The SR Ca2+ ATPase of the Antarctic scallop Adamussium colbecki: cold adaptation and heavy metal effects

Cell calcium is accumulated in intracellular stores by sarco-endoplasmic re ticulum Ca2+ ATPases functionally interacting with the membrane lipid envir onment. Cold adaptations of membrane lipids in Antarctic Sea organisms sugg est possible adaptive effects also on sarco-endoplasmic reticulum Ca2+ ATPa ses. We investigated the SR Ca2+ ATPase of an Antarctic scallop, Adamussium colbecki, by characterising the enzyme activity and studying temperature e ffects. Ca2+ ATPase, assayed by following ATP hydrolysis, was thapsigargin- and vanadate-sensitive, showed maximum activity under 2 mu M Ca2+, 200 mM KCl and pH 7.2, and had a K-M for ATP of 22 +/- 7 mu M. Temperature effects showed an Arrhenius inversion between -1.8 and 0 degrees C, indicating col d adaptation, an Arrhenius break at 10 degrees C, and a collapse above 20 d egrees C. A. colbecki accumulates high amounts of cadmium in the digestive gland; heavy metal effects on sarco-endoplasmic reticulum Ca2+ ATPases were therefore tested, finding an IC50 = 0.9 mu M for Hg2+ and 3 mu M for Cd2+. Finally, SDS-PAGE analysis showed a main band at about 100 kDa, which was identified as sarco-endoplasmic reticulum Ca2+ ATPase after trypsin digesti on, and accounted for 60% total protein.