Characterization of a homologue of mammalian serine racemase from Caenorhabditis elegans: the enzyme is not critical for the metabolism of serine in vivo.

Free d-serine (d-Ser) plays a crucial role in regulating brain function in mammals. In various organisms, including mammals, d-Ser is biosynthesized by Ser racemase, a synthetic enzyme that produces d-Ser from l-Ser. Ser racemase also exhibits dehydratase activity toward several hydroxyamino acids. Thus, this enzyme is unique in that it possesses the capability to both synthesize and degrade d-Ser; however, the physiological significance of its degradative activity remains unclear. In contrast to the physiological roles of d-Ser in mammals, little is known about the role of this amino acid in lower organisms, including the nematode Caenorhabditis elegans. It is known that a mammalian Ser racemase homologue (T01H8.2) from C. elegans exhibits racemase activity. Here, the enzymatic properties of recombinant T01H8.2 were characterized and compared with those of recombinant human Ser racemase. Furthermore, the levels of several d- and l-amino acids were measured in wild-type C. elegans and in a mutant in which the T01H8.2 gene is partially deleted and thereby inactivated. The results indicate that T01H8.2 also shows dehydratase activity toward several hydroxyamino acids, although the enzyme is not critical for Ser metabolism in vivo. The possible physiological roles of T01H8.2 are discussed.