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D P Ringer

Publications and source records attributed to D P Ringer.

At least 37 records · Page 2Linked to original sources
PubMed1993

Changes in levels of ADP-ribose polymers in rat liver during 2-acetylaminofluorene-induced hepatocarcinogenesis.

The exposure of rats to the carcinogen 2-acetylaminofluorene (2-AAF) results in the accumulation of DNA-damaging adducts. The inability of cells to repair such damage adequately is a putative causal event in chemical carcinogenesis. It has been shown that one cellular response to DNA damage that leads to DNA repair is poly(ADP-ribosyl)ation of nuclear proteins. To examine the possible existence of an altered poly(ADP-ribosyl)ation response to 2-AAF-mediated damage of rat liver DNA, tissue ADP-ribose polymer levels were determined during various stages of 2-AAF-mediated carcinogenesis. 2-AAF was administered to rats in a discontinuous feeding regimen comprised of five consecutive cycles, each cycle consisting of 3 weeks on 2-AAF diet followed by 1 week of recovery on a control diet without 2-AAF. During cycle one of 2-AAF administration, rat liver ADP-ribose polymer levels increased 3-fold over that found in livers of rats fed only the control diet. In contrast, when rats were administered the non-genotoxic liver mitogen 4-AAF for one cycle, no significant elevation occurred in ADP-ribose polymer levels. Elevated ADP-ribose polymer production was also observed during cycles two and three of 2-AAF administration. However, during cycles four and five of 2-AAF administration, a period when rats administered 2-AAF acquire a high risk for hepatocarcinogenesis, an altered pattern of ADP-ribose polymer production occurred in rat livers. ADP-ribose polymer levels in these rat livers remained low, similar to levels found in control rat livers, despite the administration of 2-AAF. When the livers from rats fed either one or five cycles of 2-AAF were analyzed for possible decreases in the levels of tissue NAD+, the substrate for poly(ADP-ribose) polymerase, no changes in relative abundance were found. In addition, analysis of poly(ADP-ribose) polymerase activity showed no decrease at five cycles of 2-AAF administration. These results indicated that at late stages of 2-AAF-induced hepatocarcinogenesis, 2-AAF does not induce an expected increase in ADP-ribose polymer levels, and suggested that significant changes in DNA repair may occur at a time just preceding an increased risk for developing liver cancer.

2-Acetylaminofluorene↗
PubMed1992

Characterization of a complementary DNA for rat liver aryl sulfotransferase IV and use in evaluating the hepatic gene transcript levels of rats at various stages of 2-acetylaminofluorene-induced hepatocarcinogenesis.

A complementary DNA (cDNA) for rat hepatic aryl sulfotransferase IV (AST IV) was isolated, characterized, and used as a hybridization probe to evaluate the molecular basis for the differential expression of AST IV during 2-acetylaminofluorine (2AAF)-induced hepatocarcinogensis. The AST IV cDNA clone was obtained by immunochemical screening of a male Sprague-Dawley rat liver cDNA library. The AST IV cDNA was found to be 1.3 kilobases long and to encode a fusion protein which was reactive with an antibody to AST IV and enzymatically able to generate the sulfuric acid ester of N-hydroxy-2AAF. Sequence analysis of the AST IV cDNA showed it to be 1127 residues in length and to have essentially complete homology with PST-I cDNA, a previously reported (S. Ozawa, et al., Nucleic Acids Res., 18: 4001, 1990), 1028-base cDNA for an uncharacterized rat liver aryl sulfotransferase. Comparison of the PST-I/AST IV cDNA-deduced amino acid sequence with data from a partial (51%) amino acid sequence analysis of purified AST IV showed complete amino acid homology, confirming the identity of the cDNA and establishing that AST IV was an N-blocked, 291-amino acid protein with a molecular mass of 33,909 daltons. The AST IV cDNA sequence differed from the PST-I cDNA in two principal ways: the 5' end lacked 18 coding bases, and the 3' end contained a 190-base extention in the untranslated region, including a consensus sequence for signalling polyadenylation. Studies of AST IV gene transcript levels showed that the livers of rats fed 2AAF for 3 wk (early stage hepatocarcinogenesis) and hyperplastic nodules from the livers of rats fed 2AAF for 19 wk (intermediate stage hepatocarcinogenesis) displayed transcript levels similar to those of livers from normal rats. This contrasted with the 60 to 70% lower than normal capacity of the mRNA fractions to express AST IV observed during in vitro translation. These results indicated that modulation of AST IV expression at early and intermediate stages of hepatocarcinogenesis involved regulatory mechanisms at the translational level. In contrast, mRNA fractions isolated from some 2AAF-induced liver tumors or from known chemical carcinogen-derived rat hepatoma cell lines showed losses of both AST IV transcript level and in vitro translation capacity, suggesting that regulation at the transcriptional level may become important at late stages of 2AAF-induced hepatocarcinogenesis. These results indicated that the molecular mechanisms for the 2AAF-mediated down regulation of AST IV expression during 2AAF-induced hepatocarcinogenesis involved alterations in regulation at both translational and transcriptional levels.

