Accumulation of hypothiocyanite ion during peroxidase-catalyzed oxidation of thiocyanate ion.

Abstract

An oxidized form of thiocyanate accumulated during incubation of lactoperoxidase or myeloperoxidase with thiocyanate ion and low concentrations of peroxide. Addition of one mole of peroxide resulted in loss of one mole of thiocyanate and formation of one mole of the oxidation product. The oxidation product reacted with two moles of sulfhydryl to yield one mole of disulfide and one mole of thiocyanate. The same product was obtained by alkaline hydrolysis of thiocyanogen. The product is proposed to be hypothiocyanite ion. The peroxidases may catalyze the oxidation of thiocyanate to hypothiocyanite, or to thiocyanogen. Hypothiocyanite may be the antimicrobial agent formed during peroxidase‐catalyzed oxidation of thiocyanate.Hypothiocyanite decomposed slowly with a second‐order rate constant of 0.1 M−1 s−1 at 25°C. When the amount of peroxide added approached the amount of thiocyanate, the yield of hypothiocyanite reached a maximum and then declined. This result suggested that hypothiocyanite was oxidized by excess peroxide. Efforts to produce high concentrations of hypothiocyanite were unsuccessful, even when thiocyanate was well in excess of peroxide. As the hypothiocyanite concentration approached 0.5 mM, decomposition was accelerated and small amounts of cyanide were produced. Hypothiocyanite reacted stoichiometrically with cyanide to give products that did not oxidize sulfhydryls. Hydrolysis of thiocyanogen at neutral pH also resulted in rapid decomposition and formation of cyanide.