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. Sin ce glutathione S-transferases (GST) can utilize GTN as substrate and G ST inhibitors can attenuate GTN-induced relaxation of rabbit aorta in vitro it has been suggested that these enzymes are involved in the bio activation of GTN in rabbit aorta. Because GSTs are multifunctional en zymes and a multitude of GST isozymes with varying substrate preferenc es are present in mammalian tissues, the role of specific GST isozymes in bioactivation of GTN in rabbit aorta needs to be established. Ther efore, during the present studies we have purified and characterized G ST isozymes from rabbit aorta and evaluated their possible roles in th e biotransformation of GTN. The results of these studies showed that r abbit aorta contained three GST isozymes having pI values of 9.4, 7.7, and 5.4. Structural, immunological, and kinetic studies showed that G ST 9.4, GST 7.7, and GST 5.4 belonged to the alpha-, pi-, and mu-class es, respectively. The relative abundance of these enzymes in rabbit ao rta was alpha > pi > mu. The alpha- and mu-class GST isozymes had simi lar activities toward GTN (0.71 U/mg and 0.86 U/mg, respectively) whil e the pi-class GST showed much lower activity toward GTN. The catalyti c efficiency k(cat)/K-m of the mu- and alpha-class GSTs toward GTN wer e similar but these activities were differentially inhibited by ethacr ynic acid, its GSH conjugate, bromosulfophthalein (BSP), and hematin. These results suggest that in rabbit aorta GSTs may be involved in bio activation of GTN, and because of their higher abundance the a class G STs may be more important for the pharmacological effects of GTN than the mu-class GSTs. The results on kinetics of inhibition by various in hibitors suggest that hematin may be an effective inhibitor to delinea te the role of specific GST isozymes in the bioactivation of GTN. (C) 1996 academic Press, Inc.