Disulfide reduction and sulfhydryl uptake by Streptococcus mutans.

Incubation of Streptococcus mutans cells with certain disulfide compounds resulted in accumulation of reduced sulfhydryl compounds in the extracellular medium or in both the medium and the cells. Oxidized lipoic acid and lipoamide competed for reduction. At high concentrations, these compounds were reduced at rates comparable to that of glucose metabolism, and all of the increase in sulfhydryls was in the medium. Cystamine did not compete with these compounds for reduction but was also reduced at high rates and low apparent affinity, and all of the cysteamine produced from cystamine accumulated in the medium. In contrast, glutathione disulfide (GSSG) and L-cystine were reduced slowly but with high apparent affinity, and 60 to 80% of the increase in sulfhydryls was intracellular. NADH-dependent lipoic acid or lipoamide reductase activity was present in the particulate (wall-plus-membrane) fraction, whereas NADPH-dependent GSSG reductase activity was present in the soluble (cytoplasmic) fraction. Two transport systems for disulfide and sulfhydryl compounds were distinguished. GSSG, L-cystine, and reduced glutathione competed for uptake. L-Cysteine was taken up by a separate system that also accepted L-penicillamine and D-cysteine as substrates. Uptake of glutathione or L-cysteine, or the uptake and reduction of GSSG or L-cystine, resulted in up to a 10-fold increase in cell sulfhydryl content that raised intracellular concentrations to between 30 and 40 mM. These reductase and transport systems enable S. mutans cells to create a reducing environment in both the extracellular medium and the cytoplasm.