2' Derivatives of guanosine and inosine cyclic 3',5'-phosphates. Synthesis, enzymic activity, and the effect of 8-substituents.

A series of representative derivatives of guanosine cyclic 3',5'-phosphate (cGMP) and inosine cyclic 3',5'-phosphate (cIMP) which contained modifications in either the 2' position or the 8 and 2' positions were synthesized. Three types of derivatives were investigated: (1) derivatives in which the 2' position has been altered to produce a 2'-deoxynucleoside cyclic 3',5'-phosphate or a 9-beta-D-arabinofuranosylpurine cyclic 3',5'-phosphate; (2) 2'-omicron-acyl derivatives; and (3) doubly modified derivatives containing a 2' modification [as in (1) and (2)] and an 8-substitution. 2'-Deoxyinosine cyclic 3',5'-phosphate and 9-beta-D-arabinofuranosylhypoxanthine cyclic 3',5'-phosphate were obtained by HNO2 deamination of 2'-deoxyadenosine cyclic 3',5'-phosphate and 9-beta-D-arabinofuranosyladenine cyclic 3',5'-phosphate (ara-cAMP), respectively. Treatment of 8-bromo-2'-omicron-(p-toluenesulfonyl) adenosine cyclic 3',5'-phosphate with NaSH yielded the intermediate 8,2'-anhydro-9-beta-D-arabinofuranosyl-8-mercaptoadenine cyclic 3',5-phosphate, which was converted directly to 2'-deoxyadenosine cyclic 3',5'-phosphate (dcAMP) by treatment with Raney nickel. 8-Bromo-2'-omicron-(p-toluenesulfonyl) guanosine cyclic 3',5'-phosphate was converted to 8,2'-anhydro-9-beta-D-arabinofuranosyl-8-mercaptoguanine cyclic 3',5'-phosphate, and the latter was desulfurized with Raney nickel to give 2-deoxyguanosine cyclic 3',5'-phosphate. Ara-cAMP, 9-beta-D-arabinofuranosylguanine cyclic 3',5'-phosphate, and 9-beta-D-arabinofuranosyl-8-mercaptoguanine cyclic 3',5'-phosphate have been previously reported (Mian et al. (1974), J. Med. Chem. 17, 259). 8-Bromo-2'-omicron-acetylinosine cyclic 3',5'-phosphate and 8-[(p-chlorophenyl)thio]-2'-omicron-acetylinosine cyclic 3',5'-phosphate were produced by acylation of 8-bromoinosine cyclic 3',5'-phosphate and 8-[(p-chlorophenyl)thio]inosine cyclic 3',5'-phosphate, respectively; while 8-bromo-2'-omicron-butyrylguanosine cyclic 3',5'-phosphate was synthesized by bromination of 2'-omicron-butyrylguanosine cyclic 3',5'-phosphate.     

The degradation of ribonucleic acid in the cotyledons of Pisum arvense

1. A ribonuclease has been partially purified from the cotyledons of germinating seed of Pisum arvense. 2. The enzyme degrades ribopolynucleotides to adenosine 3'-phosphate, guanosine 3'-phosphate and the cyclic nucleotides cytidine 2',3'-phosphate and uridine 2',3'-phosphate; no resistant ;core' remains. 3. The activity of RNA-degrading enzymes in the cotyledons increases to a maximum during the first 5 days of germination, passes through a minimum around the eighth day, and thereafter increases again. 4. Ion-exchange chromatography of methanol-soluble extracts of cotyledons revealed the presence, amongst other components, of the 2'-, 3'- and 5'-phosphates of cytidine and uridine, the 3'- and 5'-phosphates of adenosine, and guanosine 5'-phosphate. 5. Seed soaked in a solution containing [(32)P]orthophosphate gave a methanol-soluble fraction containing labelled nucleoside 5'-phosphates, but nucleoside 2'- and 3'-phosphates were not labelled. 6. It is believed that the nucleoside 2'- and 3'-phosphates arise by the action of ribonuclease on cotyledon RNA.     

THE DISTRIBUTION OF CYCLIC NUCLEOTIDE PHOSPHODIESTERASE IN THE HYPOTHALAMUS

Abstract— Quantitative measurements of capacities of microgram-sized samples of tissue to hydrolyze adenosine 3',5'-phosphate (cyclic AMP) and guanosine 3',5'-phosphate (cyclic GMP) have been made in 32 discrete regions of the rabbit hypothalamus. The capacities to hydrolyze the two cyclic nucleotides were found to be distributed over a 3-fold range and to vary in parallel from region to region. Notable exceptions, however, were observed. A positive association was found between the capacity to hydrolyze cyclic AMP and the reported density of catecholaminergic innervation. No clear relationship was observed between the distributions of the cyclic nucleotide phosphodiesterase activities and that of other enzymes and neurotransmitters.     

