THIOCYANATE AND CHLORIDE AS COMPETING SUBSTRATES FOR MYELOPEROXIDASE

The neutrophil enzyme myeloperoxidase uses H2O2 to oxidize chloride, b romide, iodide and thiocyanate to their respective hypohalous acids. C hloride is considered to be the physiological substrate. However, a de tailed kinetic study of its substrate preference has not been undertak en. Our aim was to establish whether myeloperoxidase oxidizes thiocyan ate in the presence of chloride at physiological concentrations of the se substrates. We determined this by measuring the rate of H2O2 loss i n reactions catalysed by the enzyme at various concentrations of each substrate. The relative specificity constants for chloride, bromide an d thiocyanate were 1:60:730 respectively, indicating that thiocyanate is by far the most favoured substrate for myeloperoxidase. In the pres ence of 100 mM chloride, myeloperoxidase catalysed the production of h ypothiocyanite at concentrations of thiocyanate as low as 25 mu M. Wit h 100 mu M thiocyanate, about 50% of the H2O2 present was converted in to hypothiocyanite, and the rate of hypohalous acid production equalle d the sum of the individual rates obtained when each of these anions w as present alone. The rate of H2O2 loss catalysed by myeloperoxidase i n the presence of 100 mM chloride doubled when 100 mu M thiocyanate wa s added, and was maximal with 1 mM thiocyanate. This indicates that at plasma concentrations of thiocyanate and chloride, myeloperoxidase is far from saturated. We conclude that thiocyanate is a major physiolog ical substrate of myeloperoxidase, regardless of where the enzyme acts . As a consequence, more consideration should be given to the oxidatio n products of thiocyanate and to the role they play in host defence an d inflammation.