Abstract
In this paper, we examined the distribution pattern of D‐aspartic acid (D‐Asp), as well as D‐aspartate oxidase (D‐AspO), D‐amino acid oxidase (D‐AAO), and L‐amino acid oxidase (L‐AAO) activities in different tissues of frog, Rana esculenta. High concentrations of free D‐Asp were found in the testes (0.21±0.02 µmol/g b.w), in the liver (0.20±0.03 µmol/g b.w), and in the Harderian gland (HG) (0.19±0.03 µmol/g b.w). A higher activity of both D‐AspO and D‐AAO with respect to L‐AAO was endogenously present in all examined frog tissues, particularly within the kidney, liver, and brain. Our in vivo experiments, consisting of i.p. injections of 2.0 µmol/g b.w. D‐Asp in frogs, revealed that all examined tissues can take up and accumulate D‐Asp and that this amino acid specifically triggers D‐AspO activity. Indeed, no increase in both D‐AAO and L‐AAO was found in all frog tissues after D‐Asp treatment. The optimum pH for D‐AspO activity was around 8.2 and the optimum temperature was about 37°C. Furthermore, its activity linearly increased with increasing D‐Asp incubation times. In vitro experiments assaying the substrate specificity of D‐AspO indicated that the enzyme had greater affinity for N‐methyl‐D‐aspartate than for D‐Asp and D‐glutamate. This study provides evidence of the presence of free D‐Asp in frog R. esculenta tissues, along with its role in triggering D‐AspO activity. These findings suggest that D‐AspO could play an essential role in decreasing excessive amounts of D‐Asp in frog tissues, a phenomenon that, if left unchecked, could have detrimental physiological effects on the animal. J. Exp. Zool. 313A:137–143, 2010. © 2010 Wiley‐Liss, Inc.