Carbon dioxide metabolism by Actinomyces viscosus: pathways for succinate and aspartate production

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Abstract
14C-labeled bicarbonate was incorporated into trichloroacetic acid[TCA]-insoluble material by cell suspensions of A. viscosus strain M100 and into the 4-C fermentation product, succinate, but not into the 3-C fermentation product, lactate. The initial step in the conversion of 14C-labeled bicarbonate into insoluble material and succinate was catalyzed by the enzyme phosphoenolypyruvate [PEP] carboxylase, which served to convert the glycolytic intermediate, PEP, and bicarbonate to the 4-C compound, oxaloacetate. The metabolic fate of oxaloacetate was its conversion to TCA-insoluble material or succinate. One pathway by which oxaloacetate may be metabolized into acid-insoluble material is via its conversion to the biosynthetic precursor aspartate by the action of glutamate aspartate aminotransferase. One source of the .alpha.-amino group of aspartate was NH4+ which could be incorporated into glutamate, the substrate of the glutamate aspartate aminotransferase reaction, by the action of a NADPH-dependent glutamate dehydrogenase whose reducing equivalents could be derived from the NADP-dependent oxidative reactions of the hexose monophosphate pathway catalyzed by glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. Alternatively, oxaloacetate was converted to the fermentation product, succinate, through the sequential action of malate dehydrogenase, fumarase and succinic dehydrogenase. The resolution and partial purification of PEP carboxylase, glutamate aspartate aminotransferase, glutamate dehydrogenase, malate dehydrogenase, fumarase and succinic dehydrogenase are reported. [The pathogenesis of dental caries and periodontal disease is discussed].