Mechanism of hypochlorous acid-mediated heme destruction and free iron release

Here, we show that hypochlorous acid (HOCl), a potent neutrophil-generated oxidant, can mediate destruction of free heme (Ht) and the heme precursor, protoporphyrin IX (PPIX). Ht displays a broad Soret absorbance peak centered at 365 and 394 nm, indicative of the presence of monomer and μ-oxo-dimer. Oxidation of Ht by HOCl was accompanied by a marked decrease in the Soret absorption peak and release of free iron. Kinetic measurements showed that the Ht-HOCl reaction was triphasic. The first two phases were HOCl concentration dependent and attributable to HOCl binding to the monomeric and dimeric forms. The third phase was HOCl concentration independent and attributed to Ht destruction with the release of free iron. HPLC and LC-ESI-MS analyses of the Ht-HOCl reaction revealed the formation of a number of degradation products, resulting from the cleavage or modification of one or more carbon-methene bridges of the porphyrin ring. Similar studies with PPIX showed that HOCl also mediated tetrapyrrole ring destruction. Collectively, this work demonstrates the ability of HOCl to modulate destruction of heme, through a process that occurs independent of the iron molecule that resides in the porphyrin center. This phenomenon may play a role in HOCl-mediated oxidative injury in pathological conditions.