Kinetic studies on degradation of nucleotides catalyzed by phosphodiesterase-phosphomonoesterase from Fusarium moniliforme

Degradation of the 2'-phosphates, 3'-phosphates, 5'-phosphates, 2':3'-cyclic phosphates, 3':5'-cyclic phosphates, and 5'-(p-nitrophenylphosphates) of adenosine, guanosine, cytidine, and uridine catalyzed by Fusarium phosphodiesterase-phosphomonoesterase was followed by means of high performance liquid chromatography. All the nucleotides were susceptible to the enzyme to a greater or lesser degree, and the kinetic constants, Km and kcat, were determined at pH 5.3 and 37 degrees C. These constants were affected by both the nucleoside moiety and the position of the phosphate. Judged from kcat/Km, the 3'-phosphates, 2':3'-cyclic phosphates, and 5'-(p-nitrophenylphosphates) were good substrates, whereas the 2'-phosphates, 5'-phosphates, and 3':5'-cyclic phosphates were poor substrates except for adenosine 2'-phosphate, adenosine 5'-phosphate, and cytidine 5'-phosphate, which were hydrolyzed relatively easily. Among the phosphodiesters, the 2':3'-cyclic phosphates of adenosine, guanosine, and cytidine; and the 3':5'-cyclic phosphates of adenosine and cytidine were degraded into nucleoside and inorganic phosphate without release of intermediary phosphomonoester into the medium. Other phosphodiesters were degraded stepwise releasing definite intermediates.     

3',5'-Cyclic nucleotide phosphodiesterase activity of the sulphatase A of ox liver

The sulphatase A (aryl-sulphate sulphohydrolase, EC 3.1.6.1) of ox liver hydrolyses adenosine 3',5'-monophosphate (cyclic AMP) to adenosine 5'-phosphate at an optimum pH of approx. 4.3, close that for the hydrolysis of cerebroside sulphate, a physiological substrate for sulphatase A. The Km is 11.6 mM for cyclic AMP. On polyacrylamide gel electrophoresis sulphatase A migrates as a single protein band which coincides with both the arylsulphatase and phosphodiesterase activities, suggesting that these are due to a single protein. Cyclic AMP competitively inhibits the arylsulphatase activity of sulphatase A, showing that both activities are associated with a single active site on the enzyme. sulphatase A also hydrolyses guanosine 3',5'-monophosphate, but not uridine 3',5'-monophosphate nor adenosine 2',3'-monophosphate.     

Formation of Adenosine Cyclic 3′,5′-Phosphate by Nonproliferating Cells and Cell-free Extract of Brevibacterium liquefaciens

The formation of adenosine cyclic 3',5'-phosphate by Brevibacterium liquefaciens ATCC 14929 was studied with the use of nonproliferating cells and cell-free extract. With nonproliferating cells provided by deprivation of sulfate, the formation of this nucleotide was accelerated by adding some amino acids and sugars. Among amino acids tested, alanine and asparagine were most effective. Pentoses were more favorable than hexoses and other sugars. Formation of adenosine cyclic 3',5'-phosphate was observed also with chloramphenicol-treated cells. Experiments on cell-free extract showed that addition of alanine or pyruvate stimulated the formation of adenosine cyclic 3',5'-phosphate from adenosine-5'-triphosphate. When alanine was added to the cell-free system, shaking of the reaction mixture further increased the amount of the nucleotide, but pyruvate was far more effective than alanine. No synergistic effect of alanine and pyruvate was observed. Some enzyme activity was observed which decomposed adenosine cyclic 3',5'-phosphate, but it was weak as compared with adenyl cyclase activity in the presence of pyruvate. From the results obtained, it appears that pyruvate may act as an activating factor of adenyl cyclase in Brevibacterium liquefaciens.     

Interaction of "aza" and "deaza" analogs of adenosine cyclic 3', 5'-phosphate with some enzymes of adenosine cyclic 3', 5'-phosphate metabolism: evidence that the lone pair electrons of N-3 are involved in the binding of adenosine cyclic 3', 5'-phosphate to type II adenosine cyclic 3', 5'-phosphate-dependent protein kinase.

