D-Aspartate racemase activity in rat tissues: Synthesis of D-Asp. Homogenized tissue were incubated with L-Asp for 60 min at 37°C and then the amount of D-Asp synthesized was detected. The D-Asp that developed was determined by a colorimetric method based on the use of D-AspO. One enzyme unity (EU) was defined as the amount of the enzyme capable of generating 1 nmol of D-Asp in 60 min of incubation at 37°C and in the above assay conditions. The specific activity was defined as the EU/mg protein. The values are the mean ± SEM as obtained from five determinations, each carried out on a different rat.