Physiologic concentrations of homocysteine inhibit the human plasma GSH peroxidase that reduces organic hydroperoxides

The plasma reduced glutathione (GSH) selenoperoxidase is a highly conserved enzyme. Furthermore, a small clinical study reported that patients with se vere atherosclerosis had low peroxidase activities. Together these observat ions suggest that the peroxidase is important in preventing atherosclerosis . Yet others have reported that when the assay was run in Tris buffer, it w as inactive with the concentrations of GSH found in the plasma. Second, it is known that hyperhomocysteinemia increases the rate of atherogenesis. Bec ause there is some homology between homocysteine and the cysteine in GSH, t he question is whether the hyperhomocysteinemia effect may be due to inhibi tion of the peroxidase. We purified the peroxidase from human plasma and de termined its activity by a coupled spectrophotometric assay and a substrate disappearance chemiluminescence assay. When the peroxidase activity was de termined in phosphate-buffered saline solution (PBS), there was significant activity with the reported plasma GSH concentrations (5 to 20 mu mol/L). T he peroxidase was exclusively in the HDL fraction. There was no correlation between the peroxidase activity and the HDL or LDL cholesterol concentrati ons. Finally, at physiologic concentrations of GSH (9 mu mol/L), the peroxi dase was inhibited by physiologic, free homocysteine concentrations (1 to 5 mu mol/L). These data suggest that the peroxidase is active in vivo and ma y be important in protecting the endothelium from atherosclerosis by preven ting oxidant injury. The homocysteine inhibition of the peroxidase suggests a possible biochemical basis for the observed association between hyperhom ocysteinemia and cardiovascular disease. Our studies imply that low concent rations of this peroxidase may be an independent risk factor for atheroscle rosis.