two ca2+-dependent atpase components were found in a plasma membrane fraction : a high affinity component ( half-saturation constant for ca2+ of 280 nm , vmax of 13.5 nmol/mg per min ) and a low affinity component , which was in reality a divalent cation atpase , since mg2+ could replace ca2+ without loss of activity .
C0001476
ca2
C0001473
atpase