Spectral properties of myeloperoxidase compounds II and III

In order to resolve the confusion about the spectral properties of myeloperoxidase Compound II and Compound III (myeloperoxidase is donor:hydrogen-peroxide oxidoreductase, EC 1.11.1.7), the absorbance spectra in the visible and ultraviolet regions were measured under conditions where either Compound II or Compound III was present. Peak positions, isosbestic points and absorption coefficients are presented. We conclude that in most studies on Compound II or Compound III, mixtures of these compounds had been present. Our data indicate that the relative contributions of Compound II and Compound III in a sample can be determined from the absorbance ratio A625nm/A456nm. The optical absorbance spectrum of myeloperoxidase compound III was not affected by pH (pH 3-8). The absorbance spectrum of Compound II, however, was dependent on pH. The absorbance spectrum of Compound II at high pH is described.