D-Aspartate is one of a few D-
amino acids that attracted attention at an early date, since it was detected in various tissues of mammals as a
protein component. The occurrence of free
D-aspartate in nature was recognized a little later, and raised questions about its physiological functions and metabolism. This
amino acid has been gradually accepted, based on various experimental observations, to be a physiological substrate of
D-aspartate oxidase, whose role had been considered enigmatic since its early discovery in the 1940s. Mammalian
enzymes that serve to liberate D-aspartyl residue in
proteins have been identified. One
enzyme hydrolyzes
peptide bond at the amino side of D-aspartyl residue in a
dipeptide and another
enzyme hydrolyzes that at the carbonyl side of the residue in
proteins. The first
pyridoxal 5'-phosphate-dependent
aspartate racemase has been purified and cloned from a bivalve species. The
enzyme supports the high contents of
D-aspartate comparable to those of
L-aspartate in the bivalve, and the enantiomers are consumed when
hypoxia is imposed on the bivalve. In some yeast species, assimilation of
D-aspartate has been found to depend on inducible
D-aspartate oxidase, which also serves to detoxify acidic D-
amino acids.