Five hetercyclic analogs of adenosine cyclic 3',5'-phosphate (cyclic AMP) were examined for their ability (1) to stimulate type II cyclic AMP-dependent kinases from bovine brain, bovine heart, and rat liver; (2) to serve as substrates for "high Km" (Km for cyclic AMP = 0.13-0.43 mM) cyclic nucleotide phosphodiesterases from bovine heart, rabbit kidney, and rat liver; and (3) to inhibit the hydrolysis of cyclic AMP catalyzed by "low Km" (Km for cAMP = 0.32-1.5 muM) cyclic nucleotide phosphodiesterases from bovine brain, bovine heart, dog heart, rabbit liver, rat brain and rat liver. The analogs all had a purine ring system which had been modified by replacement of a ring carbon with nitrogen or vice versa to yield 2-aza-cAMP (7-amino-4-beta-D-ribofuranosylimidazo [4,5-d] -v-triazine cyclic 3',5'-phosphate); 8-aza-cAMP (7-amino-3-beta-D-ribofuranosyl-v-triazolo-[4,5-d]-pyrimidine cyclic 3',5'-phosphate); 1 deaza-cAMP (7-amino-3-beta-D-ribofuranosylimidazo [4,5-b[pyridine cyclic 3',5'-phosphate); 3-deaza-cAMP (4-amino-1-beta-D-ribofuranosylimidazo[4,5-c]pyridine cyclic 3',5'-phosphate) and 7-deaza-cAMP (7-amino-4-beta-D-ribofuranosylpyrrolo[2,3-d]pyrimidine cyclic 3',5'-phosphate).     

Cyclic adenosine 3':5'-monophosphate and the induction of deoxyribonucleic acid synthesis in liver.

A mixture containing glucagon and thyroid hormone was previously devised that enhances markedly nuclear DNA replication and mitosis in the parenchymal liver cells of the unoperated rat. It is now shown that the glucagon of the stimulatory solution can be completely replaced by a mixture of a butyryl derivative of cyclic adenosine 3':5'-monophosphate and theophylline. Cyclic guanosine 3':5'-monophosphate and its butyryl derivatives and insulin and high levels of glucose are inactive. The inactivity of N2-monobutyryl cyclic guanosine 3':5'-monophosphate cannot be ascribed to rapid breakdown in the animal or to the impenetrability of the liver cell since the coumpound elevates the rate of hepatic amino acid transport and the activity of ornithine decarboxylase. The observation of others (MacManus, J.P., Franks, D.J., Youdale, T. & Braceland, B.M. (1972) Biochem. Biophys. Res. Commun. 49, 1201-1207) that the level of cylcic adenosine 3':5'-monophosphate is raised during most of the prereplicative period after 70% hepatectomy is confirmed. The evidence supports a positive role for adenosine 3':5-monophosphate in regulating DNA synthesis in the liver.     

Induction of phosphodiesterase by cyclic adenosine 3':5'-monophosphate in differentiating Dictyostelium discoideum amoebae.

Cyclic adenosine 3':5'-monophosphate added to the starvation media of Dictyostelium discoideum amoebae induces both intracellular and extracellular phosphodiesterase activities of these cells. The induced enzyme activity appears several hours earlier than that in starved cells which have not been induced with cyclic nucleotide. In both cases, the appearance of enzyme is inhibited by cycloheximide, and actinomycin D, and daunomycin. The KmS for the extracellular enzyme(s) of nucleotide-induced and uninduced control cells are identical. The induction of enzyme activity seems specific for cyclic adenosine 3':5'-monophosphate since cyclic guanosine 3':5'-monophosphate, as well as other nucleotides, have no effect. No differences in the activity or excretion of either N-acetylglucosaminidase or the inhibitory of the extracellular phosphodiesterase are observed between cyclic adenosine 3':5'-monophosphate-induced and control cells. A direct activation of phosphodiesterase by cyclic adenosine 3':5'-monophosphate can be excluded, since the addition of this nucleotide to cell lysates has no effect on the enzyme activity.     

Inhibition of endoribonuclease VI from Artemia larvae by cytidine 2'-phosphate

The endoribonuclease VI from Artemia larvae is non-competitively inhibited by cytidine 2'-phosphate with a Ki ca 1 microM. Neither of the cytidine monophosphates isomers with the phosphate group in the 3' or 5' position nor the cyclic 2':3' phosphate are inhibitors at concentrations up to 100 microM. Adenosine, guanosine and uridine 2' or 3' phosphates are also ineffective in this range of concentrations. Certain polyribonucleotides are potent competitive inhibitors of the ribonuclease activity.     