2-Acetylaminofluorene↗
PubMed1992

Reaction product inactivation of aryl sulfotransferase IV following electrophilic substitution by the sulfuric acid ester of N-hydroxy-2-acetylaminofluorene.

Rat liver N-hydroxy-2-acetylaminofluorene (N-OH-2AAF) sulfotransferase activity is mediated by aryl sulfotransferase IV (AST IV) and causes the bioactivation of N-OH-2AAF to a highly reactive sulfuric acid ester form putatively capable of inducing liver cancer. Dietary administration of 2-acetylaminofluorene (2AAF) to induce hepatocarcinogenesis in rats has been shown to cause a rapid loss in N-OH-2AAF sulfotransferase activity. A possible mechanism for the in vivo loss in sulfotransferase activity may be the PAPS-dependent, sulfotransferase-catalyzed, reaction product inactivation of the enzyme by covalent reaction with the N-OH-2AAF sulfuric acid ester. In vitro studies to evaluate this possibility utilized a highly purified form of AST IV and measured the extent of PAPS-dependent interaction between the enzyme and N-OH-2[9-14C]AAF. The results showed the presence of a adenosine-3'-phospho-5'-phosphosulfate (PAPS)-dependent 14C-labeling of AST IV. The labeling could be blocked if the sulfotransferase inhibitor pentachlorophenol was present. Analysis of 14C-labeled AST IV following alkaline digestion and chromatography of digestion products indicated that AST IV cysteine and methionine residues were primary sites of 2[9-14C]AAF adduction. Studies involving the pretreatment of AST IV with PAPS and N-OH-2AAF prior to the measurement of N-OH-2AAF sulfotransferase activity showed a close parallel between formation of the AST IV cysteine-2AAF adduct and loss of activity. Similar studies showed that enzyme inactivation and cysteine-2AAF adduct formation could be blocked when excessive amounts of a competing nucleophile, methionine, were present during the pretreatment step, suggesting that inactivation does not proceed by a mechanism-based process. Finally, experiments involving prior reaction of AST IV with the thiol-blocking agent, N-ethylmaleimide, before measurement of enzyme activity showed essentially full loss of sulfotransferase activity and suggested that formation of AST IV cysteine-2AAF adducts could be a mechanism for enzyme inactivation. These results indicate that the in vitro inactivation of AST IV by the reactive N-OH-2AAF sulfuric acid ester is accompanied by covalent binding to AST IV, possibly through the formation of cysteine-2AAF adducts, and suggests that this mechanism merits further consideration as a basis for the loss of N-OH-2AAF sulfotransferase activity in vivo.

Animals↗
PubMed1992

Sulfation of hydroxylamines and hydroxamic acids in liver cytosol from male and female rats and purified aryl sulfotransferase IV.

Sulfation activity towards hydroxamic acids and hydroxylamines was determined in liver cytosols for juvenile and adult males and female rats, as well as in purified rat liver aryl sulfotransferase IV preparations. Sulfation activity towards the hydroxamic acids N-hydroxy-2-acetylaminofluorene, N-hydroxy-2-acetylaminophenanthrene, N-hydroxy-4-acetylaminobiphenyl, N-hydroxy-4'-fluoro-4-acetylaminobiphenyl, N-hydroxy-2-acetylamino-5-phenylpyridine, was higher in cytosols derived from adult males (two or three times) than in those from adult females and juveniles (both sexes). N-Hydroxy-2-acetylamino-3-methyl-5-phenylpyridine (N-OH-2AAMPP), however, was poorly sulfated by any of the cytosols. Sulfation activity towards the hydroxylamines N-hydroxy-2-aminofluorene, N-hydroxy-2-aminophenanthrene, N-hydroxy-4-aminobiphenyl, N-hydroxy-4'-fluoro-4-aminobiphenyl was much lower. N-Hydroxy-2-amino-5-phenylpyridine (N-OH-2APP), however, was sulfated much better than the other hydroxylamines. No higher sulfation activity in adult male cytosols for hydroxylamines was found, except for N-OH-2APP and N-hydroxy-2-amino-3-methyl-5-phenylpyridine (N-OH-2AMPP). Purified aryl sulfotransferase IV (AST IV) converted all hydroxamic acids; N-OH-2AAMPP was a poor substrate. Of the hydroxylamines only N-OH-2APP and N-OH-2AMPP were conjugated. These results suggest that hydroxylamines and hydroxamic acids are converted by different sulfotransferases in the rat in vivo. They also indicate that AST IV may be the major enzyme responsible for sulfation of a variety of aromatic hydroxamic acids in the male rat liver. The results presented here are discussed in relation to the carcinogenic effects of some of these compounds.

