Radioimmunoassay of cytidine 3',5' monophosphate (cCMP). I. Development of the assay

Medicinal Chemistry of the Noncanonical Cyclic Nucleotides cCMP and cUMP

After decades of intensive research on adenosine-3',5'-cyclic monophosphate (cAMP)- and guanosine-3',5'-cyclic monophosphate (cGMP)-related second messenger systems, also the noncanonical congeners cyclic cytidine-3',5'-monophosphate (cCMP) and cyclic uridine-3',5'-monophosphate (cUMP) gained more and more interest. Until the late 1980s, only a small number of cCMP and cUMP analogs with sometimes undefined purities had been described. Moreover, most of these compounds had been rather synthesized as precursors of antitumor and antiviral nucleoside-5'-monophosphates and hence had not been tested for any second messenger activity. Along with the recurring interest in cCMP- and cUMP-related signaling in the early 2000s, it became evident that well-characterized small molecule analogs with reliable purities would serve as highly valuable tools for the evaluation of a putative second messenger role of cyclic pyrimidine nucleotides. Meanwhile, for this purpose new cCMP and cUMP derivatives have been developed, and already known analogs have been resynthesized and highly purified. This chapter summarizes early medicinal chemistry work on cCMP and cUMP and analogs thereof, followed by a description of recent synthetic developments and an outlook on potential future directions.

Radioimmunoassay of cytidine 3',5'-cyclic monophosphate: unambiguous assay by means of an optimized protocol incorporating a trilayer column separation to obviate cross-reactivity problems

Previous assays for cytidine 3', 5'-cyclic monophosphate (cyclic CMP) have been criticized as being ambiguous. Here a modified RIA protocol, in which the production of assay components has been optimized and a novel trilayer chromatography column separation introduced which successfully separates cyclic CMP from compounds, endogenous to living tissues, which cross-react with anti-cyclic CMP sera, is described. The assay is capable of assaying cyclic CMP between 0.1 and 5 pmol, can be increased in sensitivity by means of an additional acetylation step, and enables the separation of cyclic CMP, cyclic AMP and cyclic GMP so that all three can be estimated in a single sample.

Cytidylate cyclase: development of assay and determination of kinetic properties of a cytidine 3',5'-cyclic monophosphate-synthesizing enzyme

A method is described for the separation of cytidine 3',5'-cyclic monophosphate (cyclic CMP) from cytidine tri-, di- and mono-phosphates and from cytidine 3',5'-cyclic pyrophosphate, cytidine 2'-monophosphate-3',5'-cyclic monophosphate, cytidine 2'-O-aspartyl-3',5'-cyclic monophosphate and cytidine monophosphate, compounds previously shown to be the result of putative cytidylate cyclase activity. This separation, involving elution of a novel bilayer column of QAE-Sephadex and alumina with 0.03 M-HCl, has been incorporated into an assay protocol to determine the enzyme-catalysed conversion of radiolabelled CTP to cyclic CMP. By this assay, cytidylate cyclase activity has been shown to be present in rat lung, spleen, ovary, testes, brain, stomach, liver, heart and kidney preparations; the activity was of a similar order in each tissue and had a sharp pH optimum of 7.0-7.5. The liver preparation had a Vmax. of 1.2 nmol of cyclic CMP formed/min per mg, and a Km of 220 microM-CTP, and although active in the absence of added cations, it was stimulated by Fe2+ and Mn2+ ions. In several of the tissues examined, the cytidylate cyclase activity was inversely proportional to age of the animals.

Cytidylate cyclase activity in mouse tissues: the enzymatic conversion of cytidine 5'-triphosphate to cytidine 3',5'-cyclic monophosphate (cyclic CMP)

Cytidylate cyclase activity, which enzymatically converts cytidine 5'-triphosphate (CTP) to cytidine 3',5'-cyclic monophosphate (cyclic CMP), has been demonstrated in mouse tissue homogenates by use of a highly sensitive enzyme immunoassay (EIA) specific for cyclic CMP. Cyclic CMP formation is dependent on the amount of homogenate and on the incubation time. Although the enzyme activity was detected at wide ranges of pH from 6.8 to 11.5, the maximal activity was observed at around pH 9.4. The optimal temperature was 37 degrees C. Cytidylate cyclase activity was almost completely lost if the homogenates were heated at 90 degrees C for 3 min prior to use. The enzyme reaction exhibited typical Michaelis-Menten kinetics with an apparent Km for CTP of approx. 0.31 mM. Cyclic CMP formation was greatly enhanced with 4 mM Mn2+, Mg2+, Co2+; Mn2+ was the most effective. Fe2+ and Ca2+ were without effect. Cu2+ and Zn2+ at a concentration of 0.1 to 0.5 mM were inhibitory to Mn2+-dependent activity. Moreover, the enzyme activity was inhibited by several nucleotides including ATP, ADP, 5'-AMP, and GTP. Cytidylate cyclase activity was found to be present in all homogenates from a variety of mouse tissues examined except heart, with the highest level found in brain, and the lowest in liver.