Elevation of guanosine 3',5'-monophosphate level by adenosine in cerebellar slices of guinea pig.

Effect of adenosine on the level of guanosine 3',5'-monophosphate in guinea pig cerebellar slices was investigated. Adenosine increased the concentration of guanosine 3',5'-monophosphate in the slices 3--4 fold. Upon removal of adenosine from the medium, the concentration of guanosine 3',5'-monophosphate returned to the initial level. AMP, ADP or ATP also increased the guanosine 3',5'-monophosphate level to the same extent as adenosine, while adenine or other nucleosides were not effective. In the absence of Ca2+ in the incubation medium, adenosine did not increase the concentration of guanosine 3',5'-monophosphate in cerebellar slices although level of adenosine 3',5'-monophosphate was elevated by adenosine. Anticholinergic agents, adrenergic blocking agents or antihistaminics did not prevent the increase of guanosine 3',5'-monophosphate by adenosine indicating that the effect of adenosine was not mediated by the release of neurotransmitters. The combination of adenosine with depolarizing agents showed an additive effect on the level of guanosine 3',5'-monophosphate indicating that adenosine increased the level of guanosine 3',5'-monophosphate by a different mechanism from the depolarization.     

Intracellular Levels of Cyclic Nucleotides during Cell Enlargement and Cell Division in Excised Tobacco Pith Tissues

Two lines of evidence, one of which is based on the radioimmunoassay and the other on adenosine 3', 5'-cyclic monophosphate (cyclic AMP)-dependent histone phosphorylation, are presented to demonstrate the presence of cyclic AMP in excised tissues of higher plant species. Intracellular levels of this cyclic nucleotide appear to be determined by auxin and a positive correlation has been found to exist between cell enlargement and chromosomal DNA replication, both auxin-dependent processes, and the level of cyclic AMP in tobacco pith cells. Intracellular guanosine 3', 5'-cyclic monophosphate (cyclic GMP) levels, while measurable, did not appear to be affected by either auxin or kinetin, or both, during the cell enlargement or cell division phases of the cell cycle in the tobacco pith system.     

Scanning calorimetric study of the thermal unfolding of catabolite activator protein from Escherichia coli in the absence and presence of cyclic mononucleotides

The thermal unfolding of the catabolite activator protein (CAP) of Escherichia coli and the complexes it forms with adenosine cyclic 3',5'-phosphate (cAMP) and guanosine cyclic 3',5'-phosphate (cGMP) was studied by high-sensitivity differential scanning calorimetry (DSC). The thermal denaturation of CAP at pH 7.00 gave an irreversible, symmetrical denaturation curve with a single peak. Distinctly different, more complex DSC curves were obtained for the thermal denaturation of the cAMP-protein and cGMP-protein complexes. The DSC data indicate intermolecular cooperation among CAP dimers, with the extent of oligomerization remaining unchanged during unfolding of the protein. The DSC curves for the thermal denaturation of the cAMP-protein complex and cGMP-protein complex have been resolved into three and two components, respectively, according to the model of independent two-state processes. Analysis of the DSC data suggests two and three independent domains for cGMP-protein and cAMP-protein complexes, respectively, with dissociation of mononucleotide occurring in the second component in both cases during protein denaturation. Furthermore, our studies indicate that the presence of either ligand alters the degree of oligomerization of CAP dimers, cAMP having a greater effect than cGMP.     

Properties of enzyme fraction A from Chlorella and copurification of 3' (2'), 5'-biphosphonucleoside 3' (2')-phosphohydrolase, adenosine 5'phosphosulfate sulfohydrolase and adenosine-5'-phosphosulfate cyclase activities.

