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PubMed1988

Deamination of methylamine by semicarbazide-sensitive amine oxidase in human umbilical artery and rat aorta.

The deamination of methylamine (MA) by amine oxidase enzymes has been studied and compared with that of benzylamine (BZ) in homogenates of rat aorta and human umbilical artery by means of a radiochemical assay to estimate the radiolabelled deaminated metabolites produced, and also a spectrophotometric assay to measure H2O2 formation during the metabolism of these substrates. The effects of various inhibitors used in these assays suggest that a semicarbazide-sensitive amine oxidase (SSAO) is predominantly if not wholly responsible for the deamination of both MA and BZ in these tissues. MA was found to have a relatively higher apparent Km (102 microM in aorta; 779 microM in umbilical artery) than BZ (6.8 microM in aorta; 207 microM in umbilical artery) for metabolism by SSAO in these tissues. However, these large differences between species in the apparent Km values for each amine indicate that the biochemical properties of SSAO in human and rat vasculature are not identical. SSAO in human umbilical artery was particularly active towards MA, with a Vmax which was approximately 70% greater than that for BZ as substrate, whereas in rat aorta the Vmax for MA was around 60% of that for BZ. MA is known to occur endogenously in man and other species, and the possibility that it may be a physiological substrate in vivo for SSAO is discussed.

Amine Oxidase (Copper-Containing)↗
PubMed1982

The role of dietary nitrate and nitrite in the reductive deamination of sulfadiazine by the rat, guinea pig, and neonatal calf.

Previous metabolic depletion studies of 14C-sulfadiazine (SDZ) in the neonatal calf led to identification of two novel metabolites, 2-benzenesulfonamidopyrimidine (desNH2SDZ) and 2-benzenesulfonamido-4-hydroxypyrimidine. The novelty of the biotransformation prompted examination of mechanisms for the reductive deamination of SDZ in vivo. In subsequent work, it was found that neonatal calves did not consistently convert SDZ to desNH2SDZ; however, calves that were treated simultaneously with nitrite did. Further, when SDZ was given orally to guinea pigs, whose diet is high in nitrate and who demonstrate the capacity to reduce nitrate to nitrite in the oral cavity, SDZ was transformed to desNH2SDZ. Rats did not reductively deaminate SDZ even if they consumed a diet high in nitrate for two weeks prior to treatment with SDZ. However, they did so when nitrite was added to their diet. These observations strongly suggest that reductive deamination of sulfonamides is dependent on the ingestion of nitrite or the reduction of dietary nitrate to nitrite. This reduction of nitrate to nitrite proceeds in the oral cavity, presumably via microflora residing there.

Animal Feed↗
PubMed1992

Differential in vivo and in vitro effect of gentamicin on glutamate synthesis and glutamate deamination in rabbit kidney-cortex tubules and mitochondria.

The effect of gentamicin on both glutamate synthesis and glutamate deamination was studied in kidney-cortex mitochondria and tubules isolated from both control and gentamicin-treated animals. In kidney-cortex mitochondria which were permeabilized in order to make a free access of substrates and antibiotic to the glutamate dehydrogenase, gentamicin appeared to be a very potent inhibitor of glutamate synthesis, resulting in about 60% decrease of the enzyme activity at 5 mM concentration. Other aminoglycoside antibiotics decreased the enzymatic activity, in the following order: gentamicin > neomycin = tobramycin = kanamycin > biodacyna > amikacin > streptomycin. This, in principle, corresponds to their known nephrotoxic potential observed in vivo. The inhibitory action of antibiotics was abolished by neither ADP nor leucine, allosteric activators of glutamate dehydrogenase. Surprisingly, gentamicin did not decrease the rate of ammonia formation from glutamate when added to both renal tubules and mitochondria isolated from control rabbits. This indicates that the antibiotic exerts its inhibitory effect on glutamate dehydrogenase activity in the direction of glutamate synthesis only. In contrast, the rate of both glutamate deamination and glutamate synthesis was about 40% lower in renal tubules and mitochondria isolated from kidney-cortex of animals which were given antibiotics for 10 days. In view of these results it seems that (i) the depression of ammoniagenesis in gentamicin-treated animals may be due to a decrease of glutamate dehydrogenase content and (ii) under conditions in vitro the aminoglycoside inhibits the enzyme activity in the direction of glutamate synthesis while it does not affect the glutamate deamination.

