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Chymotryptic activation of adenylate cyclase from rat heart was examined with respect to guanosine triphosphate (GTP), adenosine triphosphate (ATP) and Mg2+ concentrations. The activation was inhibited by aprotinin. In the absence or presence of GTP, chymotrypsin stimulated the cyclase at the same ratio. GTP exerted stimulatory (up to 0.1 mM) and inhibitory (at 1 mM) effects on the cyclase. Both effects were not affected by chymotrypsin. ATP at low concentrations increased the cyclase activity dose dependently and at high concentrations decreased the cyclase activity. Chymotrypsin retarded the appearance of the decreasing phase. This effect of chymotrypsin was similar to that of increasing the concentration of Mg2+ in the reaction mixture. According to double-reciprocal plots of Mg2+ concentrations and the cyclase activity, chymotrypsin diminished Km for Mg2+ with little effect on the Vmax. Hill plots of the same data showed that the Hill coefficient was about 1 and was not altered by chymotrypsin. These results suggest that chymotryptic activation of adenylate cyclase is mainly due to increasing the affinity for Mg2+ of the system.

Increase in Mg2+ affinity as a possible mechanism of chymotryptic activation of adenylate cyclase in rat heart membranes.