Abstract
Among the many d-amino acids which have healthy functions, D-serine reportedly prevents cognitive impairment of the human brain. After lactic acid bacteria capable of producing D-serine were selected, we investigated the bacteria cultivation conditions for production of D-serine. Many microorganisms that have serine racemase produce D-serine from L-serine. For this study, D-serine was assayed using colorimetry, for which peroxide produced by amino acid oxidase reacting with D-serine was colored using methyl benzothiazolinonehydrazone (MBTH) and dimethylaniline (DMA). Lactic acid bacteria (129 strains) were cultivated in media containing L-serine. After cultivation, the bacteria-converted D-serine was assayed. Results demonstrated high D-serine conversion potential for the H74 strain, which was identified as Lactococcus lactis ssp. lactis according to its morphological, physiological, and molecular biological characteristics. The H74 strain converted efficiently L-serine to D-serine in a medium containing 1.0% L-serine. The strain converted efficiently L-serine to D-serine at 15°C, and the conversion rate at 15°C was found to be four times higher than that found at 30°C. The high conversion rate at 15 °C continued for 8 days.
Serine racemase activity in the strain was found in the cell membrane and cytoplasm fractions. These findings suggest that serine racemase is a membrane-binding cytoplasmic enzyme. From these findings, we infer that the strain has high potential for the production of D-serine and foods that contain D-serine as supplements supporting health.
