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

Abstract —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 Mg 2+ or Mn 2+ in addition to ATP. In the presence of Mg 2+ , 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 Mn 2+ cause inhibition. Prostaglandin E 1 and 2‐chloroadenosine activate the enzyme. The K m of the cyclase for 2‐chloroadenosine is 6 μ m . 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 E 1 and 2‐chloroadenosine is inversely related to the Mg 2+ concentration. Calcium ion causes inhibition of adenylate cyclase from 0.1 to 4mM with a K i of 5 ± 10 −4 m . Ca 2+ interaction with the enzyme in the absence or presence of either 2‐chloroadenosine or prostaglandin E 1 appears cooperative (i.e. Hill coefficients of ˜2). Ca 2+ inhibition is non‐competitive with respect to either ATP or 2‐chloroadenosine but is progressively diminished by increasing Mn 2+ concentrations. Divalent cation effects and activation by 2‐chloroadenosine and prostaglandin E 1 of the neuroblastoma adenylate cyclase are compared with ion effects and hormone activation of the enzyme obtained from non‐neuronal tissue.