Glutathione reductase deficiency and platelet dysfunction induced by 1,3-bis(2-chloroethyl)-1-nitrosourea

Human platelets exposed in vitro to increasing amounts of BCNU rapidly develop a progressive, relatively selective, and almost complete deficiency of GSSG-R activity. Several other enzymes are not inhibited when intact platelets are exposed to the nitrosourea; lipoamide dehydrogenase was investigated because of the remarkable similarity of the structure of its active site with that of GSSG-R. BCNU inhibits lipoamide dehydrogenase and GSSG-R only when they are in the reduced state; in the intact platelet, lipoamide dehydrogenase (unlike GSSG-R) is oxidized and is therefore unaffected. This is the first documentation of lipoamide dehydrogenase activity in platelets. After BCNU exposure, there is a reduced release of 14C-serotonin in response to collagen; the cells become incapable of aggregating in response to even large doses of epinephrine, ADP, collagen, or arachidonic acid, with loss of both primary and secondary waves of aggregation. At higher doses of BCNU, there is also a diminished PF-3 activity of intact platelets; sonication of drug-treated platelets normalizes coagulant activity. The drug-induced functional abnormalities occur despite preservation of the number of platelets, their electron microscopic appearance, and their capacity to take up 14C-serotonin. BCNU induced GSSG-R deficiency precedes the development of the earliest evidence of platelet dysfunction, and almost all of the enzyme's activity must be abolished before any functional abnormality becomes detectable. A small fraction of GSSG-R activity is essential for platelet function, and BCNU provides a powerful new tool to investigate the role of the enzymatic reduction of glutathione in platelet physiology and pathology.