Inhibition of mitochondrial respiration by sodium nitroprusside and the mechanism of cyanide liberation.

The influence of sodium nitroprusside (SNP) on mitochondrial respiration was examined in rat liver mitochondria. The addition of SNP 1 mmol litre-1 during state 3 respiration inhibited the oxygen uptake by 63.4%. A mixture of SNP 1 mmol litre-1 and glutathione (GSH) 1 mmol litre-1 inhibited the oxygen uptake more markedly (by 75.9%). The cyanide concentrations were 0.01 mmol litre-1 with SNP alone and 0.15 mmol litre-1 with the mixture of SNP and GSH. Cyanide production from SNP in the presence of various reducing agents was studied in potassium phosphate 0.1 mol litre-1 buffer solution (pH 7.4) incubated at 37 degrees C. Cyanide was liberated markedly from SNP in the presence of GSH or ascorbate. Less cyanide was produced in the presence of NADH or NADPH. The rate of production of cyanide was dependent entirely upon the concentration of each reducing agent added. No cyanide was liberated when sodium dithionite or the oxidized forms of GSH, NAD or NADP were used. It was concluded that SNP is degradated to cyanide by a hydrogen donor and that the cyanide liberated in this manner inhibits the cytochrome oxidase activity of mitochondria in vivo.