Pyruvate oxidase in Streptococcus sanguis under various growth conditions.

Abstract

Streptococcus sanguis ATCC 10556 was grown in batch culture under aerobic and anaerobic conditions. The organism was also kept in aerated nitrogen‐ and sugar‐limited continuous cultures. Pyruvate oxidase was induced under aerobic conditions and the highest specific activity of the enzyme was found in aerated batch cultures in the early phase of exponential growth. A steady‐state growth of the organism could not be established in aerated continuous culture, unless catalase was included in the culture medium. In aerated nitrogen‐limited continuous culture with an excess of sugar there was very low pyruvate oxidase activity, and lactate was the only fermentation product. In aerated sugar‐limited continuous culture there was high pyruvate oxidase activity, and acetate was the main fermentation product. This indicated that pyruvate oxidase was the key enzyme in converting pyruvate into fermentation products in the aerated sugar‐limited continuous culture. The pyruvate oxidase reaction requires high concentrations of pyruvate, phosphate and oxygen for maximum velocity. These concentrations are probably never reached intracellularly. The limited supply of substrates may thus be the most important factor in modulating the activity of the enzyme. The enzyme is hysteretic and this may also contribute to the adaptation of the enzyme activity to the metabolic processes of the cell. The combination of pyruvate oxidase and NAD(P)H‐OSCN‐oxidoreductase in S. sanguis and the presence of thiocyanate and salivary peroxidase in saliva may provide an ecological advantage to S. sanguis in habitats exposed to saliva.