The peroxidation of thiocyanate catalysed by myeloperoxidase and lactoperoxidase. 
Peroxidation of SCN- to OSCN-, catalysed by myeloperoxidase and lactoperoxidase, was studied.  The rate of this reaction showed sharp optima between pH 5 and 7.5, the position of which is determined by the concentrations of both SCN- and H2O2.  At low pH values, both SCN- and H+ inhibited myeloperoxidase and lactoperoxidase competitively with respect to H2O2.  The inhibition constants of SCN- for myeloperoxidase and lactoperoxidase (2 and 6 mM, respectively) are independent of pH. For these enzymes a Ki for H+ of 1 microM was found that corresponded to an ionisable group on the enzymes (pKa = 6) which controls the enzymic activity.  A kinetic expression is proposed that explains most of the data.  The physiological consequences of the corresponding mechanism are discussed.