Hydrogen sulfide bypasses the rate-limiting oxygen activation of heme oxygenase.

Discovery of unidentified protein functions is of biological importance because it often provides new paradigms for many research areas. Mammalian heme oxygenase (HO) enzyme catalyzes the O-dependent degradation of heme into carbon monoxide (CO), iron, and biliverdin through numerous reaction intermediates. Here, we report that HS, a gaseous signaling molecule, is part of a novel reaction pathway that drastically alters HO's products, reaction mechanism, and catalytic properties. Our prediction of this interplay is based on the unique reactivity of HS with one of the HO intermediates. We found that in the presence of HS, HO produces new linear tetrapyrroles, which we identified as isomers of sulfur-containing biliverdin (SBV), and that only HS, but not GSH, cysteine, and polysulfides, induces SBV formation. As BV is converted to bilirubin (BR), SBV is enzymatically reduced to sulfur-containing bilirubin (SBR), which shares similar properties such as antioxidative effects with normal BR. SBR was detected in culture media of mouse macrophages, confirming the existence of this HS-induced reaction in mammalian cells. HS reacted specifically with a ferric verdoheme intermediate of HO, and verdoheme cleavage proceeded through an O-independent hydrolysis-like mechanism. This change in activation mode diminished O dependence of the overall HO activity, circumventing the rate-limiting O activation of HO. We propose that HS could largely affect O sensing by mammalian HO, which is supposed to relay hypoxic signals by decreasing CO output to regulate cellular functions. Moreover, the novel HS-induced reaction identified here helps sustain HO's heme-degrading and antioxidant-generating capacity under highly hypoxic conditions.