Oxygen dependence of nitric oxide-mediated signaling

Nitric oxide (center dot NO) is a biologically important short-lived free radical signaling molecule. Both the enzymatic synthesis and the predominant forms of cellular metabolism of center dot NO are oxygen-dependent. For these reasons, changes in local oxygen concentrations can have a profound influence on steadystate center dot NO concentrations. Many proteins are regulated by center dot NO in a concentration-dependent manner, but their responses are elicited at different thresholds. Using soluble guanylyl cyclase (sGC) and p53 as model center dot NO-sensitive proteins, we demonstrate that their concentration-dependent responses to center dot NO are a function of the O-2 concentration. p53 requires relatively high steady-state center dot NO concentrations ( > 600 nM) to induce its phosphorylation (P-ser-15), whereas sGC responds to low center dot NO concentrations ( < 100 nM). At a constant rate of center dot NO production (liberation from center dot NO-donors), decreasing the O-2 concentration (1%) lowers the rate of center dot NO metabolism. This raises steady-state center dot NO concentrations and allows p53 activation at lower doses of the center dot NO donor. Enzymatic center dot NO production, however, requires O-2 as a substrate such that decreasing the O-2 concentration below the K-m for O-2 for nitric oxide synthase (NOS) will decrease the production of center dot NO. We demonstrate that the amount of center dot NO produced by RAW 264.7 macrophages is a function of the O-2 concentration. Differences in rates of center dot NO production and center dot NO metabolism result in differential sGC activation that is not linear with respect to O-2. There is an optimal O-2 concentration (approximate to 5-8%) where a balance between the synthesis and metabolism of center dot NO is established such that both the center dot NO concentration and sGC activation are maximal. (C) 2013 The Authors. Published by Elsevier By. Open access under CC BY-NC-ND liGense