Purification and properties of an asparagine aminotransferase from Pisum sativum leaves

Abstract

The enzyme responsible for the transamination of L-asparagine in pea leaves has been partially purified. It appears to be the same protein as the serine-glyoxylate aminotransferase. It is able to use serine or asparagine as amino donors and pyruvate or glyoxylate as amino acceptors. The reaction is reversible but the equilibrium is toward glycine or alanine production. The favored substrates are serine and glyoxylate: serine shows competitive inhibition toward asparagine, as does pyruvate toward glyoxylate. Substrate interaction and product inhibition patterns are consistent with a ping-pong mechanism. The enzyme has a pH optimum at 8.1. Gel filtration indicates a Mr of 105,000. Inhibition was caused by aminoxyacetate and hydroxylamine, but the enzyme was unaffected by isonicotinic acid hydrazide. The apoenzyme was resolved and was inactive: addition of pyridoxal 5'-phosphate restored 85% of the original activity.