HEME AND THE ENDOTHELIUM - EFFECTS OF NITRIC-OXIDE ON CATALYTIC IRON AND HEME DEGRADATION BY HEME OXYGENASE

We studied the effects of nitric oxide (NO) on the control of excess c ellular heme and release of catalytically active iron. Endothelial cel ls (ECs) exposed to hemin followed by a NO donor have a ferritin conte nt that is 16% that of cells exposed to hemin alone. Hemin-treated ECs experience a 3.5-fold rise in non-heme, catalytic iron 2 h later, but a hemin rechallenge 20 h later results in only a 24% increase. The ad dition of a NO donor after the first hemin exposure prevents this adap tive response, presumably due to effects on ferritin synthesis. NO don ors were found to reduce iron release from hemin, while hemin accumula ted in cells. A NO donor, in a dose-dependent fashion, inhibited heme oxygenase activity, measured by bilirubin production. Using low temper ature EPR spectroscopy, heme oxygenase inhibition correlated with nitr osylation of free heme in microsomes, Nitrosylation of cellular heme p revented iron release, for while there was heme oxygenase-dependent re lease of iron in cells incubated with hemin for 24 h, the addition of a NO donor blocked iron release. This indicates that NO readily nitros ylates intracellular free heme and prevents its degradation by heme ox ygenase. Nitrosylation of heme was found to reduce sensitization of ce lls to oxidative injury.