Substrate interactions with brain (Ca + Mg)-ATPase.

ATP hydrolysis by a partially purified (Ca + Mg)-ATPase preparation from rat brain increased with substrate concentration in a biphasic fashion, with apparent Km values of 3 microM and 0.1 mM. Ca-dependent phosphorylation, however, had only a single Km value, 3 microM. KCl increased ATPase activity in both concentration ranges, but the K0.5 for KCl decreased from 7 mM to 0.3 mM as the ATP concentration was reduced from 1 mM to 10 microM. The K0.5 for MgCl2 decreased somewhat less, from 3 mM to 0.6 mM with ATP concentrations from 1 mM to 1 microM, but was far lower for steady-state phosphorylation, 0.03 mM. (Ca + Mg)-dependent hydrolysis was not demonstrable with other nucleotide triphosphates or p-nitrophenyl phosphate, and these substances, as well as a reaction product, Pi, were also inhibitors. On the other hand, ADP inhibited at both ATP concentration ranges, and also stimulated dephosphorylation. This pattern of responses to substrate and cations is reminiscent of that of well-characterized transport ATPases, suggesting similar roles and mechanisms.