Interactions of fluoride and guanine nucleotides with thyroid adenylate cyclase

The activation of bovine thyroid adenylate cyclase (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1) by Gpp(NH)p has been studied using steady-state kinetic methods. This activation is complex and may be characterized by two Gpp(NH)p binding sites of different affinities with measured constants: Ka1 = 0.1 micro M and Ka2 = 2.9 micro M. GDP beta S does not completely inhibit the Gpp(NH)p activation: analysis of the data is consistent with a single GDP beta S inhibitory site which is competitive with the weaker Gpp(NH)p site. Guanine nucleotide effects upon F- activation of adenylate cyclase have been studied. When App(NH)p is the substrate, 10 micro M GTP along with 10 mM NaF gives higher activity than NaF alone, while GDP together with NaF inhibits the activity by 50% relative to NaF. These features are not observed when the complex is assayed with ATP in the presence of a nucleotide regenerating system or when analogs Gpp)NH)p or GDP beta S are used along with NaF. These effects were studied in three other membrane systems using App(NH)p as substrate: rat liver, rat ovary and turkey erythrocyte. No consistent pattern of guanine nucleotide effects upon fluoride activation could be observed in the different membrane preparations. Previous experiments showed that the size of soluble thyroid adenylate cyclase changed whether membranes were preincubated with Gpp(NH)p or NaF. This size change roughly corresponded to the molecular weight of the nucleotide regulatory protein. This finding, coupled with the present data, suggests that two guanine nucleotide binding sites may be involved in regulating thyroid cyclase and that these sites may be on different protein chains.