A proton-relaxation-time study of the conformation of some purine and pyrimidine 5'-nucleotides in aqueous solution

The measurement of proton relaxation time T1 in a series of purine and pyrimidine 5'-nucleotides has been carried out to investigate their conformation in dilute neutral aqueous solutions. The interpretation of relaxation data has been performed with the help of computer calculations taking into account the different conformers of the ribose ring and of the exocyclic group. It has been found that all nucleotides under study show nearly the same preferential orientations of the base defined by gamma0 = 70 degrees +/- 10 degrees in the syn range. A more elaborate treatment, using an angular distribution derived from calculated potentials on 5" -GMP gives theoretical relaxation times in close agreement with experimental ones.

Biochemical properties of cyclic nucleotide phosphodiesterase in metastasizing and nonmetastasizing rat mammary carcinomas

The biochemical properties of cyclic nucleotide phosphodiesterases in a nonmetastasizing and a spontaneously metastasizing rat mammary carcinoma were compared. The phosphooiesterases in both tumors had a pH optimum of around 8.0 and preferentially hydrolysed cyclic purine nucleotides. The rate of hydrolysis of purine nucleotides in the nonmetastasizing tumor was two times higher than in the metastasizing tumor, but the rate of pyrimidine nucleotide hydrolysis was equal in both tumors. Theophylline, caffeine, and D,L-4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro20-1724) inhibited the enzyme activity in both tumors; the percent inhibition was the same by each inhibitor. The cyclic nucleotie phosphodiesterase activity in either tumor was stimulated by Mg++, Mn++, and Co++ and suppressed by Ca++, Zn,++, and Ni++. EDTA inhibited the activity below the basal level (activity in the absence of added cation), an this inhibition could be recovered up to the basal level by an equimolar quantity of either Mn++ or Mg++. Further stimulation of the enzyme activity with increasing concentrations of divalent cations was observed only with Mn++. Similar effects were observe with ethylene glycol bis(beta-aminoethyl ether)-tn,n-tetraacetic acid. The stimulatory cations affected both the low and high Michaelis constant (tkm) enzymes in these tumors by increasing the maximum velocity. In the low Km enzyme, the Km was also slightly increased. Neither guanosine 3',5'-cyclic monophosphate nor adenosine 3',5'-cyclic monophosphate had any effect on the hydrolysis of the other at physiologic levels.

Affinity labelling to - SH groups in adenosine - triphosphate - phosphoribosyl transferase with the dinitrophenyl group from S-dinitrophenyl-6-mercaptopurine-riboside 5'-phosphate

Adenosine-triphosphate-phosphoribosyl transferase from Escherichia coli reacts with S-dinitrophenyl-6-mercaptopurine-riboside 5'-phosphate. In this reaction the dinitrophenyl group becomes attached to the enzyme, while the nucleotide is split off. Most aliphatic high and low-molecular-weight-SH compounds react with the thioether in the opposite way, i.e. bind the nucleotide and split off dinitrothiophenol. It appears that the dinitrophenyl moiety of the thioether interacts with the enzyme in a specific way, and that this interaction activates the bond between the dinitrophenyl group and the sulfur atom. In support of this it was found that dinitrophenol inhibits the transferase reaction with half maximal effect at 0.4 mM. The inhibition is competitive with ATP. Dinitrophenol also competes with ATP in binding studies.

Nuclear magnetic resonance studies of 2'- and 3'-ribonucleotide structures in solution

A systematic 220-MHz proton nuclear magnetic resonance (nmr) study has been made of all common purine and pyrimidine 2'(3')-ribonucleotides in D2O solutions at 20 plus or minus 2 degrees. Spectra for the entire series were measured under similar conditions of concentration, temperature, and ionic strength, thereby facilitating intercomparisons of spectral properties. Spectral assignments were accomplished with the aid of selected 31P-1H decoupling experiments and accurate values of nmr parameters were derived by simulation-iteration procedures. A detailed analysis of the coupling constants and chemical shifts permitted a determination of conformational properties for ribose rings, exocyclic carbinol and phosphate groups, and the orientation of base-ribose rings. Following procedures described elsewhere, an evaluation was made of ribose ring pseudorotational parameters for each 2'(3')-nucleotide. The results show that both the degree of pucker and pseudorotational angle vary only slightly throughout the entire series of molecules, and lie within the ranges found in the crystalline state. Furthermore, the ribose rings are in rapid equilibrium between N type (C(3')-ENDO, C(2')-exo) and S type (C(2')-endo, C(3')-exo) conformers, N forms and is formed by S, with an S type conformer favored in purine 2'(3')-ribonucleotides (-60:40) while pyrimidines exhibit approximately equal compositions. Thus, the phosphate location on the ring has less of an effect on ring properties than the nature of the base ring. An analysis is also reported of rotamer equilibria about C(4')-C(5'), C(2')-O(2'), and C(3')-O(3') bonds. For the former the nmr coupling constant data are consistent with a predominant gg rotamer (-70%) with gt and tg rotamers populated to the extent of -20 and -10%, respectively. No correlation of the type seen for 5'-nucleotides appears to exist between C(4')-C(5') gg population and ribose ring equilibrium composition. For 2'-nucleotides the 31P-H(2') coupling data indicate a preferred C(3')g, C(1')t conformer about C(2')-O(2') in agreement with 13C nmr results. A less definitive rotamer analysis follows from observed J31P,H(3') values, but when these results are combined with relevant chemical shift data for deoxynucleosides and nucleotides the evidence strongly points to essentially free rotation and approximately equal rotamer populations about C(3')-O(3'). Chemical shift differences between purine and pyrimidine 2'(3')-ribonucleotides are qualitatively accounted for by "in-plane" purine diamagnetic anisotropy effects. Also, the greater magnitude for purine deshieldings in 2'(3')-nucleotides relative to 5'-nucleotides is explained by a more favored syn:anti ratio in the former in line with recent nucle

Breaking action of ascorbic acid on nucleic acids

The lowering of the viscosity of DNA solution was caused by the action of AsA or EA and facilitated in the presence of CU2+. However, the action of AsA-3-P was very weak. By sucrose density gradient centrifugation, it was observed that single- and double-strand scissions of DNA were provoked by AsA or EA and enhanced with Cu2+, while only a single-strand scission was caused by AsA-3-P and Cu2+. Similar action of AsA or AsA-3-P was also observed for RNA. Thus, the result indicates that the enediol group of AsA takes an essential part in the breakage of nucleic acids, and Cu2+ enhances the action. It was shown that Apu was mainly decomposed by AsA, whereas Apy not, suggesting that some pyrimidine cluster may be one of the regions attacked by AsA. During the reaction with DNA, the reducing activity of AsA decreased first to some extent and then increased, whereas the reducing activity of AsA-3-P was much lower than that of AsA and decreased steadily. The priming activity of DNA for DNA polymerase was changed after treatment with AsA according to the condition. It was enhanced when DNA was treated under mild conditions but decreased with severer action.