CHARACTERIZATION OF PLATELET GAMMA-GLUTAMYLTRANSFERASE AND ITS ALTERATION IN CASES OF ATHEROSCLEROSIS

Among the various functional and biochemical alterations in the platel ets of cases of atherosclerosis, the membrane alterations occupy an im portant place. The platelet intrinsic membrane protein gamma glutamylt ransferase (GGT), which is involved in glutathione metabolism, has sho wn decreased activity in cases of atherosclerosis. To add new insights into the pathogenesis of atherosclerosis, GGT is characterized and co rrelated with other alterations. Triton X-100 solubilized membrane fra ctions of frozen and thawed platelets of atherosclerotic and normal su bjects had 18.66 +/- 2.86 mU/10(9) platelets and 35.67 +/- 3.01 mU/10( 9) platelets, respectively. The K-m values were the same, 2.08 mmol/L for gamma glutamyl-p-nitroanilide and 5.87 mmol/L for glycylglycine. T he V-max values were reduced from 100 mU/10(9) platelets to 41.66 mU/1 0(9) platelets for gamma glutamyl-p-nitroanilide and from 45.45 mU/10( 9) platelets to 38.46 mU/10(9) platelets for glycylglycine. Optimum pH of GGT activity was 8.2, and optimum temperature was 37 degrees C. It had thermal stability with a 64% relative activity at 56 degrees C fo r 30 min. Serine against berate was detected as the competitive inhibi tor and bromcresol green as the noncompetitive inhibitor. In vivo admi nistration of the antithrombotic drug defibrotide increased the platel et GGT levels to those of normals, from 14.72 +/- 7.27 mU/10(9) platel ets to 31.80 +/- 12.21 mU/10(9) platelets in 2 hs. Cholesterol, high-d ensity lipoprotein cholesterol in the membrane fractions, and platelet glutathione levels were unaltered. The lipid peroxidation (membrane m alondialdehyde) level was increased, and glucose and histidine active transport systems were impaired in atherosclerotics. All of these chan ges are discussed in relation to GGT.