Inhibition of nitric oxide synthase by cobalamins and cobinamides

Abstract

Cobalamins (Cbl) are important co-factors for methionine synthase and methylmalonyl-coA mutase. Certain corrins also bind nitric oxide (NO), quenching its bioactivity. To determine if corrins would inhibit NO synthase (NOS), we measured their effects on 14-C-L-arginine-to-14-C-L-citrulline conversion by NOS1, NOS2, and NOS3. Hydroxocobalamin (OH-Cbl), cobinamide (Cbi), and dicyanocobinamide (CN 2 -Cbi) potently inhibited all isoforms, whfile cyanocobalamin, methylcobalamin, and adenosylcobalamin had much less effect. OH-Cbl and CN 2 -Cbi prevented binding of the oxygen analog carbon monoxide (CO) to the reduced NOS1 and NOS2 heme active site. CN 2 -Cbi did not react directly with NO or CO. Spectral perturbation analysis showed that CN 2 -Cbi interacted directly with the purified NOS1 oxygenase domain. NOS inhibition by corrins was rapid and not reversed by dialysis with L-arginine, tetrahydrobiopterin. Molecular modeling indicated that corrins could access the unusually large heme and substrate-binding pocket of NOS. Best fits were obtained in the "base-off" conformation of the lower axial dimethylbenzimidazole ligand. CN 2 -Cbi inhibited interferon-γ-activated Raw264.7 mouse macrophage NO production. We show for the first time that certain corrins directly inhibit NOS, suggesting that these agents (or their derivatives) may have pharmacological utility. Endogenous cobalamins and cobinamides might play important roles regulating NOS activity in normal and pathological conditions.

Temperature Dependence of Henry’s Law Constant for Hydrogen Cyanide. Generation of Trace Standard Gaseous Hydrogen Cyanide

“…For structures of cobalamins and cobinamide, see (43). Hydroxycobalamin binds cyanide with a relatively high affinity (it is used to treat smoke inhalation victims (44)).…”

Is Endothelial Nitric Oxide Synthase a Moonlighting Protein Whose Day Job is Cholesterol Sulfate Synthesis? Implications for Cholesterol Transport, Diabetes and Cardiovascular Disease

“…Our suggestion of CblSSH as a candidate substrate for eNOS-catalyzed sulfate synthesis is based on evidence that a reasonably stable cobalamin persulfide derivative can be prepared in aqueous solution at pH 7 [64]. Moreover, structural studies indicate that cobalamin, an inhibitor of NO synthesis, can fit well into the heme pocket of NOS isoforms [65]. In addition, a cobalt-phthalocyanine complex (a nitrogen-containing ring system comparable to a heme group) can facilitate photocatalyic oxidation of sulfur dioxide, in an aqueous environment, to produce sulfate.…”

Biphasic Modulation of NOS Expression, Protein and Nitrite Products by Hydroxocobalamin Underlies Its Protective Effect in Endotoxemic Shock: Downstream Regulation of COX-2, IL-1β, TNF-α, IL-6, and HMGB1 Expression

“…Ubiquitous and continuous Cbl scavenging of • NO, on the other hand, may pose biochemical hazards. For • NO has important antioxidant and cell-signalling actions [25] which might be obstructed by HOCbl's previously proposed, indiscriminate • NO scavenging, or even just by a recently proposed, Cbl structural-based, direct inhibition of the NOS tout court and nothing else [45]. There is some evidence that HOCbl can discriminate between exogenous • NO donors and the natural endogenous donor, S -nitrosoglutathione, GSNO, actually prolonging only GSNO-induced, gastric fundus relaxations [46].…”