Fig. 4. O₂-dependent H₂S production and consumption measured in real-time with a polarographic H₂S sensor. (A,B) H₂S production (as total sulfide) by minced trout heart increases with increasing concentrations of cysteine (mmol l^–1) at P_O20 and is transiently inhibited by injection of oxygen (O₂) into the reaction chamber. H₂S production resumes, presumably after the oxygen has been depleted (post O₂). (A) A single representative trace, (B) pooled results expressed as means ± s.e.m. (N=4). (C,D) H₂S is consumed by addition of purified trout heart mitochondria (M). The rate of sulfide consumption is greater when the reaction mixture is equilibrated with room air (O₂) than with nitrogen (N₂) or if the mitochondria are heat denatured (Heat). w, wash. (C) Single representative trace, (D) pooled results expressed as means ± s.e.m. (N=4 replicates). A, B adapted from Whitfield et al. (Whitfield et al., 2008), with permission; C, D adapted from Olson et al. (Olson et al., 2008b), with permission.