Respiration of Malate and Glutamate in SymbioticFrankiaPrepared fromAlnus incana

Abstract

Frankia vesicle clusters were prepared from root nodules of Alnus incana (L.) Moench inoculated either with a local source of Frankia or with Frankia Cpll. The capacity of vesicle clusters to respire was investigated by respirometric and enzymological studies. Simultaneous addition of malate, glutamate, and NAD+ supported respiration in both types of Frankia, though at a smaller rate compared to the substrates NADH or 6-phosphogluconate. The saturating concentrations of malate and glutamate were also much higher than with the other substrates. No respiration was supported by succinate. Activity of the enzymes malate dehydrogenase (EC 1.1.1.37) and glutamate oxaloacetate transaminase (EC 2.6.1.1) was demonstrated in crude extracts from both types of symbiotic Frankia. Their maximum rates were high enough to account for the respiration of malate and glutamate. This respiration was inhibited by mersalylic acid, an inhibitor of the dicarboxylate shuttle in mitochondria, but it was shown that inhibition of respiration could be due to a direct effect on the enzymes. We conclude that respiration of malate and glutamate is most likely mediated by malate dehydrogenase and glutamate oxaloacetate transaminase, but no explicit evidence for or against the presence of a dicarboxylate carrier was found. The utilization of respiratory substrates was largely similar in the two types of Frankia, except for some differences in maximum rates and cofactor dependency.