Thiocyanate and chloride as competing substrates for myeloperoxidase.

Biochem J

Free Radical Research Group, Department of Pathology, Christchurch School of Medicine, P.O. Box 4345, Christchurch, New Zealand.

Published: October 1997

AI Article Synopsis

  • Myeloperoxidase, an enzyme in neutrophils, utilizes hydrogen peroxide (H2O2) to oxidize different substrates like chloride, bromide, iodide, and thiocyanate, with chloride being the primary one studied.
  • Research showed that thiocyanate is a significantly preferred substrate over chloride and bromide, with specific constants indicating a strong preference (1:60:730).
  • The study found that even at high chloride concentrations (100 mM), myeloperoxidase effectively produced hypothiocyanite from thiocyanate, suggesting its vital role in physiological conditions related to inflammation and host defense.

Article Abstract

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.

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Source
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1218820PMC
http://dx.doi.org/10.1042/bj3270487DOI Listing

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