Purified aspartic acid β-decarboxylase from Clostridium perfringens is markedly activated by catalytic amounts of pyridoxal 5′-phosphate or α-keto acids. The effects of these activators are not additive, and the added activators may be removed readily by dialysis. Assay of the purified enzyme by microbiological procedures indicates the presence of enzyme-bound pyridoxal 5 -phosphate. Radiation of the purified enzyme with ultraviolet light yields an apoenzyme preparation that no longer responds to the addition of α-keto acid, but which is still activated by pyridoxal 5′-phosphate; microbiological studies show that the radiated enzyme contains no pyridoxal 5′-phosphate. Incubation of the radiated enzyme with pyridoxal 5′-phosphate followed by exhaustive dialysis gives an enzyme preparation which is indistinguishable from the original preparation in that it is activated by both pyridoxal 5′-phosphate and α-keto acids. The experimental data lead to the conclusion that pyridoxal 5′-phosphate functions not only as the prosthetic group of aspartic acid β-decarboxylase but also as a less tightly bound co-factor. The data are consistent with the hypothesis that the enzyme-bound pyridoxal 5′-phosphate is linked to the enzyme in a form which does not react with substrate; reaction of the enzyme with catalytic quantities of an α-keto acid or pyridoxal 5′-phosphate converts the inactive prosthetic group to one capable of forming a Schiff base with aspartate.
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