Evidence for vasopressin activation of adenylate cyclase by subunit dissociation

Radiation inactivation is used to probe the sequence of subunit interactions involved in the activation of adenylate cyclase by vasopressin in cultured renal epithelial cells (LLC-PK₁) based on our previous analysis of the radiation inactivation of multimeric enzymes. For basal adenylate cyclase activity, a concave downward ln(activity) vs. dose relation was observed with limiting slope corresponding to a molecular weight of (169-196) x 10³. Similar results were obtained with NaF. In contrast, addition of vasopressin, guanylyl imidodiphosphate, or forskolin resulted in transition to a linear ln(activity) vs. dose relation with a slope corresponding to a molecular weight similar to that observed for basal activity. These findings were incorporated into a cyclic dissociation model for the hormonal activation of adenylate cyclase where H is hormone, R is receptor, C is catalytic unit, α and β are subunits of guanyl nucleotide-regulatory protein (G), GTP is guanosine triphosphate, and GDP is guanosine diphosphate. The addition of H favors the dissociation of G into α and β subunits by providing a rapid pathway for addition of GTP to dissociated α subunits. The observed target size of the active enzyme of α(GTP) with C. This model consolidates the radiation inactivation findings as well as the known biochemical characteristics for adenylate cyclase.