Adenosine Diphosphate↗
PubMed2006

Dehydration versus deamination of N-terminal glutamine in collision-induced dissociation of protonated peptides.

Some of the most prominent "neutral losses" in peptide ion fragmentation are the loss of ammonia and water from N-terminal glutamine. These processes are studied by electrospray ionization mass spectrometry in singly- and doubly-protonated peptide ions undergoing collision-induced dissociation in a triple quadrupole and in an ion trap instrument. For this study, four sets of peptides were synthesized: (1) QLLLPLLLK and similar peptides with K replaced by R, H, or L, and Q replaced by a number of amino acids, (2) QLnK (n = 0, 1, 3, 5, 7, 9, 11), (3) QLnR (n = 0, 1, 3, 5, 7, 9), and (4) QLn (n = 1, 2, 3, 4, 8). The results for QLLLPLLLK and QLLLPLLLR show that the singly protonated ions undergo loss of ammonia and to a smaller extent loss of water, whereas the doubly protonated ions undergo predominant loss of water. The fast fragmentation next to P (forming the y5 ion) occurs to a larger extent than the neutral losses from the singly protonated ions but much less than the water loss from the doubly protonated ions. The results from these and other peptides show that, in general, when N-terminal glutamine peptides have no "mobile protons", that is, the number of charges on the peptide is no greater than the number of basic amino acids (K, R, H), deamination is the predominant neutral loss fragmentation, but when mobile protons are present the predominant process is the loss of water. Both of these processes are faster than backbone fragmentation at the proline. These results are rationalized on the basis of resonance stabilization of the two types of five-membered ring products that would be formed in the neutral loss processes; the singly protonated ion yields the more stable neutral pyrrolidinone ring whereas the doubly protonated ion yields the protonated aminopyrroline ring (see Schemes). The generality of these trends is confirmed by analyzing an MS/MS spectra library of peptides derived from tryptic digests of yeast. In the absence of mobile protons, glutamine deamination is the most rapid neutral loss process. For peptides with mobile protons, dehydration from glutamine is far more rapid than from any other amino acid. Most strikingly, end terminal glutamine is by far the most labile source of neutral loss in excess-proton peptides, but not highly exceptional when mobile protons are not available. In addition, rates of deamination are faster in lysine versus arginine C-terminus peptides and 20 times faster in positively charged than negatively charged peptides, demonstrating that these formal neutral loss reactions are not "neutral reactions" but depend on charge state and stability.

Amino Acid Sequence↗
PubMed1998

Deamination of methylamine and aminoacetone increases aldehydes and oxidative stress in rats.

Semicarbazide-sensitive amine oxidase (SSAO)-mediated deamination of methylamine and aminoacetone in vitro produces carbonyl compounds, such as formaldehyde and methylglyoxal, which have been proposed to be cytotoxic and may be responsible for some pathological conditions. An HPLC procedure was developed to assess different aldehydes, which were derivatized with 2,4-dinitrophenylhydrazine (DNPH). We have demonstrated in vivo deamination of methylamine and aminoacetone by examining the excretion of formaldehyde and methylglyoxal, respectively, in rats. Following chronic administration of methylamine, the urinary level of malondialdehyde (MDA), an end product of lipid peroxidation, was also found to be substantially increased. A selective SSAO inhibitor blocked the increase of MDA. The results support the idea that increased SSAO-mediated deamination of methylamine and aminoacetone can be a potential cytotoxic risk factor.

Acetaldehyde↗
PubMed2001

Endonuclease V of Escherichia coli prevents mutations from nitrosative deamination during nitrate/nitrite respiration.

