Differential effects of procarbazine and methylnitrosourea on the accumulation of O6-methylguanine and the depletion and recovery of O6-alkylguanine-DNA alkyltransferase in rat tissues

The kinetics of accumulation of the premutagenic DNA adduct O6-methylguanine (O6-meG) in the liver, blood leukocytes, lymph nodes and bone marrow of rats was examined and compared after single or multiple doses of procarbazine, a methylating cytostatic drug employed in the treatment of Hodgkin's lymphoma patients, and methylnitrosourea (MNU), an experimental methylating agent and carcinogen. Maximal O6-meG levels occurred 1-2 h after administration of single doses of procarbazine (10 mg/kg) or MNU (1 mg/kg), thereafter decreasing with half-lives of approximately 20-45 h, depending on the tissue. A relatively uniform tissue distribution was observed with both agents, with the liver generally showing highest adduct levels, followed by the lymph nodes, bone marrow and blood leukocytes which contained broadly similar amounts of O6-meG. During daily, oral administration to rats of procarbazine for 10 days at dose rates of 2.5, 5, 10 or 20 mg/kg/day (treatment analogous to that of the MOOP chemotherapy protocol for Hodgkin's lymphoma) followed by animal death on different days (in each case 24 h after the last treatment), a biphasic mode of O6-meG induction was observed: an initially steep build-up during the first 3-4 days was followed by a transient decline in the rate of accumulation, in turn followed by a second wave of accumulation and then a further slow-down. During the same treatment, liver O6-methylguanine-DNA alkyltransferase (AGT) declined in a dose-related manner. AGT recovery after the end of treatment was slow, taking nearly 20 days after the end of the high-dose treatment to return to control levels, despite the fact that all detectable adducts had been lost from DNA within 3 days after the end of treatment. A similar depletion and slow recovery of AGT in the liver, blood lymphocytes, bone marrow and lymph nodes was observed after treatment with a single dose of 100 mg/kg procarbazine. In contrast to these observations, O6-meG accumulated smoothly during a 10 day administration of MNU (1 or 10 mg/kg/day) to reach a steady-state within 5-6 days, while liver AGT was partially depleted after the high dose and recovered fully within 72 h of cessation of treatment. Similarly, a single dose of MNU (35 mg/kg) resulted in AGT depletion followed by rapid recovery in all four tissues examined. It is concluded that procarbazine (but not MNU) causes a decrease in cellular AGT concentrations by a mechanism additional to suicide repair of O6-meG.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative dosimetry of O6-methylguanine in humans and rodents treated with procarbazine

The accumulation of O6-methylguanine (O6-meG) in the DNA of blood leukocytes of 21 Hodgkin's lymphoma patients (followed for up to 12 cycles of treatment) treated in the context of MOPP combination chemotherapy with 150 mg procarbazine daily for 10 days was examined and compared to that observed in rats treated with different doses of procarbazine as a single agent once per day for 10 days. In humans, the adduct accumulated in a dose-related fashion and appeared to approach a steady-state after 7-8 days of treatment. Adduct levels on day 10 of the treatment cycle averaged 0.25 +/- 0.09 (mean +/- SD) mumol/molG and, for different individuals, covered a 3-fold range. Intra-individual variability between different treatment cycles was much more limited than inter-individual variability, the two parameters accounting for 8.9% and 84.5% respectively of adduct variance at a constant cumulative dose. Comparison of the dose-response relationships for humans and rats indicates that, under conditions of no depletion of O6-alkylguanine-DNA alkyltransferase (AGT), O6-meG accumulates in blood leukocyte DNA of humans at a rate which is only approximately 2-fold lower than in rats, implying that, to the extent to which O6-meG contributes to the genotoxic activity of procarbazine, human susceptibility to it is likely to be comparable to that of the rat. This is likely to be true also of the bone marrow (the tissue of interest as a target tissue for leukaemogenesis), since the tissue distribution of O6-meG induced by low doses of procarbazine in rats, mice and rabbits indicated that blood leukocyte levels of this adduct closely reflect those in the bone marrow. Based on these results, it is estimated that by the end of a MOPP chemotherapy cycle O6-meG reaches levels of the order of 0.2-0.3 fmol/microgram DNA (0.3-0.5 mumol/molG) in human bone marrow (the target tissue of leukaemogenesis observed after such treatment).

