Lactoferrin enhances hydroxyl radical production by human neutrophils, neutrophil particulate fractions, and an enzymatic generating system.

Abstract

During phagocytosis, neutrophils take oxygen from the surrounding medium and convert it to superoxide anion (O2-) and hydrogen peroxide (H2O2). Hydroxyl radical (.OH), a particularly potent oxidant, is believed to be produced by interaction between O2- and H2O2 in the presence of iron, according to the Haber-Weiss reactions. Production of .OH by whole human neutrophils, by particulate fractions from human neutrophils disrupted after stimulation, and by a xanthine oxidase system was measured by conversion of alpha-keto-gamma-methiol butyric acid to ethylene. FeCl3 or ferric EDTA enhanced ethylene production in all three systems by 155--406% of base line at a concentration of 50--100 microM. Iron-saturated human milk lactoferrin, 100 nM, increased ethylene generation by 127--296%; and purified human neutrophil lactoferrin, 10 nM, enhanced ethylene production by 167--369%. Thus, iron bound to lactoferrin was approximately 5,000 times more effective in producing an enhancement in ethylene generation than iron derived from FeCl3 or ferric EDTA. O2- and H2O2 were required for ethylene production in the presence of lactoferrin, since superoxide dismutase inhibited ethylene formation in the three systems by 76--97% and catalase inhibited by 76--98%. Ethylene production in the presence of lactoferrin was inhibited by the .OH scavengers mannitol, benzoate, and thiourea by 43--85, 45--94, and 76--96%, respectively. Thus, most of the ethylene production could be attributed to oxidation of alpha-keto-gamma-methiol butyric acid by .OH. The ability of neutrophil lactoferrin to provide iron efficiently to the oxygen radical-generating systems is compatible with a role for lactoferrin as regulator of .OH production. As such, lactoferrin may be an important component in the microbicidal activity of neutrophils.