Endonuclease V (Endo V) of Escherichia coli participates in the excision repair of hypoxanthine and xanthine (deaminated adenine and guanine) in DNA. It thereby reduces the mutagenic effects of nitrous acid by attacking lesions caused by nitrosative deamination. Nitrosating agents may be produced endogenously when E. coli is grown in oxygen-poor cultures, during which nitrate and nitrite replace oxygen as preferred electron acceptors. In this study, the protective effect of Endo V was observed under such conditions. During micro-aerobic growth, an nfi (Endo V) mutation enhanced the frequency of nitrate- and nitrite-induced A:T-->G:C and G:C-->A:T transition mutations, which are consistent with a defect in the removal of DNA hypoxanthine and xanthine, respectively. Similar effects were observed in saturated, aerobic cultures but not in well-aerated, logarithmically growing ones. A narG (nitrate reductase) mutation blocked the mutagenesis of the nfi mutant by nitrate but not by nitrite. These results differed from those of previous studies in which cell suspensions generated an exogenous nitrosating agent from nitrite, but not from nitrate, in a reaction that was narG-dependent. Nitrate/nitrite metabolism is also known to generate endogenous alkylating agents through N-nitrosation. However, an nfi mutation did not appreciably enhance mutagenesis by N-methyl-N-nitrosourea, suggesting that the mutator effect of nfi is not due to a defect in alkylation repair. The overall results indicate that Endo V functions during normal growth by helping to repair nitrosatively deaminated bases in DNA, which are by-products of anaerobic nitrate/nitrite respiration.

Aerobiosis↗
PubMed2003

Immunity through DNA deamination.

Functional antibody genes assembled by V(D)J joining are subsequently diversified by somatic hypermutation, gene conversion and class-switch recombination. Recent evidence indicates that all three processes are caused by the deamination of cytosine to uracil at sites within the immunoglobulin (Ig) loci, with the pattern of diversification depending on the pathway used for resolving the initiating dU-dG lesion. Whereas DNA deamination targeted to the endogenous Ig locus triggers a program of somatic gene diversification that underpins adaptive immunity, deamination targeted to foreign DNA might have arisen initially as a form of innate immunity. Furthermore, the observation that members of the DNA deaminase family can target inappropriate genes suggests they might also contribute to mutations during genome evolution, as well as in cancer.

Animals↗
PubMed1999

A minor fraction of basic fibroblast growth factor mRNA is deaminated in Xenopus stage VI and matured oocytes.

Adenosine deaminases that act on RNA (ADARs) convert adenosine to inosine in double-stranded regions of RNA. ADAR activity is in the nucleus in Xenopus laevis stage VI oocytes, and released into the cytoplasm at oocyte maturation. We previously demonstrated that a cytoplasmic double-stranded RNA (dsRNA) binding factor(s), cyto-dsRBP, protects microinjected dsRNA from the ADAR released at maturation. Here we describe experiments to determine whether an endogenous dsRNA, the duplex formed between sense and antisense transcripts of basic fibroblast growth factor (bFGF), is protected in a similar manner. Consistent with the presence of cyto-dsRBP, we observed that the majority of bFGF RNA was not deaminated, before or after maturation. However, a minor fraction of the bFGF RNA was deaminated whether the RNA was isolated from stage VI oocytes or matured oocytes. Since ADAR activity is in the nucleus in stage VI oocytes, our results suggest that a fraction of the bFGF RNAs are hybridized in the nucleus and are ADAR substrates. Adenosine deaminations result in A-to-G changes in cDNAs, so we quantified the fraction of modified molecules using restriction-enzyme assays of RT-PCR products. Caveats due to recombination during RT-PCR are discussed.

Adenosine Deaminase↗
PubMed1993

Kinetic analysis of the deamination reactions of cyclobutane dimers of thymidylyl-3',5'-2'-deoxycytidine and 2'-deoxycytidylyl-3',5'-thymidine.

