Specific assays for peroxidases in human saliva

Abstract

The peroxidase activity in human whole saliva is due to salivary peroxidase and, in some cases, myeloperoxidase; it is usually determined by spectrophotometric methods based on the rate of oxidation of chromogen substrates. Thiocyanate ion, a normal component of saliva, interferes with these kinetic assays by competing with the chromogen for the available oxidizing equivalents; this results in underestimation of peroxidase activity. Both salivary peroxidase and myeloperoxidase will catalyse the peroxidation of the thiocyanate ion; the product, hypothiocyanite ion, is a reactive oxidizing agent. We have developed an assay for total peroxidase activity in saliva, based on the rate of formation of hypothiocyanite, which is not affected by the concentrations of thiocyanate found in saliva. Myeloperoxidase will catalyse the peroxidation of the chloride ion but salivary peroxidase will not; the product of this in neutral solution is the hypochlorite ion, which is also a reactive oxidizing agent. The specific contribution was determined of myeloperoxidase to total peroxidase activity in saliva by measuring the rate of both hypochlorite and hypothiocyanite formation. Because the thiocyanate ion will compete with the chloride ion, the concentration of thiocyanate in saliva samples must be reduced below 0.05 mM prior to measurements of the rate of hypochlorite formation.