Interaction of cyanide and nitric oxide with cytochrome c oxidase: implications for acute cyanide toxicity.

Acute cyanide toxicity is attributed to inhibition of cytochrome c oxidase (CcOX), the oxygen-reducing component of mitochondrial electron transport; however, the mitochondrial action of cyanide is complex and not completely understood. State-3 oxygen consumption and CcOX activity were studied in rat N27 mesencephalic cells to examine the functional interaction of cyanide and nitric oxide (NO). KCN produced a concentration-dependent inhibition of cellular respiration. Cyanide's median inhibitory concentration (IC50) of oxygen consumption (13.2 +/- 1.8microM) was higher than the CcOX IC50 (7.2 +/- 0.1microM). Based on respiratory threshold analysis, 60% inhibition of CcOX was necessary before oxygen consumption was decreased. Addition of high levels of exogenous NO (100microM S-nitroso-N-acetyl-DL-penicillamine) attenuated cyanide inhibition of both respiration and CcOX. On the other hand, when endogenous NO generation was blocked by an NOS inhibitor (N(omega)-monomethyl-L-arginine ester), the cyanide IC50 for both respiration and CcOX increased to 59.6 +/- 0.9microM and 102 +/- 10microM, respectively, thus showing constitutive, low-level NO production enhanced cyanide inhibition. Laser scanning cytometry showed that cyanide elevated mitochondrial NO, which then was available to interact with CcOX to enhance the inhibition. It is concluded that the rapid, potent action of cyanide is due in part to mitochondrial generation of NO, which enhances inhibition of CcOX. At low mitochondrial oxygen tensions, the cyanide-NO interaction would be increased. Also, the antidotal action of sodium nitrite is partly explained by generation of high mitochondrial levels of NO, which antagonizes the CcOX inhibition.