Does hydrophobic isothiocyanate really uncouple oxidative phosphorylation in mitochondria?

A wide variety of organic compounds are known to be uncouplers of oxidative phosphorylation in mitochondria [l] . Since uncouplers have a dissociable proton and act as proton conductors in mitochondria [2-41 and model membrane systems [S-7] , their action is thought to be due to collapse of the proton gradient as a result of proton transfer across the mitochondrial membrane. Alternatively they may have a direct interaction with some mitochondrial protein involved in oxidative phosphorylation [8-lo] . Hydrophobic isothiocyanates, such as BrPh-NCS, were reported [ 1 l] as effective uncouplers in mitochondria, stimulating the respiration of state 4 mitochondria, releasing oligomycin-inhibited respiration and activating ATPase, BrPh-NCS, the most effective isothiocyanate, stimulated state 4 respiration of rat liver mitochondria maximally at -50 PM with succinate as substrate, and activated ATPase at -15 PM [ 1 l] . If BrPh-NCS really uncouples oxidative phosphorylation in mitochondria, it would be the only known potent uncoupler that is not weakly acidic. In this case isothiocyanate would act as an SH-reagent directly reacting with an SH-group in mitochondrial protein as proposed [ 1 l] . However, as shown in [4] , isothiocyanates are very reactive with non-proteinous -SH or -NH2