MOUSE NEUROBLASTOMA CELL ADENYLATE CYCLASE: REGULATION BY 2-CHLOROADENOSINE, PROSTAGLANDIN E1 AND THE CATIONS Mg2+, Ca2+ AND Mn

—Some basic kinetic properties of adenylate cyclase in cell free preparations of mouse neuroblastoma were investigated. Production of cAMP from ATP by the enzyme requires the presence of either Mg²⁺ or Mn²⁺ in addition to ATP. In the presence of Mg²⁺, the K_m for ATP is 120 ± 15 μM and the interaction of ATP and adenylate cyclase appears to be non-cooperative (Hill coefficient of 1). Magnesium ion concentrations in excess of the ATP concentration cause stimulation although similar excess concentrations of Mn²⁺ cause inhibition. Prostaglandin E₁ and 2-chloroadenosine activate the enzyme. The K_m of the cyclase for 2-chloroadenosine is 6 μm . Activation by 2-chloroadenosine leads to an increase in V_max but does not effect the K_m for ATP. At a fixed ATP concentration, the extent of activation caused by prostaglandin E₁ and 2-chloroadenosine is inversely related to the Mg²⁺ concentration. Calcium ion causes inhibition of adenylate cyclase from 0.1 to 4mM with a K_i of 5 ± 10^?4m . Ca²⁺ interaction with the enzyme in the absence or presence of either 2-chloroadenosine or prostaglandin E₁ appears cooperative (i.e. Hill coefficients of ?2). Ca²⁺ inhibition is non-competitive with respect to either ATP or 2-chloroadenosine but is progressively diminished by increasing Mn²⁺ concentrations. Divalent cation effects and activation by 2-chloroadenosine and prostaglandin E₁ of the neuroblastoma adenylate cyclase are compared with ion effects and hormone activation of the enzyme obtained from non-neuronal tissue.