Enzyme fraction A from Chlorella which catalyzes the formation of adenosine 5'-phosphosulfate from adenosine 3'-phosphate 5'-phosphosulfate is further characterized. Fraction A is found to contain an Mg2+ -activated and Ca2+ -inhibited 3' (2')-nucleotidase specific for 3' (2'), 5'-biphosphonucleosides. This activity has been named 3' (2), 5'-biphosphonucleoside 3' (2')-phosphohydrolase. The A fraction is also found to contain an activity which catalyzes the formation of adenosine 3':5'-monophosphate (cyclic AMP) from adenosine 5'-phosphosulfate (adenosine 5'-phosphosulfate cyclase). Under the same conditions of assay, 5'-ATP and 5'-ADP are not substrated for cyclic AMP formation. Unlike the 3' (2'), 5'-biphosphonucleoside 3' (2')-phosphohydrolase activity, the adenosine 5'-phosphosulfate cyclase activity does not require Mg2+, requires NH+4 or Na+, and is not inhibited by Ca2+. The A fraction also contains an adenosine 5'-phospho sulfate sulfohydrolase activity which forms 5'-AMP and sulfate. The three activities remain together during purification and acrylamide gel electrophoresis of the purified preparation yields a pattern where only one protein band has all three activities. The phosphohydrolase can be separated from the other two activities by affinity chromatography on agarose-hexyl-adenosine 3'n5'-bisphosphate yielding a phosphohydrolase preparation showing a single band on gel electrophoresis. The adenosine 5'-phosphosulfate cyclase may provide an alternate route of cyclic AMP formation from sulfate via ATP sulfurylase, but its regulatory significance in Chlorella, if any, remains to be demonstrated. In sulfate reduction, the phosphohydrolase may serve to provide a readily utilized pool of adenosine 5'-phosphosulfate as needed by the adenosine 5'-phosphosulfate sulfotransferase. The cyclase and sulfohydrolase activities would be regarded as side reactions incidental to this pathway, but may be of importance in other metabolic and regulatory reactions.     

Hyperkalemia, hyperglycemia, and plasma levels of cyclic AMP and cyclic GMP induced by portal vein injection of cyclic nucleotides and their butyryl derivatives into dogs

The levels of serum potassium, blood glucose, and plasma adenosine cyclic 3':5'-monophosphate (cAMP) and guanosine cyclic 3':5'-monophosphate (cGMP) were studied after the portal vein injection of cyclic nucleotides and their derivatives, (cAMP, cGMP, N6, O2'-dibutyryl adenosine 3':5'-monophosphate (DBcAMP), N6-monobutyryl adenosine cyclic 3':5'-monophosphate (NMBcAMP), and O2'-monobutyryl adenosine cyclic 3':5'-monophosphate (OMBcAMP), into dogs. Dose-related hyperglycemic responses were observed after the injection of DBcAMP (1-8 mg/kg). Transient and prominent hyperkalemia and hyperglycemia were caused by the injection of DBcAMP, NMBcAMP, and OMBcAMP (4 mg/kg). The hyperkalemic response was highest with NMBcAMP (1.22 mequiv./L), followed by OMBcAMP (0.64), DBcAMP (0.54), cGMP (0.47), and cAMP (0.41), whereas the hyperglycemic response was highest with NMBcAMP (146 mg/100 mL), followed by DBcAMP (93.6), OMBcAMP (77.1), and cAMP (56.0), and there was only a slight change with cGMP (28.4) compared with the control. The plasma level of cAMP was maximal with DBcAMP (1.92 nmol/mL), followed by NMBcAMP (1.28) and OMBcAMP (0.76), whereas the plasma levels of cGMP showed no evident change, except that caused by DBcAMP (0.27). Of the cyclic nucleotides tested, NMBcAMP was found to be most potent in causing both hyperkalemia and hyperglycemia. Based on these results, possible correlations between hyperkalemia, hyperglycemia, and plasma levels of cAMP and cGMP are discussed.     

A specific cyclic guanosine 3':5'-monophosphate-binding protein in Caulobacter crescentus.

A binding protein specific for cyclic guanosine 3':5'-monophosphate (cyclic GMP) has been partially purified from extracts of the eubacterium Caulobacter crescentus and resolved from cyclic adenosine 3':5'-monophosphate (cyclic AMP)-binding activity. Binding of cyclic GMP is not affected by the addition of cyclic AMP or 5'-GMP, but is inhibited about 50 percent by a 50-fold molar excess of dibutyryl cyclic GMP or cyclic hypoxanthine 3':5'-monophosphate. The apparent dissociation constant for the cyclic GMP-binding protein complex is 1.1 X 10(-6) M.     

