title: The peroxidation of thiocyanate catalysed by myeloperoxidase and lactoperoxidase.
abstract: 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.
mesh_terms: D000818:Animals; D002417:Cattle; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007784:Lactoperoxidase / Q000378:metabolism*; D007962:Leukocytes / Q000201:enzymology; D008892:Milk / Q000201:enzymology; D010084:Oxidation-Reduction; D009195:Peroxidase / Q000097:blood*; D010544:Peroxidases / Q000097:blood* / Q000378:metabolism*; D013861:Thiocyanates / Q000378:metabolism*
chemical_list: D013861:Thiocyanates; D007784:Lactoperoxidase; D010544:Peroxidases; D009195:Peroxidase