TBT and calmodulin: love at first sight or occasional flirt?

TBT, widely used as antifouling biocide, is the most abundant pesticide in coastal environment. One of its main toxic effect is immunosuppression in both vertebrates and invertebrates. In the presence of sublethal doses of TBT, phagocytes loose their ability to move towards and ingest foreign particles and to trigger a respiratory burst. In cultured haemocytes of marine invertebrates, such as molluscs and ascidians, we reported a dose- and time-dependent disorganisation of cytoskeleton morphology by TBT. Since TBT can antagonise the effect of chlorpromazine, a specific calmodulin (CaM) inhibitor, and the co-presence of exogenous CaM and TBT in the incubation medium resulted in the absence of effects, we hypothesised an interaction of TBT with CaM: TBT could subtract endogenous CaM to cell proteins thus inactivating them and altering Ca2+ homeostasis. According to this view, we observed the inactivation of Ca2+-ATPase and a sustained increase in cytosolic calcium in the presence of the pesticide. In order to further support our hypothesis of a direct TBT-CaM interaction, we studied conformational changes of pure CaM in the presence of various concentrations of TBT and its degradation products, DBT and MBT, by circular dichroism (CD). Results obtained suggest a dose- and time-dependent non-specific interaction of TBT with the hydrophobic core of CaM. Conversely, DBT and MBT resulted less active in inducing CaM conformational changes, without any significant difference between the two compounds.