Pancreatic plasma membranes containing a high adenylate cyclase activity and a low contamination by cytochrome c oxidase were isolated from the rat by sucrose density centrifugation. The preparation contained an (Mg,Ca)‐ATPase of high activity with the following characteristics.
The ATPase activity was shown to have two apparent Km values for Mg‐ATP (0.24 ± 0.09 mM and 1.15 ± 0.21 mM) and two apparent Km values for Ca‐ATP (0.14 ± 0.09 mM and 0.68 ± 0.10 mM). Mg‐GTP and Ca‐GTP were also hydrolysed by the preparation. The phase transition temperature was 19.3 ± 1.0°C for the Mg‐ATPase and 22.6 ± 1.1°C for the Ca‐ATPase activities.
Three lines of evidence suggest that Mg‐ATP and Ca‐ATP were substrates for the same enzyme: Mg‐dependent and Ca‐dependent activities were not additive; the two activities showed the same pH optimum at 8.0; and the nonionic detergents Triton X‐100, Triton X‐305, Triton N‐101, Lubrol P 12 A, and digitonin, produced a parallel solubilization of the two activities.
Enzyme activities were insensitive to potassium, sodium, ouabain, pancreozymin, carbamoylcholine, secretin, concanavalin A, wheat germ agglutinin, and soybean lectin.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.