Properties of malate dehydrogenase isolated from Methanospirillum hungatii

Abstract

A NADH-linked oxygen-tolerant malate dehydrogenase was purified 270-fold from cell extracts of Methanospirillum hungatii. Inhibitors of the enzyme included ADP, α-ketoglutarate, and excess NADH. Inhibition patterns for ADP were competitive with respect to NADH and non-competitive with respect to oxalacetate. Inhibition by α-ketoglutarate was non-competitive with oxalacetate as variable substrate and uncompetitive with respect to NADH. α-Ketoglutarate is surmised to function as an end-product inhibitor of the enzyme in reactions converting oxalacetate to α-ketoglutarate. No enzyme activity was detected in the direction of malate conversion to oxalacetate, in keeping with a strictly biosynthetic function of the enzyme. An analysis of variance of initial rate data fit to sequential and ping-pong equations showed that a sequential mechanism was preferred. The malate dehydrogenase of M. hungatii resembles those of many other bacteria and eucaryotic cells with respect to molecular weight (61 700) and reaction mechanism, but may be regulated differently.