A key function of neutrophil
myeloperoxidase (MPO) is the synthesis of
hypochlorous acid (HOCl), a potent
oxidizing agent that plays a cytotoxic role against invading bacteria and viruses at inflammatory sites and in phagosomes. MPO displayed a chlorinating activity preferably at acidic pH but at neutral pH MPO catalyzes mainly reactions of the
peroxidase cycle. In the present work effects of
tyrosine on the chlorinating activity of MPO were studied. At pH 7.4 we detected an increased HOCl production in the presence of
tyrosine not only by the MPO-H(2)O(2)-Cl(-) system but also in
suspensions of
zymosan-activated neutrophils. An excess of H(2)O(2) is known to cause an accumulation of compound II of MPO blocking the generation of HOCl at neutral pH. As evidenced by spectral changes,
tyrosine-induced activation of MPO to synthesize HOCl was due to the ability of
tyrosine to reduce compound II back to the native state, thus accelerating the
enzyme turnover. MPO-induced oxidation of
tyrosine is relevant to what can be in vivo; we detected MPO-catalyzed formation of
dityrosine in the presence of plasma under experimental conditions when
tyrosine concentration was about three magnitudes of order less than the Cl(-) concentration. At acidic pH formation of compound II was impaired in the presence of
chloride and
dityrosine couldn't be detected in plasma. In conclusion, the ability of
tyrosine to increase the chlorinating activity of MPO at neutral pH and enhanced values of H(2)O(2) may be very effective for the specific enhancement of HOCl production under acute
inflammation.