Two soluble forms of guanosine 5'-(beta,gamma-imino)triphosphate and fluoride-activated adenylate cyclase.

Soluble and membrane-bound adenylate cyclase from the canine renal medulla are very slowly activated by guanosine 5'-(beta,gamma-imino)triphosphate (Gpp(NH)p) or fluoride. At 25 degrees, 8 to 10 h of incubation of the soluble enzyme, and 6 to 8 h of incubation of the membrane-bound enzyme are required to reach the maximal activity. The dependence of activation on concentration of Gpp(NH)p changes with time. Half-maximal activation of soluble adenylate cyclase occurs at 3 +/- 1 X 10(-6) M Gpp(NH)p if the activity is measured without preincubation with Gpp(NH)pp and at 4 +/- 2 X 10(-8) M if it is measured after 6 to 22 h of incubation with Gpp(NH)p at 25 degrees. The activation occurs over a rather broad range of nucleotide concentration and cannot, therefore, be simply interpreted to reflect a function of nucleotide binding. Two forms of soluble adenylate cyclase are resolved by Sepharose gel filtration. One form, with a Stokes radius of 71 A, is rapidly activated by Gpp(NH)p; the other, with a Stokes radius of 56 A, requires long incubation with Gpp(NH)p to be activated. The sedimentation coefficient of the two forms is 7.3 S. The apparent molecular weight of the rapidly activated form is 200,000 while that of the slowly activated form is 160,000. The interrelationship of these two species of adenylate cyclase is not clear and is under investigation.