Lectin affinity chromatography and electrophoretic properties of human platelet gamma-glutamyl transferase

The sialoglycoprotein, gamma-glutamyl transferase (GGT, gamma-GT, EC 2.3.2. 2) is a membrane enzyme found in many cells including platelets and leukocy tes. In platelets GGT converts leukotriene C-4 (LTC4) to leukotriene D-4 (L TD4) and is involved in glutathione metabolism. In this study, human platel et GGT was solubilized with Triton X-100 and purified by lectin affinity ch romatography on Con A Sepharose 4B to determine its electrophoretic propert ies. The specific activity of purified GGT was 236 mU/mg protein; 73.7% of human platelet GGT activity was found bound to Con A and 50% of the bound a ctivity was released with 0.3 mol/l methyl alpha-D-mannopyranoside. We obse rved that human platelet GGT has only one isoenzyme band showing a carbohyd rate stained band near the origin on polyacrylamide gel electrophoresis (PA GE). The electrophoretic mobility of papain-solubilized GGT was higher than that of Triton X-100-solubilized GGT at PAGE. Also GGT activities were det ermined on neuraminidase, trypsin or n-butanol-DIPE (diisopropyl ether)-tre ated Triton X-100-solubilized membrane fractions. This characterization may be useful when trying to establish the contribution of platelet GGT to ser um GGT activity. This marker may reflect the extent of platelet activation.