Glutamate decarboxylase side reactions catalyzed by the enzyme

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Abstract
A homogeneous glutamate decarboxylase isolated from pig brain contains 0.8 mol of tightly bound pyridoxal 5‐phosphate/enzyme dimer. Upon addition of exogenous pyridoxal 5‐phosphate (pyridoxal‐5‐P), the enzyme acquires maximum catalytic activity. Preincubation of the enzyme with l‐glutamate (10 mM) brings about changes in the absorption spectrum of bound pyridoxal‐5‐P with the concomitant formation of succinic semialdehyde. However, the rate of this slow secondary reaction, i.e. decarboxylative transamination, is 10−4 times the rate of normal decarboxylation. It is postulated that under physiological conditions enzymatically inactive species of glutamate decarboxylase, generated by the process of decarboxylative transamination, are reconstituted by pyridoxal‐5‐P produced by the cytosolic enzymes pyridoxal kinase and pyridoxine‐5‐P oxidase. The catalytic activity of resolved glutamate decarboxylase is recovered by preincubation with phospho‐pyridoxyl‐ethanolamine phosphate. The experimental evidence is consistent with the interpretation that the resolved enzyme binds the P‐pyridoxyl analog, reduces the stability of the covalent bond of the phospho‐pyridoxyl moiety, and catalyzes the formation of pyridoxal‐5‐P.