A patient on nitroprusside therapy developed cyanide toxicity. Sodium nitrite was administered i.v. to combat this poisoning. Beneficial effect of sodiumnitrite in this case is dependent on:

(Ref: KDT 6/e p527) Cyanide has very high affinity for the enzymes of respiratory chain paicularly cytochrome oxidase. Methemoglobin possesses even higher affinity for cyanides. It can free the enzymes due to competitive inhibition for cyanide. Nitrates act by reducing hemoglobin to mithenoglobin, which removes cyanide from the enzymes.