Identification of mammalian aspartate-4-decarboxylase

Several animal tissues were examined for aspartate-4-decarboxylase (EC 4.1.1.12) activity. Highest activity was seen in murine livers, in rodent livers, and in rodent kidneys. The rat liver enzyme was membrane associated and could be solubilized and partially purified with the aid of detergents. The purification studies, and studies on the stoichiometry and kinetics of the reaction, showed that aspartate is directly converted to alanine. Such a metabolic reaction had not been reported before in animals. The rat liver enzyme differed significantly from the microbial aspartate-4-decarboxylases. Among other things, the rat liver β-decarboxylase could be purified away from a cysteine sulfinate desulfinase activity. Also, unlike the bacterial enzymes, the mammalian β-decarboxylase could not be inactivated by preincubation with aspartate or cysteine sulfinate. These later observations strongly suggest that the mammalian aspartate-4-decarboxylase does not have an inherent transaminase activity. Like many decarboxylases, rat liver aspartate-4-decarboxylase could be inhibited by reagents which react with carbonyl groups; however, the enzyme showed no dependence on pyridoxal 5′-phosphate.