HEME OXYGENASE (HO-1) - EVIDENCE FOR ELECTROPHILIC OXYGEN ADDITION TOTHE PORPHYRIN RING IN THE FORMATION OF ALPHA-MESO-HYDROXYHEME

Previous studies have established that reaction of the rat heme-heme o xygenase complex with H2O2 proceeds normally to give verdoheme, wherea s reaction of the complex with meta-chloroperbenzoic acid yields a fer ryl (Fe-IV = O) species and a protein radical but no verdoheme. The he me-heme oxygenase complex is shown here to react regiospecifically wit h ethyl hydroperoxide to give alpha-meso-ethoxyheme. Formation of this product exactly parallels the formation of alpha-meso-hydroxyheme in the normal reaction supported by cytochrome P450 reductase/NADPH or H2 O2. These results rule out a nucleophilic mechanism for the alpha-meso -hydroxylation catalyzed by heme oxygenase and indicate that it involv es electrophilic (or possibly radical) addition of the distal oxygen o f iron-bound peroxide (Fe-III-OOH) to the porphyrin ring.