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V M Craddock

Publications and source records attributed to V M Craddock.

At least 55 records · Page 3Linked to original sources
PubMed1971

Methylation of nuclear proteins by dimethylnitrosamine and by methionine in the rat in vivo.

1. The incorporation of methyl groups into histones from dimethylnitrosamine and from methionine was studied by injection of the labelled compounds, isolation of rat liver and kidney histones, and analysis of hydrolysates by column chromatography. 2. Labelled methionine gave rise to labelled in-N-methyl-lysine, di-in-N-methyl-lysine and an amino acid presumed to be omega-N-methyl-arginine. 3. Administration of labelled dimethylnitrosamine gave rise to labelled S-methylcysteine, 1-methylhistidine, 3-methylhistidine and in-N-methyl-lysine derived from the alkylating metabolite of dimethylnitrosamine. In addition, labelled formaldehyde released by metabolism of dimethylnitrosamine leads to the formation of labelled S-adenosylmethionine, and hence to labelling of in-N-methyl-lysine, di-in-N-methyl-lysine and omega-N-methylarginine by enzymic methylation. 4. The formation of in-N-methyl-lysine by alkylation of liver histones was confirmed by using doubly labelled dimethylnitrosamine to discriminate between direct chemical alkylation and enzymic methylation via S-adenosylmethionine. These experiments also suggested the possibility that methionine residues in the histones were alkylated to give methylmethionine sulphonium residues. 5. The extent of alkylation of liver histones was maximal at about 5h after dosing and declined between 5 and 24h. The methylated amino acids resulting from direct chemical alkylation were preferentially lost: this is ascribed to necrosis of the more highly alkylated cells. 6. Liver histones were about four times as alkylated as kidney histones; the extent of alkylation of liver histones was similar to that of liver total nuclear proteins. 7. Methyl methanesulphonate (120mg/kg) alkylated liver histones to a greater extent than did dimethylnitrosamine. Diethylnitrosamine also alkylated liver histones. 8. The results are discussed with regard to the possible effects of alkylation on histone function, and the possible role of histone alkylation in carcinogenesis by the three compounds.

Alkylation↗
PubMed1969

Stability of deoxyribonucleic acid methylated in the intact animal by administration of dimethylnitrosamine. Rate of breakdown in vivo and in vitro at different dosages.

1. The effect of administration of various dosages of dimethylnitrosamine on the extent of methylation of liver and kidney nucleic acids in the intact rat was studied. Methylation of liver nucleic acids was linearly related to the dosage, but decreasing the dose produced relatively less lowering of the extent of alkylation of kidney nucleic acids. 2. The rates of disappearance of 7-methylguanine from DNA during the 2 days after administration of dimethylnitrosamine in the intact animal and on incubation under simulated physiological conditions in vitro were compared. At a high dosage this rate was greater in vivo than in vitro. At a low dosage the small difference between the two rates was not thought to be sufficient evidence for existence of a specific enzymic excision of the abnormal base.

Alkylation↗
PubMed1969

Study of the methylation and lack of deamination of deoxyribonucleic acid by N-methyl-N'-nitro-N-nitrosoguanidine.

1. DNA labelled with (14)C in the purine residues was prepared by treating newborn rats with [(14)C]formate and killing them for preparation of nucleic acids at 11-17 months. This DNA was incubated with N-methyl-N'-nitro-N-nitrosoguanidine, and then analysed for products of methylation and deamination reactions. 2. Evidence was found for the formation of 7-methylguanine and a smaller amount of 3-methyladenine, and, after preliminary denaturation of the DNA, 1-methyladenine was detected. The presence of cysteine increased the extent of methylation. No evidence was found for the formation of xanthine or hypoxanthine, even at pH5.5.

Adenine↗
PubMed1968

Occurrence of 7-methylguanine in nucleic acids of rat liver.

1. Microsomal and soluble RNA of rat liver have been studied by column and paper chromatography after administration of [Me-(14)C]methionine; evidence was obtained for the occurrence of 7-methylguanine, the methyl group being derived from methionine. 2. No evidence was obtained for the occurrence of 7-methylguanine in DNA.

Animals↗
PubMed1968

The fate of 7[14C]-methylguanine after administration to the rat.

