Free nitric oxide (NO) activates soluble guanylate cyclase (sGC), an enzyme, within both pulmonary and vascular smooth muscle, sGC catalyzes the cyclization of guanosine 5′-triphosphate to guanosine 3′,5′-cyclic monophosphate (cGMP). Binding rates of NO to the ferrous heme(s) of sGC have been measured in vitro. However, a missing link in our understanding of the control mechanism of sGC by NO is a comprehensive in vivo kinetic analysis. Available literature data suggests that NO dissociation from the heme center of sGC is accelerated by its interaction with one or more cofactors in vivo. We present a working model for sGC activation and NO consumption in vivo. Our model predicts that NO influences the cGMP formation rate over a concentration range of ≈5-100 nM (apparent Michaelis constant ≈ 23 nM), with Hill coefficients between 1.1 and 1.5. The apparent reaction order for NO consumption by sGC is dependent on NO concentration, and varies between 0 and 1.5. Finally, the activation of sGC (half-life ≈ 1-2 s) is much more rapid than deactivation (≈50 s). We conclude that control of sGC in vivo is most likely ultra-sensitive, and that activation in vivo occurs at lower NO concentrations than previously reported.
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