Effect of thiocyanate on nitrosation of amines.

Abstract

CARCINOGENIC nitrosamines are formed by chemical reaction between nitrous acid and secondary and some tertiary amines in the stomachs of rodents1–3. Nitrosamines could be formed similarly in the human stomach and so present a carcinogenic hazard to man. The nitrosation reaction is catalysed by some anions4 particularly thiocyanate5,6. Thiocyanate is secreted in saliva which in non-smokers normally contains about 50 mg l−1 (approximately 1 mM) but in smokers contains three to four times this concentration7. In the absence of catalyst the nitrosation of N-methylaniline and some other secondary amines is maximal at pH 3, but in the presence of thiocyanate the reaction proceeds much more rapidly in acid conditions, such as in gastric juice, between pH 1 and 2 (Fig. 1). At pH 1.5, 1.0 mM thiocyanate increases the rate of reaction about 550 times. Under these acid conditions below pH 2 the nitrosation reaction rate is proportional to the concentrations of (1), nitrite; (2), thiocyanate; and (3), amine8. At pH 3.5 and greater thiocyanate has less catalytic action and the predominant reaction is proportional to the square of the nitrite concentration, but is slow.