Partial purification of gamma-glutamyltransferase from human brain microvessels

Characterization of Platelet Gamma Glutamyltransferase and Its Alteration in Cases of Atherosclerosis

Among the various functional and biochemical alterations in the platelets of cases of atherosclerosis, the membrane alterations occupy an important place. The platelet intrinsic membrane protein gamma glutamyltransferase (GGT), which is involved in glutathione metabolism, has shown decreased activity in cases of atherosclerosis. To add new insights into the pathogenesis of atherosclerosis, GGT is characterized and correlated with other alterations. Triton X-100 solubilized membrane fractions of frozen and thawed platelets of atherosclerotic and normal subjects had 18.66 ± 2.86 mU/109 platelets and 35.67 ± 3.01 mU/109 platelets, respectively. The Km values were the same, 2.08 mmol/L for gamma glutamyl- p-nitroanilide and 5.87 mmol/L for glycylglycine. The Vmax values were reduced from 100 mU/109 platelets to 41.66 mU/109 platelets for gamma glutamyl- p-nitroanilide and from 45.45 mU/109 platelets to 38.46 mU/109 platelets for glycylglycine. Optimum pH of GGT activity was 8.2, and optimum temperature was 37°C. It had thermal stability with a 64% relative activity at 36°C for 30 min. Serine against borate was detected as the competitive inhibitor and bromcresol green as the noncompetitive inhibitor. In vivo administration of the antithrombotic drug defibrotide increased the platelet GGT levels to those of normals, from 14.72 ± 7.27 mU/109 platelets to 31.80 ± 12.21 mU/109 platelets in 2 hs. Cholesterol, high-density lipoprotein cholesterol in the membrane fractions, and platelet glutathione levels were unaltered. The lipid per-oxidation (membrane malondialdehyde) level was increased, and glucose and histidine active transport systems were impaired in atherosclerotics. All of these changes are discussed in relation to GGT. Key Words: Human platelets—Atherosclerosis—Gamma glutamyltransferase—Gamma glutamyl transpeptidase— Defibrotide—Biological membranes.

Activity Determination, Kinetic Analyses and Isoenzyme Identification of Gamma Glutamyltransferase in Human Neutrophils

Gamma-glutamyltransferase (GGT, EC 2.3.2.2) which hydrolyzes glutathione (GSH), is required for the maintenance of normal intracellular GSH concentration. GGT is a membrane enzyme present in leukocytes and platelets. Its activity has also been observed in human neutrophils. In this study, GGT was purified from Triton X-100 solubilized neutrophils and its kinetic parameters were determined. For kinetic analyses of transpeptidation reaction, gamma-glutamyl p-nitroanilide was used as the substrate and glycylglycine as the acceptor. Apparent K(m) values were determined as 1.8 mM for gamma-glutamyl p-nitroanilide and 16.9 mM for glycylglycine. The optimum pH of GGT activity was 8.2 and the optimum temperature was 37 degrees C. It had thermal stability with 58 % relative activity at 56 degrees C for 30 min incubation. L-serine, in the presence of borate, was detected as the competitive inhibitor. Bromcresol green inhibited neutrophil GGT activity as a noncompetitive inhibitor. The neutrophils seem to contain only the isoenzyme that is present in platelets. We characterized the kinetic properties and compared the type of the isoenzyme of neutrophil GGT with platelet GGT via polyacrylamide gel electrophoresis (PAGE) under a standard set of conditions.