The cyclobutane dimer photoproducts of dTpdC and dCpdT have been produced by acetophenone photosensitization and separated by reverse-phase HPLC. Each dinucleoside monophosphate was shown to produce one cis,syn isomer and two trans,syn isomers. Three of these photoproducts, namely, the cis,syn isomers of dTpdC and dCpdT and one trans,syn isomer (the syn-anti glycosidic isomer) of dTpdC were selected to study the deamination kinetics. Analysis of the pH dependence indicates that the deamination proceeds by the hydrolysis of the imido amide group of the 5,6-saturated cytosine base with the formation of a carbinolamine intermediate. Determination of the kinetic parameters showed that, for these three cyclobutane dimers, the rate-determining step at physiological pH is a cyclobutane dimers, the rate-determining step at physiological pH is a nucleophilic attack of hydroxide ion on the protonated 5,6-saturated cytosine base. The kinetic analysis showed that the cis,syn isomers deaminate approximately 3 times faster than the trans,syn isomer, which is due to a large difference in pKa of the 5,6-saturated cytosine moiety. An electrostatic interaction between the iminium group of cytosine and the carbonyl group of thymine is proposed to account for the increase in pKa for the cis,syn isomers relative to the trans,syn isomer. A similar interaction is proposed to explain the relative difference in reactivity between the cis,syn isomers and the trans,syn isomer with regard to the breakdown of the carbinolamine intermediate.

Chromatography, High Pressure Liquid↗
PubMed1994

Bisulfite induces tandem double CC-->TT mutations in double-stranded DNA. 2. Kinetics of cytosine deamination.

Deamination of cytosine to uracil in double-stranded DNA (ds DNA) by sodium bisulfite has been monitored with a sensitive genetic assay. In this system, reversion of a mutant in the lacZ alpha gene coding sequence of bacteriophage M13mp2 C141 was detected by employing an ung- bacterial strain defective in the enzyme uracil glycosylase. Within the 4-base target, it is possible to measure the rates of induction of C-->T, C-->A, C-->G, and CC-->TT mutations in DNA that has been incubated at physiological temperature and pH and then transfected into ung+ and ung- E. coli cells, respectively, for amplification and detection of the mutation. For concentrations of bisulfite from 1 to 50 mM, the reversion frequency in ung- cells increased linearly with time of incubation. The most interesting features of the bisulfite reaction were as follow: (1) Mutations were reduced 5-fold in ung+ cells, indicating ung is involved in repair of bisulfite-treated transforming DNA. (2) Sequencing of 157 revertants revealed that C-->T and tandem CC-->TT transition mutations comprised 100% of the mutations scored. (3) A unique finding was that, at the highest concentrations and longest incubation times, almost every mutant obtained in ds DNA exposed to bisulfite was found to be a CC-->TT tandem double mutation. (4) The high frequency of tandem double mutants is inconsistent with two random, independent mutational events and, coupled with the observed ung dependence, lends support to the concept of catalytic deamination, wherein bisulfite induces deamination in contiguous cytosines by a concerted mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacteriophage M13↗
PubMed2004

Sequence- and base-specific delivery of nitric oxide to cytidine and 5-methylcytidine leading to efficient deamination.

Nitric oxide (NO) is an important endogenous regulatory molecule, and S-nitrosothiols are believed to play a significant role in NO storage, transport, and delivery. On the basis of their ability to generate NO in vivo, S-nitrosothiols can be used as therapeutic drugs. In this study, we have developed an innovative method for sequence- and base-specific delivery of NO to a specific site of DNA followed by specific deamination. We designed a NO transfer reaction from S-nitroso thioguanine to an imino tautomer of cytosine. Nitrosation of the thioguanosine-containing ODN 1 was carried out with S-nitroso-N-acetylpenicillamine (SNAP) to produce ODN 2. An interstrand NO transfer reaction was performed using ODN 2 and its complementary ODN 3 having dC or dmC at the target site, and a rapid NO transfer reaction was observed. In contrast, a transfer reaction was not observed either with ODN 3 having dT, dA, or dG at the target site or with ODN 5-7 having dC at a nontarget site. In the analysis of deaminated products of the NO-transferred ODN 4, it was found that the transformation ratio from dmC to dT was as high as 42% together with the dmC-diazoate (13%). In conclusion, we have demonstrated the innovative method of sequence- and base-specific delivery of nitric oxide to cytidine and 5-methylcytidine. The selectivity and efficiency of NO transfer followed by deamination exhibited in this study are extremely high compared to those of the conventional methods.