Aging↗
PubMed1991

Modulation of hepatic mRNA translation activity and specific expression of arylsulfotransferase IV during acetylaminofluorene-induced rat hepatocarcinogenesis.

Enzymatic sulfation of N-hydroxylated arylamines by mammalian hepatic cytosol sulfotransferases (AST; EC 2.8.2.1) is an important metabolic step which generates ultimate carcinogens. The metabolic activity of AST IV, the putative isozymic form of AST primarily responsible for catalyzing N-hydroxy-2-acetylaminofluorene sulfation, is modulated during 2-acetylaminofluorene (AAF)-induced rat hepatocarcinogenesis. To characterize the molecular mechanisms regulating the differential expression of AST IV, we have assessed polyadenylated mRNA derived from the livers of Sprague-Dawley rats undergoing different stages of AAF hepatocarcinogenesis for general in vitro translation capacity and specific expression of AST IV and albumin. Following 1 and 3 cycles of a cyclical feeding regimen (3 weeks 0.05% AAF, then 1 week basal diet), the mRNA capacity for translation was lowered and the expression of AST IV and albumin was down-regulated about 2-fold each but recovered to normal levels when treated rats were subsequently placed on basal diet for 3 continuous weeks. Cytosolic albumin levels were determined by Western blot analysis to be lowered about 1.5-2-fold. In contrast, however, mRNA from rats on basal diets for 3 weeks subsequent to cycle 5 of the feeding regimen recovered only about 50% of the capacity for AST IV expression, although overall translation capacity and albumin expression returned to normal levels. This pattern of reversible expression, followed by irreversible expression of AST IV at early and late stages of AAF hepatocarcinogenesis, respectively, provides the first evidence correlating the modulation of hepatic mRNA capacity for AST IV expression with differential cytosolic AST IV activity in the AAF hepatocarcinogenesis model. The results further suggest that during early stages in hepatocarcinogenesis modulation of mRNA protein synthesis functions may be a critical factor in AAF-mediated lowering of AST IV expression, while other persistent genetic lesions are likely playing a more significant role at the late stages of the carcinogenic process leading to neoplastic transformation of initiated hepatocytes.

2-Acetylaminofluorene↗
PubMed1991

Alteration in de novo pyrimidine biosynthesis during uridine reversal of pyrazofurin-inhibited DNA synthesis.

Pyrazofurin, a pyrimidine nucleoside analogue with antineoplastic activity, inhibits cell proliferation and DNA synthesis in cells by inhibiting uridine 5'-phosphate (UMP) synthase. It has been previously shown in concanavalin A (con A)-stimulated guinea pig lymphocytes (23) that pyrazofurin-inhibited DNA synthesis could be selectively reversed by exogenous uridine (Urd). In this report, we have examined possible mechanisms for the Urd reversal with experiments that determine the ability of exogenous Urd to (a) interfere with either the intracellular transport of pyrazofurin, or the conversion of pyrazofurin to its intracellularly active form, pyrazofurin-5'-phosphate; (b) reverse the pyrazofurin block of [14C]orotic acid incorporation into DNA; and (c) alter the pattern of exogenous [3H]Urd incorporation into DNA-thymine (DNA-Thy) and DNA-cytosine (DNA-Cyt) during pyrazofurin inhibition of pyrimidine de novo biosynthesis. The results of these experiments showed that Urd reversal does not occur through altered pyrazofurin transport or intracellular conversion to pyrazofurin-5'-phosphate, nor does it alter the distribution of [3H]Urd in DNA-Thy and DNA-Cyt. Instead, these findings indicate that the primary mechanism for exogenous Urd reversal of pyrazofurin inhibition of DNA synthesis involves the reversal of pyrazofurin inhibition of UMP synthase, thus restoring orotic acid incorporation into lymphocyte DNA through the pyrimidine de novo pathway.

