Interaction of trialkyltin(IV) chlorides with sarcoplasmic reticulum calcium ATPase

The interaction of the organotin compounds trimethyltin(IV) and tributyltin(IV) chlorides with the calcium pump from sarcoplasmic reticulum membranes was studied. It was found that the presence of calcium fully protects against the inhibitory effect of both organotin compounds. However, the apparent affinity of the protein for tributyltin chloride is two orders of magnitude higher than for trimethyltin chloride (K0.5 values of 14?mu m and 1.4?m m, respectively). Studies of intrinsic fluorescence of the Ca2+-ATPase and enzyme phosphorylation by ATP and Pi support the hypothesis that the inhibitory properties of trialkyltin compounds are due to the inhibition of calcium binding to the high-affinity binding sites of the Ca2+-ATPase. This suggests that there is a specific interaction between the trialkyltin compounds and the calcium binding sites of the protein. The effect of trialkyltin compounds on Ca2+-ATPase was also addressed by differential scanning calorimetry to assess the thermal transition of the protein denaturation, and by infrared spectroscopy in the absorption region corresponding to the amide I band (16001700?cm-1) to observe changes in the secondary structure of the protein. We conclude that the interaction of trialkyltin compounds with Ca2+-ATPase reduces the affinity and cooperativity for calcium binding and, consequently, the inhibition of ATPase activity. These events are accompanied by changes in the secondary structure of the protein, including loss of alpha-helix structure and a concomitant increase in protein aggregation or unfolding. The activity of trialkyltin compounds on the Ca2+-ATPase is discussed in relation to their solubility in water and in the lipid phase. Copyright (c) 2012 John Wiley & Sons, Ltd.