Malate Metabolism by Desulfovibrio gigas and its Link to Sulfate and Fumarate Reduction: Purification of the Malic Enzyme and Detection of NAD(P)+ Transhydrogenase Activity

Malate metabolism was investigated in lactate grown cells of Desulfovibrio gigas ; 3 mol of malate are converted into 2 mol succinate and 1 mol acetate. The malic enzyme (L-malate:NADP+ oxidoreductase) was purified to homogeneity and partially characterized. The enzyme is monomeric with molecular weight of 45 kDa. Its spectrum has no visible absorption and the activity is stimulated by K+ and Mg2+. The presence of an NAD(P)+ transhydrogenase, the observation of partial reduction of adenylylsulfate reductase by NADH (via NADH-rubredoxin oxidoreductase) and evidence for NADH-linked fumarate reductase activity support the involvement of pyridine nucleotides in the electron pathway toward the reduction of sulfur compounds and/or fumarate. An electron transfer chain to fumarate is proposed, taking into consideration these results and the stoichiometry of end-products derived from malate dismutation.