Relationship between Ca-transport and ATP hydrolytic activities in guinea-pig pancreatic acinar plasma membranes.

Partially purified plasma membrane fractions were prepared from guinea-pig pancreatic acini. These membrane preparations were found to contain an ATP-dependent Ca-transporter as well as a heterogenous ATP-hydrolytic activity. The Ca-transporter showed high affinity for Ca (K = 0.04 ± 0.01 μM), an apparent requirement for Mg and high substrate specificity. The major component of ATPase activity could be stimulated by either Ca or Mg but showed a low affinity for these cations. At low concentrations, Mg appeared to inhibit the Ca-dependent ATPase activity expressed by these membranes. However, in the presence of high Mg concentration (0.5-1 mM), a high affinity Ca-dependent ATPase activity was observed (K = 0.08 ± 0.02 μM). The hydrolytic activity showed little specificity towards ATP. Neither the Ca-transport nor high affinity Ca-ATPase activity were stimulated by calmodulin. The results demonstrate, in addition to a low affinity Ca (or Mg)-ATPase activity, the presence of both a high affinity Ca-pump and high affinity Ca-dependent ATPase. However, the high affinity Ca-ATPase activity does not appear to be the biochemical expression of the Ca-pump. [ABSTRACT FROM AUTHOR]