Amides↗
PubMed1990

2-Acetylaminofluorene-mediated alteration in the level of liver arylsulfotransferase IV during rat hepatocarcinogenesis.

Rat liver cytosolic sulfotransferase activity forms the highly reactive sulfuric acid ester of N-hydroxy-2-acetylaminofluorene (N-OH-2AAF), an ultimate carcinogen in 2-acetylaminofluorene (2AAF) hepatocarcinogenesis. A previous report demonstrated that 2AAF-induced liver hyperplastic nodules displayed a persistent loss of cytosolic N-OH-2AAF sulfotransferase activity following a hepatocarcinogenesis-producing regimen of 2AAF administration. As an initial step in examining the mechanism responsible for lowering N-OH-2AAF sulfotransferase activity, a monospecific polyclonal antibody to aryl sulfotransferase IV (AST IV) was produced and used in the assessment of AST IV as a candidate enzyme for liver cytosolic N-OH-2AAF sulfotransferase activity. Studies comparing the levels of N-OH-2AAF sulfotransferase activity of highly purified AST IV and rat liver cytosols with corresponding immunochemical analysis of AST IV contents demonstrated that there was sufficient AST IV activity in liver cytosols to indicate that it was the primary enzyme catalyzing cytosolic N-OH-2AAF sulfation. A subsequent immunochemical survey of nine extrahepatic tissues showed no detectable AST IV content and indicated that AST IV expression may be tissue specific. An immunochemical comparison of AST IV levels in control liver cytosols (high in sulfotransferase activity) with cytosols from 2AAF-derived hyperplastic nodules (low in sulfotransferase activity) or liver tumors (no sulfotransferase activity) showed low or no detectable levels, respectively, of AST IV. In addition, an immunochemical analysis of four rat hepatoma cell lines showed they contained no detectable levels of AST IV. These results suggested a strong correlation existed between a decrease in AST IV expression and tumor development. When the liver cytosols of rats taken from early, intermediate, and late stages of 2AAF carcinogenesis were analyzed for the development of a persistent loss of N-OH-2AAF sulfotransferase activity, a parallel loss of cytosolic N-OH-2AAF sulfotransferase activity and AST IV content was observed in rats which had proceeded from a stage of low risk to high risk for liver cancer. These findings indicated that (a) AST IV, a liver-specific enzyme, was the principle enzyme comprising cytosolic N-OH-2AAF sulfotransferase activity and (b) the decrease in sulfotransferase activity in nodules and tumors resulted from a decrease in the level of AST IV expression. Furthermore, it is suggested that a persistent decrease in AST IV expression may reflect a role for AST IV as part of a resistance phenotype in which transforming liver cells are able to escape the cytotoxic effects of highly reactive 2AAF metabolites and progress to cancer.

2-Acetylaminofluorene↗
PubMed1990

Fluorescence of phosphotyrosine--terbium(III) complexes.

Phosphotyrosine, a biologically important protein residue, was investigated for the ability to enhance terbium (Tb3+) fluorescence. Spectroscopic analysis of the Tb3+: phosphotyrosine interaction indicated the development of a new excitation peak at 275 nm and strong Tb+ fluorescence enhancement at 488 and 540 nm that was linear over a range from 0.5 to 100 microM amino acid. Subsequent experiments comparing the ability of phosphotyrosine, phosphothreonine, phosphoserine and 20 other common non-phosphorylated amino acids showed that only phosphotyrosine produced significant Tb3+ fluorescence enhancement. Analysis of various phospho-sugars and nucleotides showed (with the expected exception of GMP) that they produced little or no significant fluorescence enhancement, indicating a further selectiveness for the phosphotyrosine: Tb3+ fluorescence enhancement event. These results establish a basis for the future use of Tb3+ fluorescence enhancement as a unique probe for the investigation of phosphotyrosine residues.

In Vitro Techniques↗
PubMed1988

Nuclear matrix associated poly(ADP-ribose) metabolism in regenerating rat liver.