Accumulation of O6-methylguanine in human DNA after therapeutic exposure to methylating agents and its relationship with biological effects

O6-Methylguanine has been measured in peripheral blood leukocytes of 14 patients during one or more cycles of treatment with procarbazine (daily treatment for 10 days) and in 12 patients during one or more cycles of treatment with dacarbazine (single dose per cycle). Adduct formation at levels up to about 0.4 fmole/microgram DNA was detected in all procarbazine- and all but one dacarbazine-treated patients at some point after treatment. O6-Methylguanine accumulated during procarbazine treatment in a dose-related manner (mean rate of accumulation 2.8 x 10(-4) fmole/microgram DNA per mg/m2 dose) and appeared to approach a plateau by the end of the cycle (above 600 mg/m2 cumulative dose). The average rate of O6-methylguanine formation 2 hr after dacarbazine treatment was 11 +/- 8 x 10(-4) fmole/microgram DNA per mg/m2 dose. Individuals examined on more than one treatment cycle with either drug showed broadly similar methylation responses. The rate of adduct accumulation showed a nonsignificant, negative correlation with the pretreatment lymphocyte levels of the repair enzyme O6-alkylguanine-DNA alkyltransferase (AGT) in the case of procarbazine and no correlation in the case of dacarbazine. No consistent lymphocyte AGT depletion was noted as a result of treatment with either drug. No correlation between O6-methylguanine formation and hematological toxicity was observed. In eight patients showing full remission after treatment with dacarbazine, the value of O6-methylguanine (averaged over all the cycles) was 0.252 +/- 0.120 fmole/microgram DNA while in four patients showing partial or no response it was 0.087 +/- 0.110 fmole/microgram DNA (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Mutagenesis by O6 meG residues within codon 12 of the human Ha-ras proto-oncogene in monkey cells

The first or/and the second guanines of the human Ha-ras codon 12 (normally GGC) were substituted by O6 meG residues and the modified sequence was subsequently introduced into an SV40-based shuttle vector able to replicate in both simian cells and bacteria. After replication in simian COS7 cells (proficient in O6-alkyl-guanine transferase), plasmid DNA was extracted and mutations were screened in E. coli DH5 alpha cells. The vast majority of the mutations induced by O6 meG were G----A transitions. The mutation frequency observed at the second guanine of codon 12 (12G2 position: 3.75% +/- 0.4) was higher than the one observed at the first guanine (12G1 position: 1.09% +/- 0.6). This difference was confirmed by the results obtained when two adjacent O6 meG residues were positioned within codon 12. The higher mutation frequency observed for the 12G2 position could be attributed to differential repair or/and variation in polymerase fidelity. These results are in agreement with animal experiments where alkylating agents gave rise to mutation on G2 position of codon 12.

Studies in gastric carcinogenesis. V. The effects of ascorbic acid on N-nitroso compound formation in human gastric juice in vivo and in vitro

