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Biomedical subjects

G N Wogan

Publications and source records attributed to G N Wogan.

At least 91 records · Page 5Linked to original sources
PubMed1983

Salt-induced conversion of B-DNA to Z-DNA inhibited by aflatoxin B1.

The carcinogen aflatoxin B1 was reacted with a polymer of alternating deoxyguanine and deoxycytosine residues to determine the effect that adduct formation has on the conversion of this polymer from the right-handed B-DNA form found at low salt concentrations to the left-handed Z-DNA form found at high salt concentrations. Reaction with aflatoxin strongly inhibited the salt-induced conversion of this polymer from B-DNA to Z-DNA. This inhibition could be detected even at relatively low binding levels.

Aflatoxin B1↗
PubMed1982

Quantitation of aflatoxin B1-modified DNA using monoclonal antibodies.

Five monoclonal antibodies were obtained after fusion of mouse P3 x 63 myeloma cells with spleen cells isolated from BALB/c mice that had been immunized with aflatoxin B1-adducted DNA complexes with methylated bovine serum albumin. Selected hybridomas were found to produce monoclonal antibodies specific for modified DNA containing both the 2,3-dihydro-2-(N7-guanyl)-3-hydroxyaflatoxin B1 and the putative 2,3-dihydro-2-(N5-formyl-2',5',6'-triamino-4'-oxo-N5-pyrimidyl)-3-hydroxyaflatoxin B1, suggesting that these DNA adducts share a common antigenic determinant. Enzyme immunoassay conditions using these monoclonal antibodies were optimized, and DNA isolated from the livers of rats given dosages of aflatoxin B1 ranging from 0.01 to 1.0 mg aflatoxin B1 per kg body weight was tested. A level of modification in DNA of 1 aflatoxin B1 residue per 1,355,000 nucleotides can be quantitatively measured. Monoclonal antibodies will be useful probes for studying the molecular interactions of aflatoxin B1 with DNA and the occurrence of aflatoxin B1:DNA adducts in tissues and cells of humans exposed to this environmental carcinogen.

Aflatoxin B1↗
PubMed1981

Determination of partition ratios for allylisopropylacetamide during suicidal processing by a phenobarbital-induced cytochrome P-450 isozyme from rat liver.

Allylisopropylacetamide is shown to be a suicide substrate for the phenobarbital-inducible cytochromes P-450. In phenobarbital-induced rat liver microsomes about 70% of the cytochrome P-450-mediated N,N-dimethylaniline N-demethylase activity is sensitive to allylisopropylacetamide inactivation; the residual 30% of the N-demethylase activity is incapable of allylisopropylacetamide turnover and insensitive to allylisopropylacetamide inactivation. The partition number for inactivation of the susceptible population of cytochrome P-450 indicates turnover of 201 molecules of allylisopropylacetamide per molecule of P-450 inactivated. A purified phenobarbital-induced isozyme of cytochrome P-450, when reconstituted with purified rat liver cytochrome P-450 reductase, is also inactivated by allylisopropylacetamide in a suicide fashion with a corrected partition ratio of 184 turnovers of allylisopropylacetamide per inactivation event. This partition number is corrected for the competing O2-dependent autoinactivation of cytochrome P-450 which we have previously shown to occur with the purified isozyme (Loosemore, M., Light, D. R., and W#alsh, C. (1980) J. Biol. Chem. 255, 9017-9020). The 201 product molecules of cytochrome P-450-mediated turnover of allylisopropylacetamide in either the microsomal or purified enzyme system are probably the epoxide, are reactive toward alkylation of cellular nucleophiles, and covalently modify protein and exogenous calf thymus DNA molecules.

Acetamides↗
PubMed1981

Monoclonal antibody to aflatoxin B1-modified DNA detected by enzyme immunoassay.

