CADMIUM ION IS A NONCOMPETITIVE INHIBITOR OF RED-CELL CA2+-ATPASE ACTIVITY

In the presence as well as in the absence of calmodulin, Cd2+ inhibits the human erythrocyte plasma membrane Ca2+-ATPase activity non-competitively with K(i) = 2 nM, whereas ATP-dependent Ca2+-transport across the red cell membrane was found to be inhibited competitively by Cd2+ (Verbost, P.M., Flik, G., Pang, P.K.T., Lock, R.A.C. and Wendelaar Bonga, S.E. (1989) J. Biol. Chem. 264, 5613-5615). In this study it will be argued that Cd2+ also inhibits Ca2+-transport non-competitively, and that the discrepancy with previous conclusions most probably relies on use of an incorrect computer program that calculates the free concentrations of Ca2+ and Cd2+ at the experimental conditions applied for measurement of Ca2+ uptake.