Objectives: Microorganisms of Micromonas micros are asaccharolytic, anaerobic gram-positive cocci and frequently isolated from human oral sites such as periodontal pockets. Our preliminary study has shown that serine enhanced the growth of M. micros, but the washed organisms couldn't degrade serine. Serine-containing-peptides seemed to be a source of serine for the organisms. Therefore, we carried out to investigate degradation of serine-containing-peptides. Methods: Metabolic end products were determined with High Performance Liquid Chromatography. The related enzymatic activities in cell-free extract were also assayed. Results: Washed organisms of M. micros degraded serine-tripeptides (Ser-Ser-Ser), and produced formate (0.7 mM), pyruvate (2.9 mM), acetate (3.3 mM) and ammonia (5.4 mM). They also degraded serinyl-tyrosine (Ser-Tyr) and produced similar products, i.e., formate (0.3 mM), pyruvate (2.6 mM), acetate (2.4 mM) and ammonia (4.5 mM). Enzymatic activities such as serine dehydratase (specific activity; SP: 2.31x 10^-2 µmol/min/mg), pyruvate formate-lyase (SP: 5.78 x 10^-2 µmol/min/mg), formate dehydrogenase (SP: 3.15 x 10^-2 µmol/min/mg), pyruvate oxidoreductase (SP: 4.86 x 10^-3 µmol/min/mg), phosphate acetyltransferase (SP: 8.61µmol/min/mg) and acetate kinase (SP: 5.08 µmol/min/mg) were detected in the cell-free extract, indicating that the organism produced ATP with serine metabolism. Conclusions: M. micros utilized serine-containing-peptides rather than serine itself, and degraded Ser-Ser-Ser and Ser-Tyr to formate, pyruvate, acetate and ammonia with ATP generation.

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