6 Enzymology of Pyrrolidone Carboxylic Acid

Abstract

Publisher Summary This chapter discusses the enzymology of pyrrolidone carboxylic acid. Pyrrolidone carboxylic acid found in the free state in many plant and animal tissues and also in peptide linkage as the N-terminal group of a number of peptides and proteins. Pyrrolidone carboxylic acid was reported as a product of glutathione metabolism in kidney extracts; however, the first clearly defined enzymic reaction in which pyrrolidone carboxylic acid is a product was described by Connell and Hanes. These workers described an enzymic activity in liver that catalyzes the conversion of γ-glutamyl amino acids to pyrrolidone carboxylic acid and the corresponding amino acid. Subsequently, the formation of L- and D-pyrrolidone carboxylic acid from the corresponding isomers of glutamic acid was found to be catalyzed by glutamine synthetase in the absence of ammonia. Other enzymes that catalyze pyrrolidone carboxylate formation include (1) an activity found in the kidney and liver of a number of animals which catalyzes the conversion of D-glutamate to D-pyrrolidone carboxylate, (2) a bacterial enzyme that forms L-pyrrolidone carboxylate from L-glutamate, and (3) an enzyme found in papaya latex that forms pyrrolidone carboxylate from free glutamine and glutaminyl peptides. The formation of pyrrolidone carboxylate from glutathione in kidney extracts may be ascribed to the combined action of two enzymes: (1) γ-glutamyl transpeptidase and (2) γ-glutamyl cyclotransferase. Preparations and extracts of nuclei have been reported to catalyze the conversion of L-glutamate to L-pyrrolidone carboxylate, and it appears that this reaction is linked to an energy yielding oxidation reaction.