Rat brain aspartate ?-decarboxylase. A comparative study with the liver enzyme

Aspartate ?-decarboxylase (AspD), which catalyses the ?-decarboxylation of aspartate (Asp) to alanine (Ala), was found in significant quantities only in the brain, kidney and liver. This enzyme has an optimum pH at 7.4. Addition of exogenous pyridoxal 5?-phosphate did not increase enzyme activity presumably because of firmly bound cofactor. However, aminooxyacetic acid is a potent inhibitor. There is an apparent 8-fold variation in AspD in the seven brain regions studied, with the highest activities in the cortex and the lowest in the striatum and hippocampus. In the presence of ?-ketoglutarate, the production of (14)CO(2) from [(14)C]Asp may no longer represent AspD activity due to active transamination of Asp, presumably by aspartate aminotransferase, to oxaloacetate. Under such conditions, comparable AspD activities were observed in all seven brain regions. Kinetic analysis showed that the liver and kidney enzymes have identical affinity for Asp (K(m) = 3.5 mM) while the brain enzyme has a higher affinit (K(m) = 1.3 mM). The V(max) values obtained indicated that the enzyme populations in liver, kidney and brain are in the ratio 18:4:1. Various amino acids were found to inhibit both brain and liver AspD. Serine, however, activated the liver enzyme but inhibited competitively the kidney and brain enzymes. These results indicate that AspD may exist as two or more isozymes.