Secretin and VIP-stimulated adenylate cyclase from rat heart. I. General properties and structural requirements for enzyme activation.

Membrane adenylate cyclase from rat heart was activated by the two gut peptides secretin and vasoactive intestinal peptide (VIP), glucagon, and the beta-adrenergic drug isoproterenol, in the presence of guanosine 5'-triphosphate (GTP). With all the stimuli tested, the optimal magnesium concentration was 5 mM, i.e. in excess over the 0.5 mM ATP substrate concentration and 0.01 mM GTP used as cofactor. Under these conditions, half-maximal adenylate cyclase activation with glucagon, secretin, and VIP was achieved at concentrations of 0.5, 0.5 and 1.0 microM, respectively. Data obtained with the secretin (7--27) fragment, a secretin antagonist, indicate that secretin and VIP acted on the same binding sites, which differed from glucagon binding sites. Structural requirements for secretin activation of cardiac adenylate cyclase were evaluated by comparing the potency and efficacy of parent peptides and synthetic analogs. The gastric inhibitory peptide GIP was inactive. When using 13 mono-or bi-substituted analogs, it appeared that amino acids in positions 1, 2, 3, 4 and 6 were of major importance while those in position 5 and 11 played a relatively minor role.