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Hydroxyl radical scavengers and xanthine oxidase inhibitors protect cultured bovine pulmonary endothelial cells (BPAEC) from lytic injury by the endotoxin lipopolysaccharide (LPS). We hypothesized that exposure of BPAEC to cytotoxic concentrations of LPS activated intracellular xanthine oxidase, and that intracellular iron-dependent hydroxyl radical formation (a Fenton reaction) ensued, resulting in cell lysis. To test this, the protective effects of deferoxamine against H2O2 and LPS-induced cytotoxicity to BPAEC was assessed by 51Cr release. Preincubation with 0.4 mM deferoxamine conferred 67 ± 15% (mean ± SE) protection from LPS-induced cytotoxicity but 48 h of preincubation were required to induce significant protection. Significant protection form a classical Fenton reaction model, injury by 50 μM H2O2, could be induced by a 1-h preincubation with a 0.4 mM deferoxamine. The dissociated time course suggested that deferoxamine might work by different mechanisms in these models. The effects of LPS and deferoxamine on BPAEC-associated xanthine oxidase (XO) and xanthine dehydrogenase (XD) activity were assessed using a spectrofluorophotometric measurement of the conversion of pterin to isoxanthopterin. BPAEC had 106 ± 7 μU/mg XD+XO activity; XO activity constituted 48 ± 1% of total XO+XD activity. LPS at a cytotoxic concentration did not alter XO, XD, or percent XO. Deferoxamine had striking proportional inhibitory effects on XO and XD in intact cells. XO+XD activity fell to 6 ± 1% of control levels during a 48-h exposure of BPAEC to deferoxamine. Deferoxamine did not inhibit XO + XD ex vivo. The time course by which deferoxamine preincubation of BPAEC inhibited XD+XO paralleled the time course for inducing protection against LPS-induced cytotoxicity. These data raise the possibility of a novel mechanism of protection by deferoxamine in the LPS and possibly other models of endothelial injury: the iron chelator may modify cellular injury by inhibiting the synthesis or bioactivity of constitutive intracellular XO.