Monoclonal antibodies were obtained after fusion of mouse P3 X 63 myeloma cells with spleen cells isolated from BALB/c mice that had been immunized with aflatoxin B1-adducted DNA complexed with methylated bovine serum albumin. Selected hybridomas were found to produce monoclonal antibodies specific for aflatoxin B1-modified DNA containing both the 2,3-dihydro-2-(N7-guanyl)-3-hydroxyaflatoxin B1 and the putative 2,3-dihydro-2-(N5-formyl-2',5',6'-triamino-4'oxo-N5-pyrimidyl) -3-hydroxyaflatoxin B1, suggesting that these DNA adducts share a common antigenic determinant. The monoclonal antibody was not reactive towards the free aflatoxin B1-guanine adducts in solution, seven other aflatoxin derivatives, or benzo[a]pyrene-adducted DNA. A noncompetitive ultrasensitive enzyme radioimmunoassay could measure 15 fmol of aflatoxin B1-DNA adducts in 10 ng of DNA and was at least 100-fold more sensitive than the standard enzyme-linked immunosorbent assay. Competitive enzyme-linked immunosorbent assay with these monoclonal antibodies reliably quantitated aflatoxin B1 adducted in vivo to rat liver DNA at adduct levels of one aflatoxin B1 residue per 250,000 nucleotides. The competitive ultrasensitive enzyme radioimmunoassay was determined to be at least 6-fold more sensitive than the competitive enzyme-linked immunosorbent assay in analysis of aflatoxin B1-adducted DNA. Therefore, enzyme immunoassay using monoclonal antibodies will be useful analytical tools for studying both the molecular interactions of aflatoxin B1 with DNA and the occurrence of aflatoxin B1-DNA adducts in biological specimens from people exposed to this environmental carcinogen.

Aflatoxin B1↗
PubMed1981

In vitro reactions of aflatoxin B1-adducted DNA.

The chemical stability of aflatoxin B1 bound to calf thymus DNA was studied over a 48-hour exposure to phosphate buffers at pH 6.8-8.0 (37 degrees C). During this time, aliquots of the aflatoxin B1-modified DNA were acid-hydrolyzed and analyzed for the presence of 2,3-dihydro-2-(N7-guanyl)-3-hydroxyflatoxin B1, 2,3-dihydro-2,3-dihydroxy-aflatoxin B1, and the tentatively identified, 2,3-dihydro-2-(N5-formyl-2',5',6'-triamino-4'-oxo-N5-pyrimidyl-3-hydroxyflatoxin B1 and 2,3-dihydro-2-(8,9-dihydro-8-hydroxy-N7-guanyl)-3-hydroxyaflatoxin B1. Initial experiments determined the stability of 2,3-dihydro-2-(N7-guanyl)-3-hydroxyaflatoxin B1 in DNA at levels of modification of one residue per 60 and 1500 nucleotides. The acid-hydrolysis products obtained from these modified nucleic acids were qualitatively similar, but their proportional concentrations were different. These quantitative differences were dependent upon both pH and the initial level of modification of the DNA. During the first 6 hr of incubation, under all conditions examined, the formation of 2,3-dihydro-2,3-dihydroxyaflatoxin B1 was responsible for the initial decrease of the 2,3-dihydro-2-(N7-guanyl)-3-hydroxyaflatoxin B1 adduct in DNA. After 48 hr of incubation at pH 7.0, the major reaction of the modified DNA was depurination of the 2,3-dihydro-2-(N7-guanyl)-3-hydroxyaflatoxin B1 adduct. However, at pH 8.0, the predominant reaction product formed in 48 hr was the putative 2,3-dihydro-2-(N5-formyl-2',5',6'-triamino-4'-oxo-N5-pyrimidyl)-3-hydroxy-aflatoxin B1. The putative DNA-bound products resulting from the elimination of the positive charge in the imidazole ring of the aflatoxin-N7-guanine adduct [2,3-dihydro-2-(N5-formyl-2',5',6'-triamino-4'-oxo-N5-pyrimidyl)-3-hydroxy-aflatoxin B1 and 2,3-dihydro-2-(8,9-dihydro-8-hydroxy-N7-guanyl)-3-hydroxyaflatoxin B1] were found to be stable in DNA for at least 24 hr at both pH 6.8 and 7.4. Taken together with observed patterns of stability of aflatoxin B1 adducts in vivo, these observations strongly suggest the involvement of enzymatic repair processes in removal of the N7-guanyl adduct and also emphasize the possible biological significance of the stable imidazole ring-opened adduct.

