Oxidation of hydroquinone, 2,3-dimethylhydroquinone and 2,3,5-trimethylhydroquinone by human myeloperoxidase

Myeloperoxidase is very susceptible to reducing radicals because the reduction potential of the ferric/ferrous redox couple is much higher compared with other peroxidases. Semiquinone radicals are known to reduce heme proteins. Therefore, the kinetics and spectra of the reactions of p-hydroquinone, 2,3-dimethylhydroquinone and 2,3,5-trimethylhydroquinone with compounds I and II were investigated using both sequential-mixing stopped-flow techniques and conventional spectrophotometric measurements. At pH 7 and 15 degrees C the rate constants for compound I reacting with p-hydroquinone, 2,3-dimethylhydroquinone and 2,35-trimethylhydroquinone were determined to be 5.6 +/- 0.4 x 10(7) M(-1)s(-1), 1.3 +/- 0.1 x 10(6) M(-1)s(-1) and 3.1 +/- 0.3 x 10(6) M(-1)s(-1), respectively. The corresponding reaction rates for compound II reduction were calculated to be 4.5 +/- 0.3 x 10(6) M-1 s(-1), 1.9 +/- 0.1 x 10(5) M(-1)s(-1) and 4.5 +/- 0.2 x 10(4) M-1 s(-1), respectively. Semiquinone radicals, produced by compounds I and II in the classical peroxidation cycle, promote compound III (oxymyeloperoxidase) formation. We could monitor formation of ferrous myeloperoxidase as well as its direct transition to compound III by addition of molecular oxygen. Formation of ferrous myeloperoxidase is shown to depend strongly on the reduction potential of the corresponding redox couple benzoquinone/semiquinone. With 2,3-dimethylhydroquinone and 2,35-trimethylhydroquinone as substrate, myeloperoxidase is extremely quickly trapped as compound III. These MPO-typical features could have potential in designing specific drugs which inhibit the production of hypochlorous acid and consequently attenuate inflammatory tissue damage.