Tri-n-butyltin binding to a low-affinity site decreases the F1FO-ATPase sensitivity to oligomycin in mussel mitochondria

In mussel digestive gland mitochondria the environmental pollutant tri-n-butyltin (TBT), other than to strongly inhibit the ATPase activity at <1.0 μM, at ≥1.0 μM concentrations was previously found to desensitize the F1FO-ATPase to the antibiotic oligomycin. While the F1FO-ATPase inhibition is widely known as one of the main mitochondrial damages caused by TBT, the enzyme desensitization to oligomycin was quite unexpected. The possible mechanisms involved are here stepwise approached, aiming at enlightening the molecular mechanism(s) of TBT toxicity and the still poorly investigated oligomycin interaction with FO. The findings strongly suggest that the oligomycin desensitization directly stems from the covalent binding of TBT to mono-thiols of the F1FO-ATPase. This binding implies sulfur oxidation, irrespective of the possible formation of radical species in mitochondria, a mechanism which does not seem to be here involved. The hypothesis that TBT interacts with the enzyme complex by at least two sites featured by different affinity is shouldered: TBT binding to the high-affinity site would lead to ATPase inhibition, while TBT binding to mono-thiols in the low-affinity site could mirror the decrease in the F1FO-ATPase oligomycin-sensitivity at ≥1.0 μM TBT. Experiments carried out on inside-out submitochondrial particles hint that TBT binding destabilizes the oligomycin-blocked FO conformation allowing proton flux recovery within FO, without uncoupling the catalytic function from proton channeling.