The neutrophil
enzyme myeloperoxidase uses H2O2 to oxidize
chloride,
bromide,
iodide and
thiocyanate to their respective hypohalous
acids.
Chloride is considered to be the physiological substrate. However, a detailed kinetic study of its substrate preference has not been undertaken. Our aim was to establish whether
myeloperoxidase oxidizes
thiocyanate in the presence of
chloride at physiological concentrations of these substrates. We determined this by measuring the rate of H2O2 loss in reactions catalysed by the
enzyme at various concentrations of each substrate. The relative specificity constants for
chloride,
bromide and
thiocyanate were 1:60:730 respectively, indicating that
thiocyanate is by far the most favoured substrate for
myeloperoxidase. In the presence of 100 mM
chloride,
myeloperoxidase catalysed the production of
hypothiocyanite at concentrations of
thiocyanate as low as 25 microM. With 100 microM
thiocyanate, about 50% of the H2O2 present was converted into
hypothiocyanite, and the rate of hypohalous
acid production equalled the sum of the individual rates obtained when each of these
anions was present alone. The rate of H2O2 loss catalysed by
myeloperoxidase in the presence of 100 mM
chloride doubled when 100 microM
thiocyanate was added, and was maximal with 1mM
thiocyanate. This indicates that at plasma concentrations of
thiocyanate and
chloride,
myeloperoxidase is far from saturated. We conclude that
thiocyanate is a major physiological substrate of
myeloperoxidase, regardless of where the
enzyme acts. As a consequence, more consideration should be given to the oxidation products of
thiocyanate and to the role they play in host defence and
inflammation.