Purification and characterization of a novel GABA-synthesizing enzyme from Lentinula edodes

Abstract

Recently, there has been growing interest in γ-aminobutyric acid (GABA) as a functional component of foods. In the general metabolic pathway, GABA is produced from decarboxylation of glutamic acid by glutamic acid decarboxylase (GAD), which is a pyridoxal 5’-phosphate (PLP)-dependent enzyme. GAD has been reported in various organisms. However, there are few reports of the purification and characterization of the enzyme in relation to GABA formation in Basidiomycetes, including edible mushrooms, even though a number of mushrooms contain GABA in the fruiting body. In this study, we described the purification and characterization of an enzyme related to GABA synthesis from Lentinula edodes. The enzyme was purified to 9.28-fold with a yield of 7.14% by 4 steps, and showed a single band on SDS-PAGE. The purified enzyme from L. edodes showed a wide range of substrate specificity, reacting not only with L-glutamic acid but also L-aspartic acid, aminoadipic acid, ibotenic acid and norvaline; whereas, it is well-known that GAD reacts only with L-glutamic acid. A homology search of the amino acid database using ForestGEN indicated that the internal amino acid sequence of the enzyme was identical to phosphatidylserine decarboxylase (PSD) of L. edodes.