Metal and metal-ATP interactions with brain and cardiac adenylate cyclases

Metal (Me) and MeATP interactions with adenylate cyclases associated with rabbit ventricular particles and with a detergent-dispersed preparation from rat cerebellum have been studied. data were simulated to fit kinetic models in which an inhibitor (HATP or ATP) is added in constant proportion to the variable substrate (MeATP). The specific models considered were that the enzyme binds (a) MeATP as the substrate; (b) MeATP as the substrate and HATP or ATP as an inhibitor; (c) MeATP as the substrate and free Me as an activator; and (d) MeATP as the substrate, free Me as an activator, and HATP or ATP as an inhibitor. Both equilibrium-ordered and random (rapid equilibrium assumption) types of sequential kinetic models were considered. The various models were tested using cardiac particulate adenylate cyclase in the presence of either a phosphoenolpyruvate-pyruvate kinase or a creatine phosphate-creatine kinase ATP-regeneration system. Although the enzyme with either system appeared to bind Mg2+ as an activator, one or both ATP-regeneration systems also seemed to interact directly with adenylate cyclase, making clear interpretations difficult. With the phosphoenolpyruvate-pyruvate kinase system, kinetic patterns on double reciprocal plots were linear as a function of MgATP, but with creatine phosphate-creatine kinase, kinetic patterns were concave downward. The kinetic models were further tested using the detergent-dispersed cerebellar enzyme, a preparation with low adenosine triphosphatase activity and not requiring the addition of an ATP-regeneration system. Reciprocal plots were linear and intersecting as a function of either MeATP or Me (Me = Mg2+ or Mn2+), and secondary replots of slopes and intersecting as function of either MeATP or Me (Me = Mg2+ or Mn2+), and secondary replots of slopes and intercepts also were linear. These data indicate that the brain detergent-dispersed enzyme conforms to a bireactant, sequential mechanism where free cation is a required activator and free ATP is not a potent inhibitor.