Occurrence of Two Pathways for Malate Oxidation in Bacteroids Isolated from Sesbania rostrata Stem Nodules during C(2)H(2) Reduction.

Malate oxidation supported C(2)H(2) reduction by bacteroids isolated from Sesbania rostrata stem nodules. Optimal activity reached 7.5 nanomoles per minute per milligram of dry weight and was in the same order of magnitude as that observed with succinate but always required a lower O(2) tension. Malate dehydrogenase (EC 1.1.1.37), purified 66-fold from bacteroids, actively oxidized malate (K(m) = 0.19 millimolar). Malic enzyme (EC 1.1.1.39) from Sesbania bacteroids had a lower affinity for malate (K(m) = 2.32 millimolar). Both enzymes exclusively required NAD(+) as cofactor and required an alkaline pH for optimal activity. 2-Oxoglutarate and oxalate, inhibiting malate dehydrogenase and malic enzyme, respectively, were used to specifically block each malate oxidation pathway in bacteroids. The predominance of malate dehydrogenase activity to support bacteroid N(2) fixation was demonstrated. The inhibition of O(2) consumption by 2-oxoglutarate confirmed the importance of the malate dehydrogenase pathway in malate oxidation. It is proposed that the utilization of malate, with regard to O(2), is important in a general strategy of this legume to maintain N(2) fixation under O(2) limited conditions.