Role of guanine nucleotides in the stimulation of thyroid adenylate cyclase by prostaglandin E 1 and cholera toxin

Abstract

Cholera toxin in the presence of GTP increased adenylate cyclase activity in a purified bovine thyroid plasma membrane preparation, whereas, in the presence of guanosine 5′-(β, γ-imido)-triphosphate (Gpp(NH)[), cholera toxin had no stimulatory effect. Similarly, prostaglandin E 1 enhanced the adenylate cyclase activity induced by GTP but not by Gpp(NH)p. Gpp(NH)p-stimulated adenylate cyclase activity, assayed with hydrolysis-resistant adenosine 5′-(β, γ-imido)-[ 32P]triphosphate as substrate and no ATP-regenerating system was inhibited by GDP in a competitive fashion. Furthermore, prostaglandin E 1, but not cholera toxin, influenced the GDP inhibition of Gpp(NH)p-stimulated activity by increasing the concentration of GDP resulting in 50% inhibition approx. 2-fold. Inosyl nucleotides mimicked the effects of guanyl nucleotides on thyroid adenylate cyclase in that ITP could substitute for GTP in enhancing cholera toxin- and prostaglandin E 1-induced activities and that inosine 5′-(β, γ-imido)-triphosphate[Ipp(NH)p] was also a potent stimulator per se. Conclusions. (1) Cholera Toxin and prostaglandin E 1 enhance thyroid adenylate cyclase activation by GTP (or ITP), but have no stimulatory effect on the Gpp(NH)p (or Ipp(NH)p) response; (2) the stimulatory effect of prostaglandin E 1 on adenylate cyclase may result from decreased affinity for GDP at the guanine nucleotide regulatory site; (3) the data regarding cholera toxin stimulation of thyroid adenylate cyclase are consistent with the hypothesis that cholera toxin exerts its effect by inhibiting an endogenous GTPase.