Aflatoxin B1↗
PubMed1981

Interaction of aflatoxin B2 with rat liver DNA and histones in vivo.

The interaction of aflatoxin B2 (AFB2) in vivo with rat liver nuclear macromolecules was examined in an attempt to correlate this binding with biological potency. The incorporation of [3H]AFB2 residues into rat liver histones and DNA was determined 2, 24 and 48 h following administration of a single i.p. dose of 1 mg [3H]AFB2/kg body weight. At each time point, histone H1 and the total histone fraction contained 5--30-fold more [3H]AFB2 moieties than did DNA on a weight basis. Analytical reversed-phase h.p.l.c. of the acid hydrolysis products resulting from AFB2 binding to DNA revealed that 85% of the radioactivity co-chromatographed with the major aflatoxin B1-DNA adduct, 2,3-dihydro-2-(N7-guanyl)-3-hydroxyaflatoxin B1. These studies revealed an apparent correlation between AFB2 derived binding to DNA in vivo in rats and its potency as a toxin and carcinogen in this species.

Aflatoxins↗
PubMed1981

Excretion of an aflatoxin-guanine adduct in the urine of aflatoxin B1-treated rats.

Administration of aflatoxin B1 (AFB1) to rats resulted in the urinary excretion of 2,3-dihydro-2-(N7-guanyl)-3-hydroxyaflatoxin B1. This is the major product formed by the interaction in vivo of AFB1 with rat liver nucleic acids. The adduct was isolated from urine by the combined use of preparative and analytical high-pressure liquid chromatography and was quantitated by measurement of absorbance at 365 nm. The method allowed reproducible quantitation of adduct in urine samples from rats treated with AFB1 by i.p. injection at levels as low as 0.125 mg/kg. Application of the method to urine samples from rats given injections of AFB1 (1 mg/kg) revealed the presence of a compound chromatographically identical to authentic 2,3-dihydro-2-(N7-guanyl)-3-hydroxyaflatoxin B1. Spectral and chemical analysis of microgram quantities of this compound provided strong evidence that this compound is identical to authentic adduct. Measurement of this adduct in the urine of rats given injections of different doses of AFB1 showed that excretion occurs in a dose-dependent manner. Comparison of the dose-response curve for adduct excretion with that previously observed for adduct formation in rat liver DNA in vivo revealed a high degree of qualitative similarity, with the levels of adduct excreted in urine representing 30 to 40% of the levels seen initially in liver DNA.

Aflatoxin B1↗
PubMed1981

Quantitative comparison of covalent aflatoxin-DNA adducts formed in rat and mouse livers and kidneys.

The covalent interactions between aflatoxin B1 (AFB1) and DNA were investigated in the inbred F344 rat and noninbred CD -1 Swiss mouse. A good correlation was found between the level of covalent modification of DNA, species sensitivity, and organ specificity, to the toxic effects of AFB1. The patterns of AFB1 acid hydrolysis products from DNA isolated from the livers and kidneys of both species were examined. The principal acid hydrolysis product in all cases was identified as 2,3-dihydro-3-hydroxy-(N7-guanyl)AFB1. Minor products consisted of adducts formed by the chemical transformation of the AFB1-N7-substituted guanine moiety forming putative formamidopyrimidine derivatives and the activation of AFB1 metabolites, which also modified the N-7 guanine atom. These last-mentioned products were present in greater amounts in resistant tissues. In vitro studies on the activation of AFB1 by microsomal fractions of mouse and rat livers found that mouse liver microsomes were rapidly inactivated.

Aflatoxins↗
PubMed1981

Temporal patterns of covalent DNA adducts in rat liver after single and multiple doses of aflatoxin B1.

