4. Catabolism of H2S
Several regulated and unregulated non-enzymatic processes participate in H2S catabolism. These pathways maintain in vivo
H2S concentrations, most likely, in the nM to low mM range. H2S
can react with heme proteins in mitochondria and therefore H2S can function as a mitochondrial respiratory toxicant [22,23]. Fatal industrial accidents have been documented in individuals exposed to high concentrations of H2S gas (e.g., 4 1000 pm) [24]. Therefore, the toxicological profile of H2S has been well-studied and docu- mented [24]. The mechanism of toxicity is through the binding of H2S to cytochrome c oxidase (CcO) mediating respiratory inhibition [22]. However, this interaction is complex and poorly understood because H2S can act as both an inhibitor and an electron donor for CcO [25]. H2S binds to the oxidized states of the heme a-a3 binuclear center, resulting in the reduction of the heme molecules [26]. Excess H2S can also reduce CuB [27]. While the stoichiometry may vary, Cooper and Brown reported that 3 molecules of H2S bind per inhibited CcO [28]. In this inhibitory reaction, H2S is oxidized to sulfane sulfur and this is coupled to consumption of molecular O2 [28]. Unlike nitric oxide (dNO), the inhibition of CcO by H2S is noncompetitive with O2 [27,29]. In addition, H2S can also directly reduce the electron carrier cytochrome c producing the one electron oxidation product, the thiyl radical (dSH) [28].
Rhodanese, a mitochondrial sulfur transferase enzyme, cata- lyzes the oxidation of H2S [30]. It is one part of three enzymatic activities characterized as a major pathway for H2S catabolism. This pathway consists of a sulfide quinone oxido-reductase (SQR), a sulfur dioxygenase, and the sulfur transferase enzyme rhoda- nese (Figs. 2 and 3). H2S reduces the external disulfide on the SQR to form a thiol (RSH) and a perthiol (RSSH). This two electron oxidation of H2S reduces the FAD prosthetic group, which uses ubiquinone (Q) as an electron acceptor [31] The second sulfur