Conversion of nitroglycerin to nitric oxide in microsomes of the bovine coronary artery smooth muscle is not primarily mediated by glutathione-S-transferases.

The pharmacological action of organic nitrate vasodilators [e.g., nitroglycerin (NTG)] is thought to be mediated through metabolic conversion to nitric oxide (NO); conversion leads to vasodilatation, whereas diminished conversion in chronic therapy may lead to pharmacological tolerance. The biochemical nature of this process, however, is poorly understood. Glutathione-S-transferases (GST) have been shown to metabolize organic nitrates in the liver, but it is not known whether these enzymes are involved in this pharmacologically relevant process. We, therefore, compared the activities of conversion of NTG to NO vs. those of GST in microsomal suspensions of bovine coronary artery smooth muscle tissue. A classical GST substrate, 1-chloro-2,4-dinitrobenzene, inhibited NO production in microsomes, suggesting possible involvement of GST in organic nitrate activation. However, GST activity derived from microsomes exhibited a different heat lability profile compared to that of NO generation. Known inhibitors of GST (viz., indomethacin and bromosulfophthalein) did not alter the NO-generating activity in microsomes. Glutathione was a critical cofactor for GST, but not for NO generation from NTG, and thiols other than glutathione (e.g., N-acetyl-L-cysteine and thiosalicylic acid) also could facilitate NO production. Moreover, comparison to a commercially available purified liver GST preparation showed that, at the same GST activity toward 1-chloro-2,4-dinitrobenzene, the microsomal incubation produced about 8 times more NO than the purified liver GST. Radiation inactivation analysis of the functional molecular sizes of GST and the NO-producing enzyme(s) suggested that the enzymes were of different molecular weights (54 kD and 160 kD, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)