The antibacterial action of lactoperoxidase. The nature of the bacterial inhibitor.

Lactoperoxidase (EC 1.11.1.7), an enzyme present in various mammalian glands and in their secretions, catalyses the oxidation of thiocyanate by hydrogen peroxide to form a compound that inhibits the growth, oxygen uptake and acid production of certain bacteria. This compound was found to be too unstable to isolate in pure form, but its properties in dilute aqueous solution were studied with a view to establishing its identity. At thiocyanate concentrations of approximately 1mm, formation of the inhibitor, which took place by a nonstoicheiometric reaction, was maximal when an approximately equimolar amount of hydrogen peroxide was added. Excess of hydrogen peroxide oxidized the inhibitor to sulphate and cyanate. The inhibitor displayed a polarographic reduction wave of which the half-wave potential was pH-dependent. Studies of the variation of the polarographic half-wave potential and of the u.v. extinction with pH indicated that the inhibitor existed in an acid-base equilibrium (pK(a) 5.1+/-0.1). The inhibitor decomposed by a mechanism involving H(+) ions and thiocyanate, the kinetics varying according to whether the inhibitor was in its acidic or basic form. From these studies it was concluded that the inhibitor was either cyanosulphurous acid (HO(2)SCN) or cyanosulphuric acid (HO(3)SCN).