Rabbit aorta glutathione S-transferases and their role in bioactivation of trinitroglycerin

The pharmacological action of glyceryl trinitrate (GTN), a widely used drug for the treatment of angina pectoris, is thought to be mediated through release of nitric oxide (NO) during its biotransformation. Since glutathione S-transferases (GST) can utilize GTN as substrate and GST inhibitors can attenuate GTN-induced relaxation of rabbit aorta in vitro it has been suggested that these enzymes are involved in the bioactivation of GTN in rabbit aorta. Because GSTs are multifunctional enzymes and a multitude of GST isozymes with varying substrate preferences are present in mammalian tissues, the role of specific GST isozymes in bioactivation of GTN in rabbit aorta needs to be established. Therefore, during the present studies we have purified and characterized GST isozymes from rabbit aorta and evaluated their possible roles in the biotransformation of GTN. The results of these studies showed that rabbit aorta contained three GST isozymes having pI values of 9.4, 7.7, and 5.4. Structural, immunological, and kinetic studies showed that GST 9.4, GST 7.7, and GST 5.4 belonged to the alpha-, pi-, and mu-classes, respectively. The relative abundance of these enzymes in rabbit aorta was alpha > pi > mu. The alpha- and mu-class GST isozymes had similar activities toward GTN (0.71 U/mg and 0.86 U/mg, respectively) while the pi-class GST showed much lower activity toward GTN. The catalytic efficiency k(cat)/Km of the mu- and alpha-class GSTs toward GTN were similar but these activities were differentially inhibited by ethacrynic acid, its GSH conjugate, bromosulfophthalein (BSP), and hematin. These results suggest that in rabbit aorta GSTs may be involved in bioactivation of GTN, and because of their higher abundance the alpha-class GSTs may be more important for the pharmacological effects of GTN than the mu-class GSTs. The results on kinetics of inhibition by various inhibitors suggest that hematin may be an effective inhibitor to delineate the role of specific GST isozymes in the bioactivation of GTN.