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. 