Sulfide affects the mitochondrial respiration, the Ca-activated FF-ATPase activity and the permeability transition pore but does not change the Mg-activated FF-ATPase activity in swine heart mitochondria.

In mammalian cells enzymatic and non-enzymatic pathways produce HS, a gaseous transmitter which recently emerged as promising therapeutic agent and modulator of mitochondrial bioenergetics. To explore this topic, the HS donor NaHS, at micromolar concentrations, was tested on swine heart mitochondria. NaHS did not affect the FF-ATPase activated by the natural cofactor Mg, but, when Mg was replaced by Ca, a slight 15% enzyme inhibition at 100 µM NaHS was shown. Conversely, both the NADH-O and succinate-O oxidoreductase activities were totally inhibited by 200 μM NaHS with IC values of 61.6 ± 4.1 and 16.5 ± 4.6 μM NaHS, respectively. Since the mitochondrial respiration was equally inhibited by NaHS at both first or second respiratory substrates sites, the HS generation may prevent the electron transfer from complexes I and II to downhill respiratory chain complexes, probably because HS competes with O in complex IV, thus reducing membrane potential as a consequence of the cytochrome c oxidase activity inhibition. The Complex IV blockage by HS was consistent with the linear concentration-dependent NADH-O oxidoreductase inhibition and exponential succinate-O oxidoreductase inhibition by NaHS, whereas the coupling between substrate oxidation and phosphorylation was unaffected by NaHS. Even if HS is known to cause sulfhydration of cysteine residues, thiol oxidizing (GSSG) or reducing (DTE) agents, did not affect the FF-ATPase activities and mitochondrial respiration, thus ruling out any involvement of post-translational modifications of thiols. The permeability transition pore, the lethal channel which forms when the FF-ATPase is stimulated by Ca, did not open in the presence of NaHS, which showed a similar effect to ruthenium red, thus suggesting a putative Ca transport cycle inhibition.