Biosynthesis of d-aspartate in mammals: the rat and human homologs of mouse aspartate racemase are not responsible for the biosynthesis of d-aspartate

D-Aspartate (D-Asp) has important physiological functions, and recent studies have shown that substantial amounts of free D-Asp are present in a wide variety of mammalian tissues and cells. Biosynthesis of D-Asp has been observed in several cultured rat cell lines, and a murine gene (glutamate-oxaloacetate transaminase 1-like 1, Got1l1) that encodes Asp racemase, a synthetic enzyme that produces D-Asp from L-Asp, was proposed recently. The product of this gene is homologous to mammalian glutamate-oxaloacetate transaminase (GOT). Here, we tested the hypothesis that rat and human homologs of mouse GOT1L1 are involved in Asp synthesis. The following two approaches were applied, since the numbers of attempts were unsuccessful to prepare soluble GOT1L1 recombinant proteins. First, the relationship between the D-Asp content and the expression levels of the mRNAs encoding GOT1L1 and D-Asp oxidase, a primary degradative enzyme of D-Asp, was examined in several rat and human cell lines. Second, the effect of knockdown of the Got1l1 gene on D-Asp biosynthesis during culture of the cells was determined. The results presented here suggest that the rat and human homologs of mouse GOT1L1 are not involved in D-Asp biosynthesis. Therefore, D-Asp biosynthetic pathway in mammals is still an urgent issue to be resolved.