The concentrations of nitrite, thermo- and acetic acid-labile TEA-responsive compounds (TACs) and N-nitroso compounds (NOCs) as a group were measured in human gastric juice collected just before and 1, 2 and 4 h after oral ingestion of 1 g ascorbic acid (AA) or 200 mg sodium nitrate, separately or in combination. Individual responses of gastric [nitrite] following ingestion of AA alone varied widely, with both decreases and increases being observed, and showed no correlation with gastric pH. While a mixed response was also noted for [NOC] and [TAC], substantial decreases were observed in 5/6 individuals with initial [NOC] greater than 0.2 microM and 3/3 individuals with initial [TAC] greater than 0.2 microM, implying that (i) AA effectively inhibited gastric nitrosation and (ii) a basal amount of NOCs and TACs was present in gastric juice which could not be lowered by AA ingestion. Statistical analysis indicated that global mean values of gastric [NOC] were significantly reduced (P less than 0.02) 1-4 h after ingestion of AA. Ingestion of 200 mg sodium nitrate alone resulted in increases in gastric [NOC], which in some cases were very substantial. While nitrosation appeared lower following ingestion of the same dose of nitrate in combination with 1 g AA, the difference from the effects of nitrate alone was not statistically significant. In aqueous buffer, pH 2.5, and in the presence of 1 mM AA, 50 microM nitrite was consumed with a t1/2 of 50 min only if molecular oxygen had first been removed from the system. In the presence of oxygen, no consumption of nitrite could be detected in 50 min, reflecting nitrite recycling (oxidation of nitric oxide to higher oxides of nitrogen and hydrolysis back to nitrite). It is likely that nitrite recycling occurring after collection of gastric juice accounted for the inconsistent responses of gastric nitrite following ingestion of AA. Incubation of human gastric juice, pH 2.5, in vitro in the presence of 50 microM sodium nitrite for 60 min resulted in an increase of [NOC] and [TAC] from 0.10 to 0.70 and 1.10 microM respectively. Nitrosation was efficiently inhibited by AA, 2.27 mM AA resulting in 87 and 100% inhibition respectively. Removal of oxygen from the reaction mixture did not have any significant effect on the extent of nitrosation in the presence or absence of AA.

In vivo formation and repair of O6-methylguanine in human leukocyte DNA after intravenous exposure to dacarbazine

Blood leukocyte DNA obtained from 11 Hodgkin's disease patients undergoing ABVD chemotherapy was analysed for the presence of the precarcinogenic adduct O6-methylguanine (O6-meG) at various times (1-2 h up to 49 h) after i.v. treatment with the methylating drug dacarbazine. Adduct formation was detected in all but one of the patients examined at levels ranging up to 0.45 fmol/micrograms DNA (7.2 x 10(-7) mol/mol guanine). The levels of the adduct decreased by approximately 30% over the 24 h following exposure and were usually not detectable 49 h after exposure. In five out of seven individuals examined after more than one treatment, consistent methylation responses were noted, while in the remaining two cases the responses were mixed. No correlation between the extent of adduct formation and lymphocyte levels of the repair enzyme O6-alkylguanine-DNA alkyltransferase was observed. The average extent of O6-meG formation 1 h after dacarbazine treatment was (4.3 +/- 3.1) x 10(-2) fmol/micrograms DNA per mg/kg dose [( 1.2 +/- 0.8) x 10(-3) fmol/micrograms DNA per mg/m2 dose)]. Following exposure of rats to similar doses of dacarbazine, the corresponding levels of adduct in blood leukocyte DNA were 1.1 x 10(-2) fmol/micrograms DNA per mg/kg dose (2.6 x 10(-3) fmol/micrograms DNA per mg/m2 dose).

Accumulation of O6-methylguanine in human blood leukocyte DNA during exposure to procarbazine and its relationships with dose and repair

O6-Methylguanine was measured by a competitive repair assay in blood leukocyte DNA of seven patients with Hodgkin's or non-Hodgkin's lymphoma during therapeutic exposure to procarbazine involving three daily p.o. doses (50 mg each) for 10 days (corresponding to 2.1 mg/kg/day for a 70-kg human). Adduct accumulation was observed in all seven cases, reaching levels up to 0.28 fmol/microgram of DNA (0.45 mumol/mol of guanine). In one individual, maximal levels of adduct were reached after 7 days of exposure, followed by a steady decline, whereas in all other individuals continuous accumulation was observed throughout the exposure period. In four individuals for which data were available for Day 11 (12 to 16 h after the final intake of procarbazine), decreased amounts of O6-methylguanine were observed relative to the last previous measurements. The accumulation of O6-methylguanine was linearly correlated (P less than 0.01) with the cumulative dose of procarbazine, with a slope of 0.011 fmol of O6-methylguanine/microgram of DNA per mg/kg of body weight or 2.68 x 10(-4) fmol of O6 methylguanine DNA per mg/m2. (Two h after the administration of single p.o. doses of 1 to 10 mg/kg of procarbazine to rats, O6-methylguanine formation in leukocyte DNA was just under half that in liver DNA and showed a linear relationship with dose with a slope of 0.017 fmol/microgram of DNA per mg/kg of body weight or 5.67 x 10(-4) fmol of O6-methylguanine/microgram of DNA per mg/m2. A negative correlation (P less than 0.05) between the rate of accumulation of O6-methylguanine in different individuals and lymphocyte O6-alkylguanine-DNA alkyltransferase (AGT) was observed, demonstrating a probable protective effect of AGT against the accumulation of O6-methylguanine during exposure to methylating agents. This observation supports the suggestion of a possible role of procarbazine-induced O6-methylguanine in the pathogenesis of acute nonlymphocytic leukemia appearing after treatment with chemotherapeutic protocols which include procarbazine, based on the finding of low lymphocyte AGT levels in patients with such therapy-related neoplastic disease (Sagher et al., Cancer Res., 48: 3084-3089, 1988). Lymphocyte AGT levels were mainly in the range of 5 to 10 fmol/micrograms of DNA and showed no consistent variation during procarbazine exposure.

