NITROGLYCERIN METABOLISM IN VASCULAR TISSUE - ROLE OF GLUTATHIONE S-TRANSFERASES AND RELATIONSHIP BETWEEN NO. AND NO2- FORMATION

Nitroglycerin is a commonly employed pharmacological agent which produ ces vasodilatation by release of nitric oxide (NO.). the mechanism by which nitroglycerin releases NO. remains undefined. Recently, glutathi one S-transferases have been implicated as important contributors to t his process. They are known to release NO2- from nitroglycerin, but ha ve not been shown to release NO2. The present studies were designed to examine the role of endogenous glutathione S-transferases in this met abolic process. Homogenates of dog carotid artery were incubated anaer obically with nitroglycerin, and NO. and NO2- production was determine d by chemiluminescence. The role of glutathione S-transferases was stu died by incubating homogenates with nitroglycerin in the presence of 1 mM GSH or 1 mM S-hexylglutathione, a potent inhibitor of glutathione S-transferases. Homogenates released 163 pmol of NO./h per mg of prote in from nitroglycerin, and 2370 pmol of NO2-/h per mg. Adding GSH decr eased NO. production by 82 % and increased NO2-production by 98 %. S-H exylglutathione-inhibited glutathione S-transferase activity by 96 % a nd decreased NO2- production by 78 % but had no effect on NO. release. A linear relationship between glutathione S-transferase activity and NO2- production was observed, whereas glutathione S-transferase activi ty and NO' release were unrelated. Western-blot analysis demonstrated that dog carotid vascular smooth muscle contained Pi and Mu forms of g lutathione S-transferases, with a predominance of the former. Purified preparations of human Pi and rat Mu isoforms metabolized nitroglyceri n only to NO2- and not to NO.. On the basis of these findings, we conc lude that (1) glutathione S-transferases do not contribute to the bioc onversion of nitroglycerin to NO., but instead act as a degradative pa thway for nitroglycerin, and (2) the release of NO. from nitroglycerin is not dependent on the formation of NO2-.