Tautomerism in cytidine

The large change in the electronic absorption spectrum of cytidine on going from an aqueous to an aprotic medium is attributed to the existence of a hydrogen bonded solvent-solute complex in solvents capable of donating a proton. The spectrum of cytidine in a variety of solvents and the spectra of a number of nontautomerising model compounds are presented. The tautomerisin of cytidine and its biological implications are discussed.     

Presence of cytidine 5'-tetraphosphate in commerical samples of cytidine 5'-triphosphate

A contaminant compound has been isolated from commercial samples of CTP by ion-exchange chromatography on a Dowex-1 column. It has been characterized as cytidine 5'-tetraphosphate from its ultraviolet spectrum, labile and total phosphate content, and periodate consumption. It is present in proportions from 0.3 to 3.9%, apparently regardless of the method of preparation, age of sample, or commericial source of CTP.     

Purification and characterization of cytidine 5''-triphosphate:cytidine 5''-monophosphate-3-deoxy-D-manno-octulosonate cytidylyltransferase

Cytidine 5'-triphosphate:cytidine 5'-monophosphate-3-deoxy-D-manno-octulosonate cytidylyltransferase (CMP-KDO synthetase) was purified 2,300-fold from frozen Escherichia coli B cells. The enzyme catalyzed the formation of CMP-KDO, a very labile product, from CTP and KDO. No other sugar tested could replace KDO as an alternate substrate. Uridine 5'-triphosphate at pH 9.5 and deoxycytidine 5'-triphosphate at pH 8.0 and 9.5 could be used as alternate substrates in place of CTP. CMP-KDO synthetase required Mg2+ at a concentration of 10.0 mM for optimal activity. The pH optimum was determined to be between 9.6 and 9.3 in tris(hydroxymethyl)aminomethane-acetate or sodium-glycine buffer. This enzyme had an isoelectric point between pH 4.15 and 4.4 and appeared to be a single polypeptide chain with a molecular weight of 36,000 to 40,000. The apparent Km values for CTP and KDO in the presence of 10.0 mM Mg2+ were determined to be 2.0 X 10(-4) and 2.9 X 10(-4) M, respectively, at pH 9.5. Uridine 5'-triphosphate and deoxycytidine 5'-triphosphate had apparent Km values of 8.8 X 10(-4) and 3.4 X 10(-4) M. respectively, at pH 9.5.     

Rare earth metal complexes of cytidine

The interactions of La(III), Pr(III), Nd(III), Sm(III), Gd(III), Dy(III) and Er(III) and cytidine with glycine, histidine and oxalic acid for the formation of binary (1:1) and ternary complexes (1:1:1) have been investigated by potentiometric equilibrium measurements at 35 °C and 0.10 mol dm⁻³ (KNO₃) ionic strength. These investigations were undertaken to assess the influences of charge on the structure and stability of metal nucleoside complexes in solution. Cytidine forms more stable complexes with trivalent lanthanones compared to bivalent transition metal ions. This is explained on the basis of the differences in the charge of the metal ions concerned. The ternary complexes of these systems are more stable than the corresponding binary complexes. This enhanced stability is measured in terms of ΔlogK (difference between the stability of overall (1:1:1) and binary (1:1)). Based on the trends in ΔlogK values, various factors that affect the stability of these complexes have been explained.     

Enzymatic synthesis of cytidine diphosphate diglyceride

Evidence is presented for the enzymatic formation of cytidine diphosphate diglyceride in microsomal preparations from guinea pig liver according to the reaction: CTP + phosphatidic acid right harpoon over left harpoon CDP-diglyceride + p-O-P. Conditions have been found in which the incorporation of labeled CTP into CDP-diglyceride is almost entirely dependent upon added phosphatidic acid. The incorporation of CMP into lipid is very slight. A substantial net synthesis of CDP-diglyceride takes place under these conditions. Some properties of the enzyme system are described.     

The transport of cytidine into rat brain in vivo,and its conversion into cytidine metabolites

Double-labeled cytidine, with a³H/¹⁴C isotope ratio of 20.00, has been intraventricularly injected into the brain of young rats, and its fate followed up to 90 min from administration together with the time-course of labeling. The injected nucleoside enters the brain as an intact molecule and is immediately utilized without prior degradation. Cytidine is actively converted into uridine and CMP, the latter being then transformed by a stepwise mechanism into CDP and CTP, and finally into CDP-choline and CDP-ethanolamine. The results indicate that administered cytidine represents a compound likely to enter metabolic events, which lead to CDP-choline and CDP-ethanolamine synthesis, and presumably to phospholipid production.     

Advantages and disadvantages of cytidine deamination

Cytidine deamination of nucleic acids underlies diversification of Ig genes and inhibition of retroviral infection, and thus, it would appear to be vital to host defense. The host defense properties of cytidine deamination require two distinct but homologous cytidine deaminases-activation-induced cytidine deaminase and apolipoprotein B-editing cytidine deaminase, subunit 3G. Although cytidine deamination has clear benefits, it might well have biological costs. Uncontrolled cytidine deamination might generate misfolded polypeptides, dominant-negative proteins, or mutations in tumor suppressor genes, and thus contribute to tumor formation. How cytidine deaminases target a given nucleic acid substrate at specific sequences is not understood, and what protects cells from uncontrolled mutagenesis is not known. In this paper, I shall review the functions and regulation of activation-induced cytidine deaminase and apolipoprotein B-editing cytidine deaminase, subunit 3G, and speculate about the basis for site specificity vis-à-vis generalized mutagenesis.     

Intersubunit interactions in human cytidine deaminase

In order to design new efficient cytidine based drugs, an intersubunit interactions study related to the active site has been performed on the wild-type cytidine deaminase (CDA) and on the mutant enzyme F137W/W113F. F137 is the homologous to the Bacillus subtilis CDA F125 involved in the subunit interactions. In presence of the dissociating agent SDS, wild-type human CDA dissociate into enzymatically inactive monomers without intermediate forms via a non-cooperative transition. Extensive dialysis or dilution of the inactivated monomers restores completely the activity. The presence of the strong human CDA competitive inhibitor 5-fluorozebularine disfavour dissociation of the tetramer into subunits in the wild-type CDA but not in mutant enzyme F137W/W113F.