Cytidine↗
PubMed2005

Oxidative deamination of benzylamine and lysine residue in bovine serum albumin by green tea, black tea, and coffee.

Oxidative deamination by various polyphenolic compounds is presumed to be due to the oxidative conversion of polyphenols to the corresponding quinones through autoxidation. Here we examined the oxidative deamination by the polyphenol-rich beverages green tea, black tea, and coffee at a physiological pH and temperature. Green tea, black tea, and coffee extracts oxidatively deaminated benzylamine and the lysine residues of bovine serum albumin to benzaldehyde and alpha-aminoadipic delta-semialdehyde residues, respectively, in sodium phosphate buffer (pH 7.4) at 37 degrees C in both the presence and absence of Cu2+, indicating the occurrence of an amine (lysyl) oxidase-like reaction. We also examined the effects of pH and metal ions on the reaction. The possible biological effects of drinking polyphenol-rich beverages on human are also discussed.

Benzylamines↗
PubMed1992

(R)-(-)- and (S)-(+)-adenallene: synthesis, absolute configuration, enantioselectivity of antiretroviral effect, and enzymic deamination.

Synthesis of optically pure (-)- and (+)-adenallene 2 and 3 is described. Racemic adenallene (1a) was subjected to deamination with adenosine deaminase monitored by HPLC using a Chiralcel CA-1 column to give (-)-adenallene (2) and (+)-hypoxallene (4). The latter compound was converted to acetate 5. The reaction of 5 with trifluoromethanesulfonic anhydride and pyridine followed by ammonolysis furnished acetate 6 or (+)-adenallene (3) depending on the solvent used in the last step. Acetate 5 was smoothly transformed to the 6-chloro derivative 7, but an attempted ammonolysis led only to racemization and decomposition. Single crystal X-ray diffraction established the R-configuration of (-)-enantiomer 2. The latter forms a pseudosymmetric dimer in the lattice with the adenine moiety in an anti-like conformation. The torsional angles of the allenic bonds show departures from 90 degrees (91 and 97 degrees, respectively) and rotameric preference of the hydroxymethyl groups is different in both molecules of the dimer. The R-enantiomer 2 inhibited the replication and cytopathic effect of human immunodeficiency virus (HIV-1) in ATH8 cell culture with an IC50 of 5.8 microM, whereas the S-enantiomer 3 was less active (IC50 > 200 microM). The enantioselectivity of the anti-HIV effect is significantly lower than that of 2',3'-dideoxyadenosine. Kinetics of deamination of R- and S-enantiomers 2 and 3 catalyzed by adenosine deaminase gave the following parameters: Km values of S-form 3 and R-form 2 were 0.41 and 0.52 mM with Vmax being 530 and 18.5 mumol/min, respectively [corrected]. Again,, a much lower level of enantioselectivity of deamination was observed than that of D- and L-adenosine. These results indicate (i) different enantioselectivity of enantiomers 2 and 3 as HIV inhibitors and adenosine deaminase substrates and (ii) both R- and S-enantiomers 2 and 3 can function as nucleoside analogues with varied enantioselectivity for different enzymes or receptors.

Adenine↗
PubMed1999

Separate deamination mechanisms for isomeric styrene oxide induced N1-adenine adducts.

[formula: see text] Styrene 7,8-oxide (SO) induced N1-2'-deoxyadenosine 5'-phosphate (AMP) adducts deaminate to corresponding inosine derivatives. For the beta-isomer of N1-SO-AMP, the chiral alpha-carbon was found to be involved in the hydrolytic deamination, suggesting formation of an oxazolinium ring as an intermediate and that a water molecule attacks the benzylic carbon. The mechanism differs from the one suggested for the alpha-isomer of N1-SO-AMP, for which deamination occurs by direct attack of water at the 6-position of purine ring.