Studies in gastric carcinogenesis. IV. O6-methylguanine and its repair in normal and atrophic biopsy specimens of human gastric mucosa. Correlation of O6-alkylguanine-DNA alkyltransferase activities in gastric mucosa and circulating lymphocytes

DNA extracted from biopsies of normal or atrophic gastric mucosa obtained from 20 individuals was analysed for the presence of the precarcinogenic alkylation lesion O6-methylguanine by a recently developed, highly sensitive assay based on repair by the Escherichia coli O6-alkylguanine-DNA alkyltransferase (AGT) enzyme in competition with a radiolabelled oligonucleotide containing O6-methylguanine (O6-meG). With a limit of detection of 0.5 fmol O6-meG in 10 micrograms DNA, only one DNA sample (derived from a region of the stomach with advanced chronic atrophic gastritis) was found marginally positive, containing 0.52 fmol/10 micrograms DNA (8.3 X 10(-8) mol O6-meG/mol guanine). Measurements of AGT in 49 biopsies of normal, atrophic, hyperplastic or dysplastic mucosa obtained from the gastric antrum or corpus of 18 individuals did not reveal any significant effects of mucosal histology on AGT. The average AGT value found was 6.9 +/- 3.5 (SD) fmol/micrograms DNA, which is lower than the values reported for a number of other human tissues (liver, small intestine and lung). Measurement of AGT levels in gastric mucosa and circulating lymphocytes of the same individuals revealed a positive correlation (P less than 0.005), suggesting that lymphocytes may serve as a useful surrogate marker for AGT activity in gastric mucosa in studies of the epidemiology of this important repair enzyme.

A novel, sensitive assay for O6-methyl- and O6-ethylguanine in DNA, based on repair by the enzyme O6-alkylguanine-DNA-alkyltransferase in competition with an oligonucleotide containing O6-methylguanine

A novel assay for O6-alkylguanine-type adducts in DNA is reported. It is based on the use of the suicide repair enzyme O6-alkylguanine-DNA-alkyltransferase (AGT) to repair such adducts in DNA in competition with an oligonucleotide containing a single residue of O6-methylguanine, end labeled to high specific activity. The stoichiometric mode of action of AGT results in decreased amounts of oligonucleotide being repaired in the presence of increasing levels of adducts in the competing DNA. The extent of oligonucleotide repair is determined by immunoprecipitation of the unrepaired form with rabbit antiserum directed against O6-alkyldeoxyguanosine and radiocounting. The amount of O6-alkylguanine in competing DNA is calculated by reference to a standard curve constructed using DNA of known alkylation. In view of its relatively wide spectrum of alkyl group specificity, use of AGT from rat liver permits the determination of both O6-methyl- and O6-ethylguanine (detection limits, 0.8 fmol and 3 fmol, respectively). On the other hand, the restricted specificity of Escherichia coli AGT to repair of O6-methylguanine makes the assay based on it specific for this type of lesion (detection limit, 0.5 fmol). The maximum amount of DNA which can be included in the assay is 15 micrograms and 10 micrograms for the rat liver and E. coli AGT-based assays, respectively, leading to a limit of sensitivity of 8 x 10(-8) mol O6-methylguanine/mol guanine (50 fmol/mg DNA) (both enzymes) and 3 x 10(-7) mol O6-ethylguanine/mol guanine (200 fmol/mg DNA) (rat liver AGT-based assay) and making this one of the most sensitive assays for these important precarcinogenic adducts. The new assay has been validated by assaying DNA from rat liver methylated in vivo with dimethylnitrosamine to a known extent and has been found to give results in close agreement with those of radioimmunoassay. Six h after i.p. administration of dimethylnitrosamine (0.01-1 mg/kg) to rats, O6-methylguanine was detectable by the competitive-repair assay in liver or lymphocyte DNA at levels of 0.14-14.4 mumol/mol guanine.

