Other Properties and Coupling of Ca2+ Transport Andatp Hydrolysis

The selectivity of the Ca²⁺ pump from plasma membranes for Ca²⁺ is not so high, since other divalent metal ions can replace Ca²⁺ in the transport cycle. In 1969, Schatzmann and Vincenzi and Alson and Cazort found that in resealed ghosts, Sr²⁺ is transported out in the same way as Ca²⁺. The effects of divalent cations other than Ca²⁺ have also been studied on the Ca²⁺- ATPase activity. Hydrolysis of ATP by a purified preparation of the enzyme from human red blood cell membranes is stimulated by alkaline metal earth cations in the order Ca²⁺ > Sr²⁺ ⪡ Ba²⁺. The high apparent affinity of the transport site for Ca²⁺ is independent of ligands like ATP and monovalent cations that participate in the transport cycle, but it can be modified by several substances and treatments. The number of high-affinity sites for Ca²⁺, present on each pump unit, is not known with certainty yet.