Distinct characteristics of the basal activities of adenylyl cyclases 2 and 6

Regulation of basal activities of adenylyl cyclase (AC) 2 and 6, expressed in Sf9 cells by infection with recombinant baculovirus, was studied. An antipeptide antibody that recognizes AC2 and AC6 with equal sensitivity was used to establish that equivalent levels were expressed. Basal activities of AC2 and AC6 were compared at varying concentrations of Mg²⁺ or Mn²⁺ ions; AC2 had 15- and 10-fold greater activity than AC6, respectively. At 20 mM Mg²⁺, the K(m) values for ATP were 88 and 39 μM for AC2 and AC6, respectively, whereas their V(max) values were 281 and 11 pmol/mg protein·min. With 100 μM forskolin and either Mg²⁺ or Mn²⁺, the difference in activities between AC2 and AC6 was reduced to approximately 2- fold. Forskolin stimulated AC6 greater than 40-fold at 0.5-2 mM Mg²⁺, whereas AC2 was stimulated 4-6-fold. At 20 mM Mg²⁺, AC2 was stimulated 2- fold by forskolin, whereas AC6 was stimulated 18-fold. With Mg²⁺ alone, activities of AC2 and AC6 were not saturable up to 20 mM and yielded curvilinear Hofstee transformations. With forskolin, activities of both AC2 and AC6 were saturable by 10 mM Mg²⁺ and yielded linear Hofstee transformations. These data indicate that there are substantial differences in the basal enzymatic activities of adenylyl cyclase isoforms, due to differential regulation by Mg²⁺ ions rather than intrinsic catalytic capabilities. Thus the presence and relative abundance of adenylyl cyclase subtypes could greatly affect the resting cellular cAMP levels with consequent effects on important biological functions, such as differentiation and proliferation.