1. To assess the significance of the methylation of nucleic acids known to be caused by certain carcinogens, the metabolic fate of 7[(14)C]-methylguanine was studied, with special reference to its possible incorporation into RNA and DNA. 2. The major part (approx. 95%) of the dose was excreted unchanged in the urine. A small amount of N-demethylation took place, as evidenced by the formation of radioactive adenine and guanine, and expiration of (14)C-labelled carbon dioxide. No evidence was obtained for the direct incorporation of 7-methylguanine into systems synthesizing nucleic acids, i.e. RNA in liver, DNA in intestine or in the foetus.

Animals↗
PubMed1967

Effect of administration of the carcinogen dimethylnitrosamine on urinary 7-methylguanine.

1. Evidence is presented for the excretion of 7-methylguanine in normal rat urine at a rate of approx. 65mug./day. Experiments with animals in which the nucleic acids had been prelabelled by treatment of the neonatal rats with [(14)C]-formate gave evidence that the methylated base originated in the nucleic acids of the rat. 2. Injection of [(14)C]dimethylnitrosamine leads to an increased excretion of 7-methylguanine, and the base becomes labelled in the methyl group. The disappearance of labelled 7-methylguanine formed in nucleic acids of rats treated with the carcinogen therefore does not take place by an N-demethylation reaction, but by liberation of the intact methylated base.

Alkylation↗
PubMed1966

Analysis of bases of rat-liver nucleic acids after administration of the carcinogen dimethylnitrosamine.

Dimethylnitrosamine is metabolized to form an alkylating intermediate, which may have significance for its carcinogenic action. However, certain other compounds that are known to be highly mutagenic, including nitrous acid and hydroxylamine, might also be formed. Owing to the general reactivity of these compounds, it would be difficult to detect their formation in the intact animal. Instead, the nucleic acids of carcinogen-treated animals were examined for products of reaction with nitrous acid and hydroxylamine, i.e. for deamination of adenine and guanine, and formation of N(6)-hydroxycytosine, respectively. A double-labelling technique was used to detect very small amounts of the abnormal bases. The purine moieties of DNA in adult rat liver were labelled either with (14)C or with (3)H, by treating the neonatal animals with [(14)C]formate or with [(3)H]formate, and then allowing a period for normal growth. During this time, in liver, the labels were largely lost by metabolic turnover from cell components other than DNA. The pyrimidine moieties in DNA were labelled by treating the neonatal animals with [(14)C]orotate. The purine constituents of RNA of adult rat liver were labelled by repeated administration of [(14)C]- or [(3)H]-formate to the adult rats. The [(14)C]nucleic acid-labelled rat could then be treated with the carcinogen, and the [(3)H]nucleic acid-labelled animal could be used as a control. By this means the experimental and control tissues could be homogenized together in a single preparation, and the nucleic acids from the two tissues could be isolated, hydrolysed and analysed in a single sample. It was therefore possible to have an internal control for artifacts due to changes taking place in the nucleic acid bases during the experimental procedures. With this technique, the formation in vivo of 7-methylguanine in rat-liver DNA and RNA after administration of dimethylnitrosamine was confirmed, and no evidence was found for the formation of xanthine, hypoxanthine, N(6)-hydroxycytosine, or any other abnormal base.

Animals↗
PubMedDate not supplied

The effects of S9 mix from rat oesophagus, salivary gland, and liver on the mutagenicity of the rat oesophageal carcinogen N-nitroso-N-methylaniline in the Salmonella typhimurium assay.

The present study examines the feasibility of using the Salmonella typhimurium plate-incorporation assay of Ames for detecting target-organ specificity with N-nitroso-N-methylaniline (NMA), a compound for which the target site for tumour formation in the rat is the oesophagus. Thus it was anticipated that the oesophagus would bioactivate this compound. The compound has been investigated using S9 from Aroclor- and NMA-induced rat oesophagus, salivary gland and liver in the presence and absence of the co-mutagen, norharman. No response to NMA was seen with oesophageal S9 even though benzo[a]pyrene produced a dose-related increase in revertants in strain TA98 and TA100. No response to NMA was seen with salivary-gland S9. However, a response was produced with Aroclor-induced rat-liver S9 in the presence of norharman and with NMA-induced rat-liver S9 in the absence of norharman.

Animals↗