Development and validation of a new assay for O6-alkylguanine-DNA-alkyltransferase based on the use of an oligonucleotide substrate, and its application to the measurement of DNA repair activity in extracts of biopsy samples of human urinary bladder mucosa

A simplified and highly sensitive assay for the determination of O6-alkylguanine-DNA-alkyltransferase has been developed and validated by the analysis of extracts of human urinary bladder mucosa. The new assay involves the use of a synthetic dodecanucleotide containing a single O6-methylguanine residue as substrate for the enzyme. This substrate is 5'-end-labelled with [35S]PO3 and separation of repaired and unrepaired oligonucleotide is achived by immuno-precipitation with polyclonal antibodies specific for O6-methyldeoxyguanosine. Kinetic analysis of the repair of the oligonucleotide by extracts of Escherichia coli and rat liver showed that the reaction is first-order in substrate and enzyme and gave the molecular rate constants 7.5 x 10(6) mol-1 1-1 sec-1 and 8.0 x 10(6) mol-1 1-1 sec-1, respectively. The rate constants for the repair of the corresponding O6-ethylguanine-containing oligonucleotide were 3.0 x 10(5) mol-1 l-1 sec-1 and 3.6 x 10(6) mol-1 l-1 sec-1, respectively. Analysis of extracts of 48 samples of normal or neoplastic human urinary bladder mucosa obtained by transurethral biopsy or at surgery, by the new method or by a method involving use of [3H]-methylated DNA as substrate and HPLC, indicated excellent agreement between the two methods. The mean AGT content of normal urinary bladder mucosa obtained from individuals without diagnosed bladder cancer was 0.181 +/- 0.081 (mean +/- SD) fmol/microgram protein, that of neoplastic samples 0.323 +/- 0.177 fmol/microgram protein and that of normal tissue obtained from patients with bladder cancer 0.183 +/- 0.068 fmol/microgram protein. The new method is convenient, rapid and extremely sensitive (it can readily measure femtomole quantities of enzyme) and should prove useful for studies of the biochemical epidemiology of DNA repair.

Studies in gastric carcinogenesis. II. Absence of elevated concentrations of N-nitroso compounds in the gastric juice of Greek hypochlorhydric individuals

The concentrations of nitrate, nitrite, N-nitroso compounds and bacteria were measured in 96 samples of fasting gastric juice, pH 0.90-8.50, obtained from 56 individuals just before or at various times (8 days - 1 year) after gastric operation. The mean pH of the post-operative samples [4.66 +/- 0.39 (SEM)] was significantly higher than that of the pre-operative ones [3.29 +/- 0.33 (SEM)]. A positive correlation with pH was observed for the concentrations of total and nitrate-reducing bacteria (median values 5.0 X 10(5) organisms/ml and 9.2 X 10(4) organisms/ml, respectively, for samples with pH greater than or equal to 1.2 X 10(3) organisms/ml and 0 organisms/ml, respectively, for samples with pH less than or equal to 2.5) and nitrite [mean values 22.5 +/- 3.1 (SEM) microM and 3.20 +/- 0.5 (SEM) microM for samples with pH greater than or equal to 6.5 and pH less than or equal to 2.5, respectively]. No correlation with pH was seen for the concentrations of nitrate [mean value 0.48 +/- 0.06 (SEM) mM] or N-nitroso compounds [mean value 0.30 +/- 0.06 (SEM) microM]. The concentrations of bacteria and nitrite, although increased in hypochlorhydric individuals, were lower than those reported for corresponding individuals in other, primarily British, studies. It is suggested that the relatively low concentrations of nitrite observed in our hypochlorhydric population may account for the absence of elevated concentrations of N-nitroso compounds and that the latter phenomenon may be related to the relatively low frequency of gastric cancer in Greece.