Adenine↗
PubMedDate not supplied

Nitric oxide induces oxidative damage in addition to deamination in macrophage DNA.

Inflammatory cells such as phagocytes, neutrophils, and macrophages have been implicated in the pathogenesis of several forms of clinical and experimental tumor development. It is hypothesized that this process is mediated by the production of reactive species including NO., O2.-, H2O2, and ONOO- which inflict DNA damage. In this study, the role of NO. in combination with oxygen radicals in DNA damage was investigated. DNA deamination (xanthine) and oxidation [5-(hydroxymethyl)uracil (5HMU), 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FAPY-G), and 8-oxoguanine (8oxoG)] products were identified in the DNA of macrophages (RAW264.7) activated with Escherichia coli lipopolysaccharide (LPS) and mouse gamma-interferon (INF-gamma). The formation of these products was inhibited by N-methyl-L-arginine (NMA), a nitric oxide synthase inhibitor. NMA inhibited only the production of nitric oxide and had no effect on superoxide production. These results demonstrate that NO. plays a dual role in damaging the DNA of activated macrophages. Autoxidation of NO. leads to nitrosating species which cause deamination of bases. Reaction of NO. with O2.- leads to DNA oxidative damage due to the formation of peroxynitrite which may have HO.-like oxidizing potential. Another possible mechanism of oxidative damage by NO. could be the mobilization of free iron by NO. which could ultimately cause Fenton-type reactions. Therefore, nitric oxide not only leads to deamination of DNA bases but is also an obligatory factor in oxidative damage to DNA.

Animals↗
PubMed2006

Relatively small increases in the steady-state levels of nucleobase deamination products in DNA from human TK6 cells exposed to toxic levels of nitric oxide.

Nitric oxide (NO) is a physiologically important molecule that has been implicated in the pathophysiology of diseases associated with chronic inflammation, such as cancer. While the complicated chemistry of NO-mediated genotoxicity has been extensively study in vitro, neither the spectrum of DNA lesions nor their consequences in vivo have been rigorously defined. We have approached this problem by exposing human TK6 lymphoblastoid cells to controlled steady-state concentrations of 1.75 or 0.65 microM NO along with 186 microM O2 in a recently developed reactor that avoids the anomalous gas-phase chemistry of NO and approximates the conditions at sites of inflammation in tissues. The resulting spectrum of nucleobase deamination products was defined using a recently developed liquid chromatography/mass spectrometry (LC/MS) method, and the results were correlated with cytotoxicity and apoptosis. A series of control experiments revealed the necessity of using dC and dA deaminase inhibitors to avoid adventitious formation of 2'-deoxyuridine (dU) and 2'-deoxyinosine (dI), respectively, during DNA isolation and processing. Exposure of TK6 cells to 1.75 microM NO and 186 microM O2 for 12 h (1260 microM x min dose) resulted in 32% loss of cell viability measured immediately after exposure and 87% cytotoxicity after a 24 h recovery period. The same exposure resulted in 3.5-, 3.8-, and 4.1-fold increases in dX, dI, and dU, respectively, to reach the following levels: dX, 7 (+/- 1) per 10(6) nt; dI, 25 (+/- 2.1) per 10(6) nt; and dU, 40 (+/- 3.8) per 10(6) nt. dO was not detected above the limit of detection of 6 lesions per 10(7) nt in 50 microg of DNA. A 12 h exposure to 0.65 microM NO and 190 microM O2 (468 microM x min dose) caused 1.7-, 1.8-, and 2.0-fold increases in dX, dI, and dU, respectively, accompanied by a approximately 15% (+/- 3.6) reduction in cell viability immediately after exposure. Again, dO was not detected. These results reveal modest increases in the steady-state levels of DNA deamination products in cells exposed to relatively cytotoxic levels of NO. This could result from limited nitrosative chemistry in nuclear DNA in cells exposed to NO or high levels of formation balanced by rapid repair of nucleobase deamination lesions in DNA.