Extraction, purification and identification of cytidine 3',5'-cyclic monophosphate from rat tissues

The large-scale extraction and purification to homogeneity of cyclic CMP and its unequivocal identification are described. Rat liver, kidney, heart, spleen and lung tissues were subjected to a sequential purification procedure involving freeze-clamping, perchlorate extraction, alumina and boronate column chromatography, polyacrylamide-gel column electrophoresis and high-voltage paper electrophoresis. The purified sample co-chromatographed with authentic cyclic CMP on t.l.c. and high-pressure liquid chromatography and was positive in a cyclic CMP radio-immunoassay. The u.v., i.r. and p.m.r. spectra were each essentially identical with those of authentic cyclic CMP. Fast-atom bombardment of authentic cyclic CMP yielded a mass spectrum containing a molecular protonated ion: mass-ion-kinetic-energy scanning of this ion produced a spectrum unique to 3',5'-cyclic CMP. The extracted nucleotide produced an identical mass-ion-kinetic-energy spectrum.

Enzyme immunoassay for cytidine 3',5'-cyclic monophosphate (cyclic CMP)

An enzyme immunoassay for cytidine 3',5'-cyclic monophosphate (cyclic CMP) is presented. This assay is based upon the principles of competitive reaction and the double antibody solid phase method. Succinyl cyclic CMP-human serum albumin conjugate was injected into rabbits. Specific anti-cyclic CMP antibodies were incubated with a mixture of succinyl cyclic CMP labeled with beta-D-galactosidase and standard or sample cyclic CMP that had been succinylated prior to assay. The antibody-bound beta-D-galactosidase-cyclic CMP conjugate was separated from that of free with a second antibody, anti-rabbit immunoglobulin G, that was immobilized to a polystyrene ball. Then, activity of the enzyme on the solid phase was fluorometrically determined. When cyclic CMP contents in biological materials were estimated, acid extracts were partially purified by Dowex 1 x 8 formate column chromatography. The present immunoassay allows the detection of as little as 0.5 fmol of cyclic CMP with practically no interference from other cyclic nucleotides and cytidine analogs. By use of the enzyme immunoassay technique, we determined the amounts of cyclic CMP in various tissues of rats. They were found to be as little as 0.24-0.51 pmol/g wet weight, which was roughly 3000 to 20,000 and 100 to 500 times less than those of cyclic AMP and cyclic GMP, respectively.

Differential effects of various phosphodiesterase inhibitors, pyrimidine and purine compounds, and inorganic phosphates on cyclic CMP, cyclic AMP and cyclic GMP phosphodiesterases

The effects of various compounds on homogeneous cyclic CMP phosphodiesterase (cyclic CMP-PDE) from pig liver were compared with the effects on cyclic AMP phosphodiesterase (cyclic AMP-PDE) and cyclic GMP phosphodiesterase (cyclic GMP-PDE). Of the conventional inhibitors for AMP-PDE and cyclic GMP-PDE, only Sch 15280 was found to inhibit cyclic CMP-PDE. Nucleoside monophosphates, orthophosphate, and 2':3'-cyclic nucleotides were rather specific and were more effective in inhibiting cyclic CMP-PDE, compared to their effects on cyclic AMP-PDE and cyclic GMP-PDE. On the other hand, nucleoside di-and triphosphates and pyrophosphate (PPi) were less effective in inhibiting cyclic CMP-PDE and were without marked effect on cyclic AMP-PDE and cyclic GMP-PDE. Orthophosphate (Pi) was more potent than CMP, CDP and CTP in inhibiting cyclic CMP-PDE, with a rank order of inhibitory potency of Pi greater than CMP greater than CDP greater than CTP. Of the 3' :5'-cyclic nucleotides examined, cyclic UMP was more specific in inhibiting cyclic CMP-PDE compared to its effect on cyclic AMP-PDE and cyclic GMP-PDE. In all experiments similar results were obtained when either cyclic CMP or cyclic AMP was used as a substrate for this multifunctional cyclic CMP-PDE, supporting the contention that a single catalytic site on the enzyme is responsible for the hydrolysis of both cyclic CMP and cyclic AMP. The present studies further support our original suggestion that cyclic CMP-PDE is a unique enzyme that is distinguishable from the conventional enzymes for purine cyclic nucleotides.

Investigations on the occurrence of cytidine 3',5' monophosphate (cCMP) in tissues

A specific and sensitive radioimmunoassay (RIA) for the detection of cCMP was developed. Antibodies were prepared by immunization of rabbits with succinyl cCMP conjugated to human albumin. In the RIA commercially available 3H-cCMP was used as labeled antigen. Low amounts (0.4-2.6 pmol/g w.wt.) of immunoreactive material could be detected in extracts of brain, heart, ileum, kidney, liver, lung, spleen, testis and uterus from rats, guinea-pig and frogs. The immunoassayable material in rat liver was subjected to column chromatography on AG 1-X8 anion exchange resin. The radioimmunoassayable material isolated from the liver did not cochromatograph with authentic tritiated or unlabeled cCMP. The chromatographic properties of the immunoreactive material did not change in regenerating livers - following partial hepatectomy, or after pronase treatment. However, after treatment with pronase the amount of assayable material, extracted from the rat liver, increased. It is suggested that cCMP - as such - is not present, in detectable amounts, in extracts prepared by commonly used procedures for isolation of cyclic nucleotides. The chemical nature of the cCMP immunoreactive material isolated from rat liver is at present unclear.