Tributyl-driven enhancement of the DCCD-insensitive Mg-ATPase activity in mussel digestive gland mitochondria

The lipophilic pollutant tributyltin (TBT), other than inhibiting the DCCD (N,N0-dicyclohexylcarbodiimide) and oligomycin-sensitive Mg-ATPase activities in digestive gland mitochondria from the Mediterranean mussel Mytilus galloprovincialis, at higher than 1.0 mM concentrations in vitro promotes an increase in the ATPase activity fraction refractory to inhibitors of FO moiety, namely oligomycin and DCCD. By exploring the mechanisms involved in the TBT promoted enzyme desensitization to DCCD, we pointed out intriguing differences in the enzyme desensitization to the two inhibitors. Differently from oligomycin, the TBT promoted enzyme desensitization to DCCD is independent of the redox state of thiol groups of the enzyme complex and strongly temperature dependent, being significantly elicited only at temperatures above the break of the Arrhenius plots (around 18 C). Such differences may cast light on multiple TBT interaction modes with the enzyme complex. The TBT-driven increase in the activation energy of the Mg-ATPase activities insensitive to inhibitors of FO sector suggests that the temperaturedependent incorporation of the lipophilic toxicant within the lipid bilayer may deeply affect the membrane-bound complex functionality. Accordingly, incorporated TBT may cause structural changes in the intramembrane FO subunits, thus weakening or even preventing DCCD binding to the enzyme complex.