Cell Line, Tumor↗
PubMed1998

Deamination and Dimroth rearrangement of deoxyadenosine-styrene oxide adducts in DNA.

In reactions between styrene oxide and the ring nitrogen at the 1-position of deoxyadenosine, the epoxide is opened at both the alpha- (benzylic) and beta-carbons. The 1-substituted nucleosides formed are unstable and subsequently undergo either Dimroth rearrangement to give N6-substituted deoxyadenosines or deamination to give 1-substituted deoxyinosines. alphaN6-Substituted compounds are also formed from direct reaction at the exocyclic nitrogen. Kinetic experiments revealed that relative rates of deamination of 1-substituted deoxyadenosine-styrene oxides and 1-substituted adenosine-styrene oxides were similar. However, the rate of Dimroth rearrangement in beta1-substituted adenosine-styrene oxides was approximately 2.3-fold greater than that of beta1-substituted deoxyadenosine-styrene oxides and approximately 1.5-fold greater in alpha1-substituted adenosine-styrene oxides relative to alpha1-substituted deoxyadenosine-styrene oxides. Analysis of the products formed from reactions of styrene oxide with [3H]deoxyadenosine and [3H]deoxyadenosine incorporated into native and denatured DNA showed that the double-helical DNA structure reduced the levels of adducts formed 5-fold relative to denatured DNA but did not present a complete barrier to formation of either N6-substituted deoxyadenosine- or 1-substituted deoxyinosine-styrene oxide adducts in native DNA. Additionally, in denatured and native DNA the product distributions were altered in favor of formation of beta1-substituted deoxyinosine-styrene oxide adducts with respect to reactions of the nucleoside. The ratio of retained to inverted configuration of alphaN6-substituted products was higher in DNA than in nucleoside reactions. These experiments indicate that in addition to the N6-position, the ring nitrogen at the 1-position of deoxyadenosine is available, to some extent, for reaction in native DNA. In styrene oxide-DNA reactions, formation of 1-substituted adenines can lead to deaminated products where both Watson-Crick hydrogen-bonding sites are disrupted.

Chromatography, High Pressure Liquid↗
PubMed2004

The cyclobutane dimers of 5-methylcytosine and their deamination products.

The photochemical reactions of 5-methylcytosine (m(5)C), a minor component of mammalian DNA, have been studied at a concentration of 2 mM in frozen 10 mM aqueous NaCl solution at dry ice temperature (194.5 K). For these studies, low-pressure lamps emitting mainly UVB radiation were used. We have isolated and characterized three cyclobutane dimers, namely the cis-anti(c,a) the cis-syn(c,s) and the trans-syn(t,s) forms. While the c,a and the t,s cyclobutane dimers are relatively stable towards deamination upon standing in solution at 277 K, the c,s isomer is gradually converted into the corresponding c,s m(5)C-thymine (Thy) mixed dimer; this latter reaction occurs considerably faster at 310 K. The t,s cyclobutane dimer is converted into the corresponding m(5)C-Thy mixed dimer upon incubation at 373 K, while the c,a dimer is converted into a mixture of m(5)C and c,a mixed dimer when incubated at 310 K. Irradiation of equimolar mixtures of Thy (1 mM) and m(5)C (1 mM) under similar conditions yields each of the three m(5)C cyclobutane dimers, as well as significant amounts of c,a, c,s and t,s m(5)C-Thy mixed cyclobutane dimers. These m(5)C-Thy dimers undergo decompositions similar in nature to the processes undergone by m(5)C cyclobutane dimers. Pseudo-first order rate constants for deamination of the c,s m(5)C homodimer and c,s m(5)C-Thy heterodimer at various temperatures and at pH 7.7 have been measured and the enthalpies and entropies of activation have been evaluated for the deamination processes for these two compounds. The two dimers have half-lives of about 14 and 22 h, respectively, at 310 K; however, at 273 K, the corresponding half-lives can be evaluated as being around 30 and 36 days, respectively.

5-Methylcytosine↗