Purification and properties of gamma-glutamylcyclotransferase from human erythrocytes

1. GAMMA-Glutamylcyclotransferase was purified 10000-fold from human erythrocytes. 2. The purification steps involved fractionation with (NH4)(2)SO(4) and chromatography on Sephadex G-75, DEAE-cellulose and hydroxyapatite. The purified enzyme was found to be homogeneous on density-gradient polyacrylamide-gel electrophoresis. 3. The maximum reaction rate was observed at pH9.0 and the apparent Km value for gamma-glutamyl-L-alanine was 2.2mM. 4. The molecular weight (25250) of the purified enzyme agreed well with the value (25500) in fresh haemolysates, indicating no apparent structural modification of the enzyme during purification. However, rapid processing of the blood through the initial (NH4)(2)SO(4) and Sephadex-chromatography steps was required to prevent formation of a high-molecular-weight aggregate with substantially lower specific activity. 5. gamma-Glutamylcyclotransferase catalyses the formation of 5-oxoproline from gamma-glutamyl dipeptides. The role of this enzyme in erythrocytes is of particular interest, because gamma-glutamyl-L-cysteine serves as a substrate for both gamma-glutamylcyclotransferase and glutathione synthetase. Thus the cyclotransferase could modulate glutathione synthesis.