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The lactoperoxidase system, defined as lactoperoxidase in the presence of hydrogen peroxide and thiocyanate, is a powerful method of host defense, particularly in mammalian secretory mucosae. The product of this system, hypothiocyanite, functions as an antimicrobial by oxidizing bacterial sulfhydryls, thereby inhibiting bacterial respiration. The lactoperoxidase system and its product, hypothiocyanite, have been the focus of study for over a century. We explore herein the function of selenocyanate as a substrate of the lactoperoxidase system and we reinterpret the mechanism of the lactoperoxidase system with is natural substrate, thiocyanate. There is new evidence in the literature that selenocyanate, the selenium derivative of thiocyanate, is present in mammals and concentrated in the secretory mucosae using the same active membrane transport mechanism as thiocyanate. However, the ability of lactoperoxidase to utilize selenocyanate as a substrate for the lactoperoxidase system has never been studied. Furthermore, the selenium derivative of hypothiocyanite, hyposelenocyanite, has not been previously characterized. We successfully synthesized hyposelenocyanite via the oxidation of selenocyanate by hypochlorous acid at alkaline pH and using the lactoperoxidase system. Hyposelenocyanite produced at alkaline pH was characterized spectrophotometrically by comparison with the known spectrum of hypothiocyanite. The rate of oxidation of selenocyanate by hypochlorous acid at alkaline pH is faster than the oxidation of thiocyanate by hypochlorous acid by two orders of magnitude which is expected given the higher reactivity of …