Comparative biochemistry of the biological reduction of fumaric acid

Abstract

The biological reduction of fumaric acid is catalysed by succinic dehydrogenase, acting in reverse, or by a similar enzyme which can be provisionally designated as a fumaric acid with molecular hydrogen, and which also contain hydrogenase can reduce fumaric acid with molecular hydrogen, and experiments with cell-free extracts from the anaerobic bacterium Micrococcus lactilyticus indicate that the coupling between hydrogenase and reductase can be mediated by free flavin. Reductase activity can be assayed in cell-free preparations by measuring utilization of molecular hydrogen in the presence of excess hydrogenase and a viologen dye serving as electron carrier. Yeast and mammalian succinic dehydrogenases reduce fumarate at a low rate as compared with their ability to oxidize succinate. On the other hand, the reductase enzyme of M. lactilyticus reduces fumarate rapidly and oxidizes succinate slowly. Although malonate and succinate markedly inhibit the reductase activity shown by yeast and mammalian succinic dehydrogenases, the activity of the M. lactilyticus reductase is in comparison not appreciably inhibited by these compounds. The present results indicate that the reductase activity of M. lactilyticus cannot be ascribed to a conventional succinic dehydrogenase such as is found in the tissues of mammals and other aerobes and suggest the possibility that the bacterial reductase may be a similar enzyme modified to meet the physiological requirements of anaerobic growth.