Search

In the present study we have demonstrated specific binding of 3H-labeled adenosine 3',5'-cyclic monophosphate (cAMP) to a nuclear extract from rat liver. GTP, GDP, and low concentrations of ATP and ADP increased nuclear binding of [3H]cAMP, and AMP inhibited [3H]cAMP binding. Photoaffinity labeling studies employing [32P]cAMP revealed four nuclear binding proteins [relative molecular weight (Mr) 36,000, 49,000, 54,000 and 57,000]. Unlabeled cAMP decreased [32P]cAMP binding to all four proteins, whereas GTP increased binding to the 57,000 protein. We also observed specific binding of [3H]cAMP in the liver cytosol, which was stimulated by GTP but not by ADP or ATP. Photoaffinity labeling studies of the cytosol in the absence of unlabeled nucleotides revealed three cAMP-binding proteins (Mr 36,000, 49,000, and 54,000). Unlabeled cAMP inhibited binding of [32P]cAMP to all three proteins, whereas in the presence of GTP there was binding of [32P]cAMP to a Mr 57,000 protein. Using DEAE-cellulose, we isolated from the nuclear extract and cytosol a cAMP-binding protein that responded to GTP with an increase in cAMP binding but was unaffected by GDP, ATP, ADP, and AMP. Guanosine imidodiphosphate did not affect cAMP binding, suggesting that the stimulatory effect of GTP may be mediated by phosphorylation. We speculate that alterations in intracellular GTP in vivo may modulate the binding of cAMP to a protein in the nucleus and cytosol.

GTP, in physiologic concentration (10(-4) mol/L), enhances cAMP binding to an Mr 57,000 binding protein (BP) in hepatic cytosol, which probably is the phosphorylated receptor subunit of protein kinase II (PK II). When we attempted to separate PK II from other hepatic cytosol proteins by DEAE-cellulose chromatography, we observed that GTP caused little stimulation of [3H]cAMP binding in an eluate fraction (110 to 170 mmol/L KCI), which was rich in PK II but did stimulate cAMP binding to the 210 to 325 mmol/L KCI fraction, which also contains PK II. This suggested that the latter fraction might contain a cofactor necessary for GTP stimulation of cAMP binding, which was lacking in the 110 to 170 mmol/L KCI fraction. Cyclic AMP BP in the 210 to 325 mmol/L fraction was removed by absorption onto cAMP agarose in the presence of 325 mmol/L KCI. When an aliquot of the BP-poor fraction, containing the putative cofactor, was added to the 110 to 170 mmol/L fraction containing PK II, the addition of 10(-4) mol/L GTP to the mixture increased [3H]cAMP binding by more than 80%. Cofactor activity could be extracted from the 210 to 325 mmol/L eluate by adsorption onto cAMP agarose in the presence of 10 mmol/L K phosphate, and eluted with 100 mmol/L KCI, suggesting that the cofactor may bind to the cAMP BP under appropriate circumstances. Addition of this eluted cofactor fraction to the 110 to 170 mmol/L fraction in the presence of 10(-4) mol/L GTP, increased the specific binding of [3H] cAMP more than twofold. Pretreatment of the cofactor fraction with trypsin eliminated this effect.(ABSTRACT TRUNCATED AT 250 WORDS)

GTP, in physiologic concentration, enhances the binding of cAMP to a protein in the hepatic cytosol that may be the regulatory subunit of protein kinase II. Ingestion of carbohydrate suppresses hepatic gluconeogenesis and glycogenolysis, two processes that are stimulated by cAMP. In this study, we have examined the possibility that carbohydrate inhibits these processes partly by decreasing the sensitivity of the GTP-responsive cAMP-binding protein to the effect of GTP. We found that 100 muM GTP was much less effective in enhancing cAMP binding in the hepatic cytosol of rats given 15% glucose for 2 days than in the cytosol of fasted rats [21 +/- 3% (mean +/- SE) increase vs. 67 +/- 6%, P less than .01]. Corresponding results were noted in diethylaminoethyl (DEAE)-cellulose extracts of the hepatic cytosol of these rats. GTP stimulation of cAMP binding was also diminished in the hepatic cytosol of diabetic rats treated for 7 days with insulin compared with that of untreated diabetic rats (29 +/- 10 vs. 81 +/- 11% increase, P less than .01), but this could have been due to increased food intake in the treated rats. We conclude that GTP stimulation of hepatic cAMP binding is decreased in the carbohydrate-fed state and that this effect may be mediated by the increase in plasma insulin induced by carbohydrate. Our observations suggest that some of the cellular effects of cAMP may be regulated by modulation of the stimulatory effect of GTP on the GTP-responsive cAMP-binding protein.