We have examined a possible role for protein poly(ADP-ribosylation) during in vivo DNA replication by studying the metabolism of poly(ADP-ribose) in the nuclear matrix fraction from normal and regenerating rat liver. This fraction contains the newly replicated DNA and thus allows for the examination of the events closely associated with the replication process. It was found that 55% of the total nuclear protein-bound poly(ADP-ribose) and 15-35% of the total nuclear poly(ADP-ribose)-polymerase activity were tightly associated with this subnuclear compartment in normal liver. Surgical removal of two-thirds of the liver initiated a time-dependent decrease in nuclear matrix associated polymers of ADP-ribose and poly(ADP-ribose) polymerase activity which reached a minimum of 40% of control livers after 24 h, before returning to normal levels at 41 h post-partial hepatectomy. In contrast, the total levels of poly(ADP-ribose) in intact liver and the total polymerase activity of isolated nuclei exhibited a 2-fold increase over basal levels. These results are consistent with the conclusion that the nuclear matrix is a major poly(ADP-ribosylation) site within the nucleus and that this metabolic reaction may be closely connected with the events modulating DNA replication in this fraction.

Animals↗
PubMed1988

Changes in rat liver N-hydroxy-2-acetylaminofluorene aryl sulfotransferase activity at early and late stages of hepatocarcinogenesis resulting from dietary administration of 2-acetylaminofluorene.

The ability of 2-acetylaminofluorene (AAF) to mediate a loss in N-hydroxy-AAF (N-OH-AAF) aryl sulfotransferase activity when fed to male Sprague-Dawley rats was examined at early and late stages of hepatocarcinogenesis. Administration of 0.05% AAF in the diet for 1 week caused liver N-OH-AAF aryl sulfotransferase activity to decrease to 15 +/- 5% of that for liver from non-carcinogen-fed rats, and the activity remained low throughout 19 weeks of AAF feeding. When rats were fed AAF diet for 3 weeks, then placed on a control diet, liver N-OH-AAF aryl sulfotransferase activity returned to normal levels within 3 weeks. In contrast, when rats were fed AAF for 19 weeks, then placed on control diet for an additional 10 weeks, little or no recovery of N-OH-AAF aryl sulfotransferase activity was observed in cytosols from whole livers or isolated hyperplastic nodules, respectively. These findings suggest two types of AAF-mediated decreases in sulfotransferase activity: (a) a decrease observed early in the initial stages of AAF feeding which returns to normal levels when AAF is removed from diet, and (b) a persistent decrease in activity following long term AAF administration.

2-Acetylaminofluorene↗
PubMed1987

Evidence for nucleoside channeling in vivo: deoxythymidine incorporation into rat liver dTTP and nuclear matrix DNA.

Previous studies in prokaryotes and in eukaryotic cell lines have indicated the possible existence of more than one dTTP pool accessible to DNA synthesis. To investigate this possibility in eukaryotes in vivo, the incorporation of [3H] deoxythymidine into nuclear matrix-attached DNA and intracellular dTTP was examined in regenerating rat liver. The labeling of matrix DNA reached a maximum after a 5 min pulse and then began to rapidly decrease. Conversely, [3H] deoxythymidine incorporation into dTTP began to increase after 5 min and peaked 10 min after injection. Since the peak specific activity for [3H] deoxythymidine incorporation into matrix DNA precedes that into dTTP, there seems to be channeling of exogenous thymidine directly to sites of DNA replication, bypassing existing nucleotide pools.

Animals↗
PubMed1987

Assessment of salvage pathways utilized for incorporation of exogenous pyrimidine nucleosides into DNA of guinea pig lymphocytes stimulated by Con A.

The organization of specific pyrimidine pathways to channel various nucleoside precursors into DNA is poorly understood. We show that concanavalin A-stimulated guinea pig lymphocytes incorporate [3H]dThd, [3H]dCyd, [3H]dUrd, [3H]Cyd and [3H]Urd into DNA-thymines and DNA-cytosines in a highly conserved distribution pattern. DNA-thymines were labeled only by dThd and dUrd, while DNA-cytosines were labeled only by dCyd, Cyd and Urd. The kinetics for the incorporation of the [3H]nucleosides were essentially identical, indicating equivalent abilities to measure DNA synthesis. Pyrazofurin inhibition of the pyrimidine de novo synthetic pathway inhibited cell proliferation and the levels of [3H]nucleoside incorporation by approx. 50%, but did not alter restricted distribution of the [3H]nucleosides among DNA-thymines and DNA-cytosines. These findings indicate the absence of Cyd and dCMP deaminase salvage pathways and suggest either subcellular compartmentalization or differential regulation of ribonucleoside diphosphoreductase which permits reduction of CDP but not UDP.

