S-adenosyl-L-methionine synthetase from human erythrocytes: role in the regulation of cellular S-adenosylmethionine levels.

The properties of human erythrocyte S-adenosyl-L-methionine synthetase (ATP:L-methionine S-adenosyltransferase, EC 2.5.1.6) were studied with respect to the role of S-adenosylmethionine in transmethylation reactions. Kinetic values obtained with both a cytosolic and a 350-fold purified preparation of enzyme were compared with measured intracellular concentrations of substrates and products. This analysis revealed that effective regulation of enzyme activity and product concentration can occur through feedback inhibition by S-adenosylmethionine (Ki = 2.0-2.9 microM; the endogenous concentration is 3.5 microM). This enzyme can be distinguished from S-adenosylmethionine synthetases found in other tissues and appears to be specialized for its role in erythrocyte methyl group metabolism, especially with regard to protein carboxyl methyl-transfer reactions.