Increase of cyclic GMP induced in murine thymocytes by thymosin fraction 5

Using a radioimmunoassay (RIA) for the determination of adenosine 3'5' cyclic monophosphate (cAMP) and an acetylation-RIA procedure to measure guanosine 3'5' cyclic monophosphate (cGMP), we observed that cGMP levels, but not cAMP levels, were significantly elevated in murine thymocytes which had been incubated with preparations containing the thymic hormone, thymosin. Stimulation of intracellular cGMP levels was seen as early as 1 minute after incubation with thymosin fraction 5 and was maximal at approximately 10 minutes. Dose response studies indicated an optimum stimulation of cGMP with a thymosin concentration of 100 microg/ml. A control spleen fraction prepared by an identical procedure as fraction 5 did not affect the levels of either cyclic nucleotide.     

Cyclic nucleotides and calcium in human lymphocytes induced to divide

Intracellular concentrations of cyclic adenosine 3'-5' monophosphate (cAMP) and cyclic guanosine 3'-5' monophosphate (cGMP) were measured in human lymphocytes induced to divide by the addition of lectins, 12-O-tetra-decanoylphorbol-13-acetate (TPA) and the calcium ionophore A 23187. cGMP levels rose within minutes without concomitant alterations in cAMP concentration. The cAMP and cGMP levels rose during the prereplicative and replicative phases respectively. Under calcium depleting conditions, both the fluctuations in cyclic nucleotide levels and the increase in [3H[ thymidine incorporation into DNA were abolished, suggesting a role for calcium ions in the regulation of lymphocyte proliferation.     

Poly(A) polymerase from Escherichia coli adenylylates the 3'-hydroxyl residue of nucleosides, nucleoside 5'-phosphates and nucleoside(5')oligophospho(5')nucleosides (NpnN).

The capacity of Escherichia coli poly(A) polymerase to adenylylate the 3'-OH residue of a variety of nucleosides, nucleoside 5'-phosphates and dinucleotides of the type nucleoside(5')oligophospho(5')nucleoside is described here for the first time. Using micromolar concentrations of [alpha-32P]ATP, the following nucleosides/nucleotides were found to be substrates of the reaction: guanosine, AMP, CMP, GMP, IMP, GDP, CTP, dGTP, GTP, XTP, adenosine(5')diphospho(5')adenosine (Ap2A), adenosine (5')triphospho(5')adenosine (Ap3A), adenosine(5')tetraphospho(5')adenosine (Ap4A), adenosine(5')pentaphospho(5')adenosine (Ap5A), guanosine(5')diphospho(5') guanosine (Gp2G), guanosine(5')triphospho(5')guanosine (Gp3G), guanosine(5')tetraphospho(5')guanosine (Gp4G), and guanosine(5')pentaphospho(5')guanosine (Gp5G). The synthesized products were analysed by TLC or HPLC and characterized by their UV spectra, and by treatment with alkaline phosphatase and snake venom phosphodiesterase. The presence of 1 mM GMP inhibited competitively the polyadenylylation of tRNA. We hypothesize that the type of methods used to measure polyadenylation of RNA is the reason why this novel property of E. coli poly(A) polymerase has not been observed previously.     

Phosphorylation in sealed rod outer segments: effects of cyclic nucleotides

Rod outer segments (ROS) from rat were purified on Percoll gradients. These ROS had intact plasma membranes since they were impermeable to small molecules. Protein phosphorylation in the purified ROS was studied after the plasma membrane was disrupted by freeze/thawing. [gamma-32P]ATP was used as phosphate donor. ATP concentration, time, temperature, and light or dark adaptation were varied in the assays. The 32P-labeled proteins were separated by polyacrylamide gel electrophoresis and autoradiographed. Rhodopsin was the dominant phosphorylated protein, and the addition of adenosine cyclic 3',5'-phosphate (cAMP) or guanosine cyclic 3',5'-phosphate (cGMP) (10(-4) M) did not qualitatively alter the ROS phosphorylation pattern. The only cyclic nucleotide effect we could establish in these experiments was the inhibition of rhodopsin phosphorylation by cGMP. This inhibition did not appear to be competitive with ATP since cAMP was much less inhibitory than cGMP and the phosphorylation in the presence of cGMP reached a plateau at a much lower level than in control conditions. Hypotheses implying an involvement of protein phosphorylation/dephosphorylation in dark adaptation have been formulated [Miller, J. A., & Paulsen, R. (1975) J. Biol. Chem. 250, 4427-4432; Kuhn, H., McDowell, J. H., Leser, K. H., & Bader, S. (1977) Biophys. Struct. Mech. 3, 175-180]; we suggest that cGMP may control this process through the modulation of the extent of inhibition of phosphorylation of the visual pigment.