I ron-dependent free radical reactions and renal ischemia are believed to be critical mediators of myohemoglobinuric acute renal failure. Thus, this study assessed whether catalytic iron exacerbates O2 deprivation-induced proximal tubular injury, thereby providing an insight into this form of renal failure. Isolated rat proximal tubular segments (PTS) were subjected to either hypoxia/reoxygenation (H/R: 27:15 min), "chemical anoxia" (antimycin A; 7.5 microM x 45 min), or continuous oxygenated incubation +/- ferrous (Fe2+) or ferric (Fe3+) iron addition. Cell injury (% lactic dehydrogenase [LDH] release), lipid peroxidation (malondialdehyde, [MDA]), and ATP depletion were assessed. Under oxygenated conditions, Fe2+ and Fe3+ each raised MDA (approximately 7-10x) and decreased ATP (approximately 25%). Fe2+, but not Fe3+, caused LDH release (31 +/- 2%). During hypoxia, Fe2+ and Fe3+ worsened ATP depletion; however, each decreased LDH release (approximately 31 to approximately 22%; P < 0.01). Fe(2+)-mediated protection was negated during reoxygenation because Fe2+ exerted its intrinsic cytotoxic effect (LDH release: Fe2+ alone, 31 +/- 2%; H/R 36 +/- 2%; H/R + Fe2+, 41 +/- 2%). However, Fe(3+)-mediated protection persisted throughout reoxygenation because it induced no direct cytotoxicity (H/R, 39 +/- 2%; H/R + Fe3+, 25 +/- 2%; P < 0.002). Fe3+ also decreased antimycin toxicity (41 +/- 4 vs. 25 +/- 3%; P < 0.001) despite inducing marked lipid peroxidation and without affecting ATP. These results indicate that catalytic iron can mitigate, rather than exacerbate, O2 deprivation/reoxygenation PTS injury.