A Novel Pyridoxal 5′-Phosphate-dependent Amino Acid Racemase in the Aplysia californica Central Nervous System*

d-Aspartate (d-Asp) is found in specific neurons, transported to neuronal terminals and released in a stimulation-dependent manner. Because d-Asp formation is not well understood, determining its function has proved challenging. Significant levels of d-Asp are present in the cerebral ganglion of the F- and C-clusters of the invertebrate Aplysia californica, and d-Asp appears to be involved in cell-cell communication in this system. Here, we describe a novel protein, DAR1, from A. californica that can convert aspartate and serine to their other chiral form in a pyridoxal 5′-phosphate (PLP)-dependent manner. DAR1 has a predicted length of 325 amino acids and is 55% identical to the bivalve aspartate racemase, EC 5.1.1.13, and 41% identical to the mammalian serine racemase, EC 5.1.1.18. However, it is only 14% identical to the recently reported mammalian aspartate racemase, DR, which is closely related to glutamate-oxaloacetate transaminase, EC 2.6.1.1. Using whole-mount immunohistochemistry staining of the A. californica central nervous system, we localized DAR1-like immunoreactivity to the medial region of the cerebral ganglion where the F- and C-clusters are situated. The biochemical and functional similarities between DAR1 and other animal serine and aspartate racemases make it valuable for examining PLP-dependent racemases, promising to increase our knowledge of enzyme regulation and ultimately, d-serine and d-Asp signaling pathways.