Purification and some properties of alanine racemase from the marine gastropod Cellana grata

High levels of free D-alanine were found in the muscle of a marine gastropod Cellana grata that inhabited the intertidal zone, and alanine racemase activity was detected in the muscle. The authors purified alanine racemase from the muscle of C. grata to characterize its enzymological properties. The molecular mass of the enzyme was estimated to be 40.5 kDa by sodium dodecylsulfate-polyacrylamide gel electrophoresis and 41.4 kDa by gel filtration, suggesting that the enzyme was monomeric in nature. Kinetic experiments, performed using the purified enzyme, revealed that the Lineweaver-Burk plot for D-alanine as a substrate resulted in a K_m value of 20.4 mM, and the value for L-alanine was 43.0 mM. Of the several types of amino acids tested, alanine was found to be the specific substrate for the enzyme. In the measurement of alanine racemase activity, exogenous pyridoxal 5′-phosphate (PLP) was not required for the enzyme activity; however, aminooxyacetic acid, hydroxylamine and phenylhydrazine, which inhibit PLP-dependent enzymes, strongly inhibited the enzyme activity. These results suggest that the enzyme is PLP-dependent. This is the first report on the purification and some properties of alanine racemase in a marine gastropod.