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 apparentK_m values of 3 M and 0.1 mM. Ca-dependent phosphorylation, however, had only a singleK_m value, 3 M. KCl increased ATPase activity in both concentration ranges, but theK_0.5 for KCl decreased from 7 mM to 0.3 mM as the ATP concentration was reduced from 1 mM to 10 M. TheK_0.5 for MgCl₂ decreased somewhat less, from 3 mM to 0.6 mM with ATP concentrations from 1 mM to 1 M, 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, P_i, 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.