Kinetic properties of "soluble" adenylyl cyclase. Synergism between calcium and bicarbonate.

"Soluble" adenylyl cyclase (sAC) is a widely expressed source of cAMP in mammalian cells that is evolutionarily, structurally, and biochemically distinct from the G protein-responsive transmembrane adenylyl cyclases. In contrast to transmembrane adenylyl cyclases, sAC is insensitive to heterotrimeric G protein regulation and forskolin stimulation and is uniquely modulated by bicarbonate ions. Here we present the first report detailing kinetic analysis and biochemical properties of purified recombinant sAC. We confirm that bicarbonate regulation is conserved among mammalian sAC orthologs and demonstrate that bicarbonate stimulation is consistent with an increase in the V(max) of the enzyme with little effect on the apparent K(m) for substrate, ATP-Mg(2+). Bicarbonate can further increase sAC activity by relieving substrate inhibition. We also identify calcium as a direct modulator of sAC activity. In contrast to bicarbonate, calcium stimulates sAC activity by decreasing its apparent K(m) for ATP-Mg(2+). Because of their different mechanisms, calcium and bicarbonate synergistically activate sAC; therefore, small changes of either calcium or bicarbonate will lead to significant changes in cellular cAMP levels.