Fig. 1.

Effect of repeated sulfide exposure on ETC flux. (A) Sulfide inhibition of complex IV (CIV) causes electron acceptor insufficiency in the ETC due to a predicted build-up of NADH, CoQH₂, and reduced cytochrome c (cyt c). IMS is intermembrane space, ETHE1 is persulfide dioxygenase, TST is thiosulfate sulfur transferase, and SO is sulfite oxidase. (B) Rotenone (0.5 µM)-treated HT29 cells exposed to low sulfide (5 to 10 µM) showed sharp increases in OCR with each aliquot of Na₂S. At higher Na₂S concentrations (15 to 30 µM), complex kinetics were observed. (C) Replotting the data in B as described under Methods showed a bell-shaped dependence of OCR on sulfide dose and an O₂:sulfide ratio of 1.0 ± 0.1 between 5 and 35 µM Na₂S. The data are representative of at least four independent experiments and represent the mean ± SD. (D) Experimental setup for removing residual sulfide and secreted oxidation products (Top). Washing did not impact the persistence of Na₂S-triggered OCR (Bottom). Data are representative of at least three independent experiments. Black and red arrows indicate when rotenone and sulfide, respectively, were added.