial systems (11, 12) has also been shown. Considerably more direct evidence has established that another quinone (coenzyme Q) serves a major role in oxidation-reduction reactions of mitochondria (13). It is interesting that various homologues of coenzyme Q have been found in microorganisms (14, 15), yet there are no reports of reduced nicotinamide adenine dinucleotide-coenzyme Q reductase (16) or succinate-coenzyme Q reductase activities (17) (with the higher CoQl homologues) in bacterial extracts. Lester and Crane (14), who analyzed Hydrogenomonas eutropha (listed by them as Hydrogenomonas species) for CoQ, found 0.1 pmole of COQ,~ per g of cells, dry weight. Since the participation of CoQ in electron transport of mammalian tissue has been demonstrated, and inasmuch as appreciable quantities of CoQa occur in extracts of H. eutropha, crude extracts were examined for reduced pyridine nucleotideCoQ reductase activity. No pyridine nucleotide-Co& reductase activity was detected. However, considerable NADH-menadione reductase activity was found, and various benzoand naphthoquinones could replace menadione as electron acceptor. This report describes the characteristics of NADH-menadione reductase from H. eutropha.