Potential of peroxynitrite to promote the conversion of oxyhemoglobin to methemoglobin

Superoxide anion and NO can react to form the highly oxidizing species peroxynitrite (ONOO-) which can react directly with hemoglobin (Hb) even in the presence of physiological concentration CO2. This research was to determine the ONOO- -mediated oxidation damage to the heme of oxyhemoglobin (oxyHb) under conditions expected in blood. Results showed that 8-10 mol ONOO- was needed to quickly and completely convert 1 mol oxyHb to methemoglobin (metHb). ONOO- (20-140 microM) caused rapid and extensive formation of metHb from oxyHb (50 microM) mainly occurring within first 5-20 min of incubation. The conversion efficiency reached 16%, 48%, 60%, 79% and 88% output of metHb after 90 min of incubation at 0, 20, 40, 100, and 140 microM ONOO- respectively. 1 mM CO2 caused a small decrease in the ability of ONOO- to oxidize oxyHb, and ONOO- -promoted conversion of oxyHb to metHb increased when pH decreased from 8.0 to 6.0. Relatively lower temperature in blood condition will inhibit this reaction in some degree. We postulate that ONOO- can mediate oxidation damage to the heme, and cause heme loss from the hydrophobic cavity of Hb when its concentration exceeded 90 microM. These results indicated that ONOO- could convert oxyHb to metHb under the conditions expected in blood, and this reaction was regulated by CO2 concentration, reaction time, temperature and pH value.