Amides↗
PubMed1987

Further characterization of the ability of hepatocarcinogens to lower rat liver aryl sulfotransferase activity.

Aryl sulfotransferase (AST) activity in rat liver is thought to be a primary pathway in the bio-activation of various hepatocarcinogens to forms which act as ultimate carcinogens in chemical hepatocarcinogenesis. In an effort to understand the significance of rapid and sustained decreases in liver AST that accompany dietary administration of hepatocarcinogens and to further assess its relationship to carcinogenic processes, we determined the abilities of various xenobiotics known to be hepatocarcinogens or non-hepatic carcinogens to lower AST activity. We also determined whether the co-administration of the AST enzyme inhibitor, pentachlorophenol, with hepatocarcinogens will abrogate the lowering of AST activity caused by hepatocarcinogens which do not utilize AST for bio-activation versus hepatocarcinogens which can utilize AST. Among carcinogens tested thus far, we have found the AST activity of liver cytosols to be lowered by the hepatocarcinogens 2-acetylaminofluorene, ethionine, 3'-methyl-4-dimethylaminoazobenzene, thioacetamide, aflatoxin B1, diethylnitrosamine and benzidine, but not by the non-hepatic carcinogens 2-acetylaminophenanthrene or 3-methylcholanthrene. Pentachlorophenol reversed activity losses when co-administered with all carcinogens which lowers AST activity with the exception of ethionine and thioacetamide. We suggest that AST activity lowering is relatively specific for liver carcinogens and involves two different mechanisms.

Animals↗
PubMed1986

Separation and quantitative analysis of O-linked phosphoamino acids by isocratic high-performance liquid chromatography of the 9-fluorenylmethyl chloroformate derivatives.

Phosphoamino acids derivatized with 9-fluorenylmethyl chloroformate were separated on an anion-exchange column (Partisil 10 SAX) at pH 3.90 using an isocratic elution with 10.0 mM potassium phosphate, 1.0% tetrahydrofuran, and 55% methanol. Phosphoamino acids were eluted with baseline resolution in the following order: phosphotyrosine, phosphothreonine, and phosphoserine. Each phosphoamino acid was separated from its parent amino acid, dicarboxylic amino acids, sugaramine phosphates, as well as the other common amino acids. The turn-around time from injection to injection was 35 min. The linearity for all three O-linked phosphoamino acids extended from 0.5-1000 pmol and has been shown to be directly applicable to the analysis of isolated phosphoproteins.

Amino Acids↗
PubMed1985

Distribution patterns for 5-methylcytosine among apurinic DNAs from several sources.

Purified DNA from the liver of rats, mice, rabbits, and guinea pigs, from guinea pig lymph nodes, from hyperplastic nodules induced in rat liver by feeding with 2-(acetylamino)fluorene, and from Escherichia coli cells was made apurinic by reaction with diphenylamine. After chromatographic separation of pyrimidine tracts (isostichs or isoplyths) according to the number of contiguous pyrimidines, semilog plots of tract frequency vs. the number of contiguous pyrimidines were linear, plots for DNA from several sources differed from one another, and all deviated significantly from randomness. Similar semilog plots for coding sequences among 60 mammalian genomes or 28 rat tissue genomes were intermediate among slopes for isolated DNA. Individual isostichs were hydrolyzed, and their constituent pyrimidine bases were analyzed by high-pressure liquid chromatography. Among isostichs from isolated DNAs, the distribution of Thy and Cyt contents differed markedly from the distribution of 5-methylcytosine (5-Me-Cyt); e.g., although isostich 1 contained 45-49% of 5-Me-Cyt, amounts of Thy or Cyt did not exceed 25%. Semilog plots of normalized values for tract frequency or the content of 5-Me-Cyt vs. isostich number were essentially superimposable; thus, among the first five pyrimidine tracts of a particular tissue or E. coli DNA, the number of tracts per 5-Me-Cyt moiety was essentially constant. The data showed that 5-Me-Cyt and/or dCyd-dGuo dinucleotides have a distribution throughout DNA structure that superimposes the distribution of pyrimidine tract frequency and suggests that regulatory 5-Me-Cyt moieties are principally located at 3' termini of pyrimidine tracts.

5-Methylcytosine↗