Vascular nitric oxide-generating activities for organic nitrites and organic nitrates are distinct

Like organic nitrates, the organic nitrites are assumed to undergo vascular metabolic activation to nitric oxide (NO) to produce vasodilating activity, but this assumption has never been examined. In this study, isobutyl nitrite and amyl nitrite were incubated with subcellular fractions of bovine coronary arterial smooth muscle at 37 degrees C, and NO generation was assessed by redox chemiluminescence assay of headspace NO. Bovine vascular subfractions exhibited substantial and significant catalytic activity for NO generation towards both organic nitrite substrates, and NO-generating activity could be inhibited by heating and irradiation. These observations suggest that organic nitrites are converted enzymatically to NO in vascular smooth muscle. Analysis of marker enzyme activities suggested that the primary NO-generating activity was associated with the cytosol, whereas a distinct, but relatively minor NO-generating activity was also identified in the microsomal fraction. The cytosolic and microsomal NO-generating activities exhibited different temperature dependencies for heat-induced inhibition of activity. The molecular weights of the NO-generating enzymes, determined by radiation inactivation target-size analysis, were 263 kDa (95% confidence interval: 236-298 kDa) for the cytosolic enzyme and 79 kDa (95% confidence interval: 54-148 kDa) for the microsomal enzyme. Previous studies in this laboratory have shown that the representative organic nitrate, nitroglycerin (NTG), is metabolized to NO by an enzyme associated with the plasma membrane of bovine vascular smooth muscle cells. Thus, the enzymes responsible for vascular bioconversion of organic nitrates and organic nitrites to NO are apparently different.(ABSTRACT TRUNCATED AT 250 WORDS)