Calcium and sodium transport processes in patients with cystic fibrosis. I. A specific decrease in Mg2+-dependent, Ca2+-adenosine triphosphatase activity in erythrocyte membranes from cystic fibrosis patients.

Calcium-ATPase activity (Mg2+-dependent Ca2+-ATPase, ATP phosphohydrolase, EC 3.6.1.3) in erythrocyte membrane preparations from cystic fibrosis (CF) patients was greatly reduced compared to erythrocyte membranes from control subjects. The Km for calcium was found to be similar in the two groups; however, the Vmax, the maximal rate of activation of the Ca2+-ATPase, is reduced by 50% in the erythrocyte membrane preparations of the CF patients (P less than 0.001). In contrast, the Mg2+-ATPase activity of erythrocyte membranes from CF patients was unchanged compared to the control subjects. No difference in the Na+,K+-ATPase activity in erythrocyte membranes from CF patients compared to control patients could be observed. This indicates that the Ca2+-ATPase activity noted in CF erythrocytes is not part of a generalized membrane or membrane-bound enzyme alteration. It remains to be determined whether this alteration in Ca2+-ATPase activity is directly related to a defect in calcium transport in these cells and is a generalized phenomenon in CF present in cell types more directly involved in secretion.