Nitric oxide (NO) performs a central role in biological systems, binding to the heine site of soluble guanylyl cyclase (sGC), leading to enzyme activation and elevation of intracellular levels of cGMP. Organic nitrates, in particular, nitroglycerin (GTN), are clinically important nitrovasodilators that function as NO-mimetics in biological systems. Comparison of sGC activation data with electrochemically measured rates of NO release for genuine NO donors, NONOates and nitrosothiols, yields an excellent correlation between the EC50 for sGC activation and the rate constant for NO release, kNo. However, activation of sGC by GTN and the nitrates has very different characteristics, including the requirement for specific added thiols, for example, cysteine. The reaction of GTN with cysteine in anaerobic solution yields NO slowly, and NO release, measured by chemiluminescence detection, is quenched by added metal ion chelator. The generation of NO under aerobic conditions is 100-fold slower than the anaerobic reaction. Furthermore, NO release from the reaction of GTN with cysteine in phosphate buffer is too slow to account for sGC activation by GTN/cysteine. The slow rate of the chemical reaction to release NO suggests that nitrates can activate sGC by an NO-independent mechanism. In contrast to the genuine NO donors, GTN behaves as a partial agonist with respect to sGC activation, but in the presence of the allosteric sGC activator, YC-1, GTN exhibits full agonist activity.
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