Studies in gastric carcinogenesis. III. The kinetics of nitrosation of gastric-juice components in vitro and their implications for the in vivo formation of N-nitroso compounds in normal and in hypochlorhydric populations

Fasting human gastric juice was treated in vitro, at pH 2-7 and 37 degrees C for 2 h, with 5-100 microM sodium nitrite. Under these conditions (which simulated those occurring in vivo in normal or hypochlorhydric individuals), the formation of total N-nitroso compounds had the following characteristics: (i) it increased greatly at pH less than 3; (ii) it showed first-order dependence on nitrite concentration; (iii) it was faster at pH 7 than at pH 5. These observations are compatible with the N-nitroso compounds formed by the interaction of nitrite with gastric juice being N-nitrosamides or related compounds. Furthermore, based on the results of this study, it is suggested that in order for hypochlorhydria to give rise to increased formation of N-nitroso compounds in the stomach, it would be necessary for it to be accompanied by a greater than 5- to 10-fold increase in gastric nitrite concentration relative to that found in the normal population, a condition which is not necessarily fulfilled in all hypochlorhydric individuals or populations. The implications of this conclusion for the assessment of the role on gastric N-nitroso compounds in the etiology of gastric cancer are discussed.

Effects of dimethylnitrosamine on RNA synthesis and metabolism in mouse liver

Following i.p. injection of dimethylnitrosamine into male C57BL mice, synthesis of liver nuclear heterogeneous RNA was inhibited significantly, reaching approximately 20% of control within 2 hr of a dose of 40 mg/kg. Synthesis of nucleolar RNA was also inhibited, although to a smaller extent, reaching about 70% of control after the same treatment. These effects were observed both during RNA synthesis in vivo and during in vitro transcription with isolated nuclei and nucleoli. Examination of RNA polymerases I and II, isolated and partially purified by diethylaminoethyl Sephadex column chromatography, did not indicate any change either in their activities in the transcription of exogenous DNA or in their in vivo binding to chromatin. On the other hand, the activity of purified chromatin as a template for transcription by added, partially purified RNA polymerase II was significantly reduced, suggesting that carcinogen-induced damage to chromatin was the cause of the observed inhibition of heterogeneous RNA synthesis. When purified DNA was used in place of chromatin as a template for transcription by partially purified RNA polymerase II, no inhibition was observed. Dimethylnitrosamine treatment had a pronounced effect on the kinetics of appearance of the cytoplasmic RNA species. Four hr after a 40-mg/kg dose of dimethylnitrosamine, the rate of appearance in the cytoplasm of polyadenylate-containing RNA was inhibited by 50%, while that of 4S, 18S, and 28S ribosomal RNA was inhibited by over 80%.

O6-Methylguanine-DNA transmethylase activity in extracts of human gastric mucosa

In view of the possible involvement of N-nitroso compounds in the etiology of gastric carcinoma in man, we have examined the abilities of extracts of human gastric mucosa for the removal from DNA of O6-methylguanine, the primary precarcinogenic lesion induced in DNA by methylating carcinogenic N-nitroso compounds. We find that all 20 specimens examined (including 14 non-neoplastic and 6 neoplastic) possess significant activity, removing on average 0.73 +/- 0.06 (s.e.m.) pmol O6-methylguanine per mg protein in the extract. This activity, which has the characteristics of an O6-methylguanine-DNA-transmethylase, while exhibiting wide interindividual variations, shows no significant differences between the groups of normal and of neoplastic tissues or between the groups of non-neoplastic tissues obtained from individuals with benign or with malignant gastric disease. It is estimated that the average O6-methylguanine-removing capacity of the cells of normal gastric mucosa (calculated as 166 000 +/- 26 000 (s.e.m.) molecules per cell) greatly exceeds the extent of DNA damage likely to be caused by gastric-juice N-nitroso compounds.

