Effect of cadmium on the ATPase activity in gills of Anodonta cygnea at different assay temperatures

1. 1. The effect of Cd 2+ on the ATPase activity of microsomal membrane preparations of Anodonta gills was studied at assay temperatures from 4°C to 39°C. 2. 2. The ATPase activity was relatively nonspecific towards divalent cations at 39°C: Cd 2+ as well as Mg 2+ and Ca 2+ activated ATPase with V max being, respectively, 25.7, 33.6 and 34.8 μmol Pi·mg −1 protein·h −1. 3. 3. Apparent K a was highest for Ca 2+ and lowest for Cd 2+ at all temperatures studied. A decrease in temperature caused a fall in V max for all three cations, while there was no effect on the apparent K a for Mg 2+ but a significant decrease in apparent K a for Cd 2+ (from 0.4 to 0.14 mM). 4. 4. At all temperatures studied Cd 2+, but not Ca 2+, could replace Mg 2+ as the cofactor for activation of the ouabain-insensitive (Na + or K +)-ATPase. 5. 5. Cd 2+ caused an inhibitory effect on ATPase in the presence of other divalent cations, Mg 2+-ATPase being more sensitive to Cd 2+ than Ca 2+-ATPase. The inhibition of Mg 2+-ATPase was competitive at 10°C and 4°C but not at higher temperatures. 6. 6. Some of the toxic effects of Cd 2+ on the Anodonta gill ATPase at ecological temperatures are probably due to competition for Mg 2+ binding sites.