The close association of the heme enzyme myeloperoxidase to phosphatidylserine epitopes on the surface of non-vital polymorphonuclear leukocytes (PMNs) and other apoptotic cells at inflammatory sites favours modifications of this phospholipid by myeloperoxidase products. As detected by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, ammonium ions inhibit in a concentration-dependent manner the hypochlorous acid-mediated formation of aldehyde and nitrile products from 1,2-dipalmitoyl-sn-glycero-3-phosphoserine (DPPS). Concomitantly, the formation of monochloramine (NH(2)Cl) raises with increasing NH(4)(+) concentrations. A transchlorination from monochlorinated O-phospho-L-serine to NH(4)(+) with the formation of NH(2)Cl occurs only when extraordinary high NH(4)(+) concentrations are applied. Due to the low rate of 0.044 M(-1) s(-1) for this process, a transhalogenation reaction from transient chlorinated intermediates of the serine moiety to NH(4)(+) can be ruled out as an important process contributing to the HOCl-mediated formation of NH(2)Cl. A significant formation of NH(2)Cl by myeloperoxidase interacting with DPPS in the presence of ammonium ions takes only place at acidic pH values around 5, a scenario that may occur in phagosomes of macrophages after the uptake of apoptotic PMNs.
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