Mutagenic and clastogenic effects of organic extracts from the Athenian drinking water

Organic extracts of chlorinated drinking water collected from the water distribution system of Athens, Greece, were tested in parallel for their ability to cause mutations in the Salmonella His+ reversion system and sister chromatid exchanges (SCE) and chromosomal aberrations in CHO cells. They were found to induce His+ revertants in frequencies similar to those detected previously with drinking water extracts from other areas around the world. We observe that organic samples from surface drinking water induce SCE and chromatid type abnormalities in CHO cell cultures while samples collected from ground drinking water did not. These findings confirm that chlorinated surface drinking water contains substances which are mutagenic and clastogenic and may be carcinogenic in humans.

Nondetection of O6-methylguanine in rat DNA following in vivo treatment with large doses of cimetidine alone or in combination with sodium nitrite

Cimetidine was administered by stomach tube to rats at 70 or 700 mg/kg, doses corresponding to 5 or 50 times, respectively, the typical daily dose of individuals on cimetidine treatment. In some cases, cimetidine (70 mg/kg) was administered in combination with a 2-fold molar excess of sodium nitrite. This treatment was carried out up to 6 times over a 3-day period, the pH of the rat stomach being maintained at 2.3 to 3.0 for about 1 hr after each treatment. The DNA of the stomach, liver, and intestines (large and small pooled together) was isolated 6 hr after cessation of treatment and analyzed for the presence of O6-methylguanine using a sensitive and specific radioimmunoassay. No evidence could be obtained for the presence of this methylated base in any of the DNA samples examined, the limit of detection being 3 mumol O6-methylguanine per mol guanine. We suggest that the observed lack of DNA methylation may be primarily due to the slow rate of nitrosation of cimetidine in combination with its rapid absorption into the blood stream.

The formation and repair of O6-methylguanine in rat liver nucleolar DNA after dimethylnitrosamine administration studied by radioimmunoassay

The extent of formation and the rate of repair of O6-methylguanine in rat liver nucleolar DNA following administration of a single dose of dimethylnitrosamine have been studied by means of radioimmunoassay for O6-methyldeoxyguanosine. No difference has been found in either the extent of formation or the rate of repair of this lesion between nucleolar and whole nuclear DNA.

Induction of sister chromatid exchanges and chromosome aberrations in cultured mammalian cells by N-nitrosocimetidine

N-Nitrosocimetidine (NC) induces significant numbers of sister chromatid exchanges (SCE) and chromosome aberrations in cultured Chinese hamster ovary (CHO) cells even at a concentration of 1.2 x 10(-7) M. Its effectiveness in SCE induction is about two thirds that of the gastric carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). These results constitute further evidence that NC possesses carcinogenic activity.

The use of radioimmunoassay to study the formation and disappearance of O6-methylguanine in mouse liver satellite and main-band DNA following dimethylnitrosamine administration

A sensitive radioimmunoassay for O6-methyldeoxyguanosine has been developed, permitting the analysis of microgram amounts of DNA. This technique has been used in the study of the formation and removal of O6-methylguanine in mouse liver satellite and main-band DNA. The results indicate a reduced extent of O6-methylguanine formation in satellite DNA but similar rates of its removal from both classes of DNA.

Thiol-blocking reagents and phosphate acetyltransferase catalysis, and the assessment of protection by adsorbed molecules

Phosphate acetyltransferase (EC 2.3.1.8) is shown to be unusually sensitive to thiol-blocking reagents, and one or more thiol groups probably catalyse the surface reaction. A novel method for assessing protection by adsorbed species is described. The adsorbed substrate molecules acetyl-CoA and phosphate are found to provide substantial protection against thiol-blocking reagents.