The possibility that endothelial cell-derived oxidants could contribute to neutrophil-mediated endothelial cell injury and cytotoxicity has been a subject of speculation. Rat pulmonary artery endothelial cells (RPAECs) were examined for the presence of xanthine oxidase (XO) activity, a well-known source of O2-. Using a sensitive assay based on measurements of radioactive xanthine conversion to uric acid by high performance liquid chromatography (HPLC), RPAEC extracts were found to contain both XO and xanthine dehydrogenase (XD) activities. Extracts from early passage cells have 55.3 +/- 11.7 (mean +/- SE) units/10(6) cells of total (XO + XD) activity, one unit of activity being defined as the conversion of 1% of substrate to product in 30 minutes of incubation. XO comprised 31.6 +/- 3.1% of this total activity. Addition of human neutrophils stimulated with phorbol myristate acetate (PMA) caused a rapid and dose-dependent increase in RPAEC XO activity from 31.6 +/- 3.1% to 71.7 +/- 4.8% of total without altering total (XO + XD) activity. The neutrophil dose-response curve for increase in XO paralleled closely the curve for neutrophil-mediated RPAEC cytotoxicity. The basal XO and XD activities and the neutrophil-induced increase in XO activity were inhibited by treating RPAECs with allopurinol, oxypurinol, and lodoxamide, which also inhibited cytotoxicity, but not by catalase, superoxide dismutase, or deferoxamine. Addition of H2O2 failed to cause an increase in RPAEC XO activity or XD to XO conversion. The results suggest that during neutrophil-mediated injury, rapid conversion of RPAEC XD to XO occurs, resulting in increased XO, catalyzed endogenous oxidant production, which may contribute to the oxidant burden in the killing mechanism initiated by activated neutrophils. Although the mechanism for conversion of XD to XO is uncertain, it appears that neutrophil-derived H2O2 is not sufficient to cause this phenomenon. Furthermore, neither O2- nor chelatable iron is required for neutrophil-induced XD to XO conversion. Supernatant fluids from activated neutrophils failed to induce XD to XO conversion in RPAECs. This in vitro system provides an opportunity to define the cellular and molecular mechanisms underlying the in vivo phenomenon of XD to XO conversion associated with ischemic/reperfusion or inflammatory tissue injury.