Electron-transport system of Vibrio succinogenes: II. Inhibition of electron transport by 2-heptyl-4-hydroxyquinoline N-oxide

Abstract

The effect of the inhibitor 2-heptyl-4-hydroxyquinoline N-oxide on the reduction by formate of fumarate, O2 and cytochromes was studied in extracts of Vibrio succinogenes. The inhibitor causes 90% inhibition of the couple between formate and fumarate but less than 10% inhibition of the formic dehydrogenase or fumarate reductase activity. Spectrophotometric studies showed that 2-heptyl-4-hydroxyquinoline N-oxide causes an appreciable delay in the reduction of both cytochromes b and c by formate. No effect of the inhibitor on the oxidation of reduced cytochrome b by fumarate could be detected. With succinate as substrate, the inhibitor causes a delay in the reduction of cytochrome c but no delay in the reduction of cytochrome b was detected. The effect of other inhibitory agents on the reduction of fumarate by formate and H2 was also studied.Oxygen consumption with formate as substrate is not inhibited by 2-heptyl-4-hydroxyquinoline N-oxide, but a marked increase in H2O2 formation occurs in the presence of this inhibitor. The formate peroxidase system of this extract is partially inhibited by 2-heptyl-4-hydroxyquinoline N-oxide.The implications of these findings with regard to the role of cytochrome b as an electron carrier between the formic dehydrogenase and the fumarate reductase, and the role of the cytochromes in the oxidation of formate by O2 are discussed. It is suggested that the presence of the H2O2-producing formate oxidase system and the formate peroxidase system explains the microaerophilic nature of this organism when grown with O2 as an electron acceptor.