We examined patterns of covalent modifications of DNA produced in rat liver after exposure to single and multiple doses of aflatoxin B1. The principal product, previously identified as 2,3-dihydro-3-hydroxy(N7-guanyl) aflatoxin B1, was removed rapidly from liver DNA in vivo after a 0.6-mg/kg dose was administered i.p. to male Fischer rats. This lesion had an apparent half-life of 7.5 hr. Similar kinetics of disappearance was seen for two other aflatoxin adducts, one of which was previously identified as an N7-guanine adduct of aflatoxin P1. The kinetics of formation and disappearance differed for two other products believed to be produced by scission of the imidazole ring of the 7-substituted guanine moiety of the principal adduct in the DNA molecule. These adducts were removed slowly, if at all, during the 72-hr period studied. Approximately 20% of the principal N7 adduct initially formed was converted to these products in 24 hr, at which time they were the predominant lesions in DNA. Administration of multiple doses of aflatoxin B1, using a regimen shown to produce a high incidence of hepatocellular carcinoma, caused accumulation of these persistent products in liver DNA over a 14-day period.

Aflatoxins↗
PubMed1980

There is a correlation between the DNA affinity and mutagenicity of several 3-amino-1-methyl-5H-pyrido[4,3-b]indoles.

3-Amino-1-methyl-5H-pyrido[4,3-b]indole, previously reported to be a component of tryptophan pyrolysates, is an intensely mutagenic compound requiring microsomal activation for expression of mutagenicity. We have found that this species and several synthetic analogs interact noncovalently with calf thymus DNA, as judged by both fluorescence quenching and differential dialysis. Remarkably, this noncovalent interaction correlated with the mutagenic potential of the compounds in a bacterial mutagenesis assay. Therefore, it is suggested that the mechanism of mutagenesis involves metabolic activation followed by physicochemical interaction with DNA; for these compounds, the latter step may be limiting for the expression of mutagenicity.

Animals↗
PubMed1980

Investigation of covalent aflatoxin B1-DNA adducts formed in vivo in rainbow trout (Salmo gairdneri) embryos and liver.

The formation of covalent aflatoxin-DNA adducts has been studied in embryo and yearling stages of the rainbow trout (Salmo gairdneri). A linear relationship was found between the amount of aflatoxin B1 (AFB1) absorbed into 21-day-old eggs and the level of covalent modification of embryo DNA after exposure to 0.25 to 0.5 p.p.m. aqueous solutions of AFB1 for 1 h; higher concentrations resulted in absorption of a greater proportion of AFB1. The principal covalent product, identified after acid and enzymatic hydrolysis of isolated embryo DNA, was chromatographically identical to 2,3-dihydro-2-(N7-guanyl)-3-hydroxy aflatoxin B1. Additional AFB1 derivatives which were present are believed to be formed by chemical transformation of this product in DNA producing an aflatoxin-formamidopyrimidine adduct and the metabolic activation of other aflatoxin metabolites, such as aflatoxin M1 and aflatoxin P1. Although the eggs were exposed to AFB1 for a short time, covalent modification of DNA was evident over an extended period. The highest level of covalent products was present at approximately 24 h after exposure to 0.5 p.p.m. AFB1. Ninety-six hours later, this level had decreased slightly; however, the distribution of covalent adducts had changed: formamidopyrimidine adducts now represented up to 50% of the hydrolyzed aflatoxin derivatives. A similar pattern of covalent aflatoxin derivatives was found in the liver DNA of yearling trout 10 h after the administration of 0.3 mg/kg AFB1.

Absorption↗
PubMed1979

Sterigmatocystin-DNA interactions: identification of a major adduct formed after metabolic activation in vitro.

Sterigmatocystin (ST), a potent hepatocarcinogen, was covalently bound to calf thymus DNA by incubation in the presence of phenobarbital-induced rat liver microsomes. Acid hydrolysis of ST-modified DNA liberated a major guanine-containing adduct, present in DNA at an estimated level of 1 ST residue per 100-150 nucleotides. The adduct was isolated by high-pressure liquid chromatography and subjected to structural analysis. Spectral and chemical data identified the adduct as 1,2-dihydro-2-(N(7)-guanyl)-1-hydroxysterigmatocystin, the guanine and hydroxyl moieties being in a trans configuration. The structure and stereochemistry of this adduct indicated that the exo-ST-1,2-oxide was the metabolite that reacted with DNA, and the quantitative yield of adduct indicated that this metabolite was a major product of the in vitro metabolism of ST.

Animals↗