Fig. 6. Mitochondrial oxygen and sulfide consumption from normoxic to anoxic conditions. (A) Oxygen partial pressure P_O2 (black line) and sulfide concentration (red line) as isolated mitochondria were exposed to repeated additions of 12.5 µmol l^–1 sulfide (arrows). (B) Time derivatives (x –1) of oxygen (black line) and sulfide (red line) lines, giving oxygen and sulfide consumption rates (_O2 and _H2S, respectively). At higher P_O2, both oxygen and sulfide consumption events are coincident. As oxygen levels decline, the events become uncoupled, with oxygen consumption being limited first. The multiphasic oxygen consumption kinetics may result from transient sulfide stimulation and inhibition of cytochrome c oxidase. Under anoxia, sulfide consumption continues at a low level.