The mutagenicity of amino acid analogues in Coprinus lagopus

Specificity of the G1 block induced by ethionine in human lymphocytes in vitro: a flow cytometric analysis

Ethionine is the ethyl analogue of the amino acid methionine. The agent is well known to have a weak demethylating activity. In addition, its capacity to reversibly block the cell cycle progression in G1 human lymphocytes (HL) without interfering with blastic transformation has been reported. In order to better understand the mechanism by which the agent is able to induce cell cycle block, experiments have been performed by using flow cytometry, in HL. In particular the hypothesis of the involvement of a specific target at the G0/G1 boundary was tested by treating HL at different times after blastic transformation. Starting from the 40th hour after PHA stimulation, ethionine loses its blocking capacity in such a way that cells challenged by the agent do not differ from controls in any one of the tested cell cycle-related parameters. We suggest the agent exerts its blocking activity at a specific stage of the transformation pathway.

Ethionine in the analysis of the possible separate roles of methionine and choline deficiencies in carcinogenesis

The importance of ethionine, the ethyl analogue of methionine, as a metabolic probe to study the possible roles of methionine and choline in liver carcinogenesis has been briefly reviewed. Ethionine-induced liver carcinogenesis is similar in many aspects, including initiation, promotion, and progression, to carcinogenesis with other agents. However, the special role of methionine in preventing virtually all metabolic and pathologic effects of ethionine, including liver cancer, places ethionine in a special position. On the basis of these observations and our current knowledge about choline deficiency in the genesis of liver cancer, we proposed that choline and methionine play separate but overlapping roles in the initiation and promotion of liver carcinogenesis.

Effect of L-ethionine on spontaneous and MMC-induced SCEs in G0 and G1 human lymphocytes under liquid-holding recovery conditions

L-Ethionine is an ethyl analogue of the amino acid, methionine, well known as a carcinogen but not as a mutagen. Its activity is clearly related to its capacity to hypomethylate DNA and RNA. At a final concentration of 5 mM, L-ethionine completely inhibits DNA synthesis in PHA-stimulated human lymphocytes, probably acting on a methylation reaction critical for the initiation of the S phase. This block can be reversed. Utilizing this capacity of L-ethionine to block cell proliferation, we have studied the influence of G0 and G1 repair of premutational damage (PMD) (equivalent to liquid-holding recovery in bacteria) on spontaneous and MMC-induced SCEs in human lymphocytes. Our results clearly show that L-ethionine in our experimental conditions significantly increases the frequencies of spontaneous and MMC-induced SCEs. In view of the hypomethylating activity of the analogue, we hypothesize that this action at the replication fork level may increase the error-prone ligation of unrepaired lesions, thus influencing the frequency of occurrence of SCEs.

Mutagenesis and anti-mutagenesis in Salmonella: influence of ethionine and caffeine on yields of mutations induced by 2-aminopurine and 9-aminoacridine

Ethionine, the ethyl analogue of methionine, slightly reduced the yield of reversions of the hisC3076 frameshift marker induced by 9-aminoacridine (9AA) in an excision-proficient strain of Salmonella typhimurium, but completely abolished mutagenesis by 9AA in the excision-deficient uvrB-deletion strain TA1537. No toxic effects of ethionine were apparent in either the excision-proficient or the excision-deficient strain. Because of the differential effects of ethionine on mutagenesis in the two strains, it seemed possible that an ethionine-sensitive step in the process(es) leading to fixation of 9AA-induced mutations might be compensated for by the uvrA,B,C+ excision-repair system. To further test this possibility, we used caffeine (a compound known to significantly reduce the efficacy of the excision-repair process) as a co-treatment with ethionine for cells of an excision-proficient strain exposed to 9AA. Treatment with caffeine alone or ethionine alone had very little effect on reversion yield, whereas co-treatment with the two agents abolished 9AA mutagenesis. It appeared, therefore, that either the caffeine-sensitive pathway or the ethionine-sensitive pathway needed to be functioning if 9AA-induced reversions of hisC3076 marker were to be detected. Addition of methionine to cells of the excision-deficient strain exposed to 9AA restored their ability to be mutated by 9AA, however. In a base-pair substitution back-mutation system, ethionine slightly enhanced the yields of revertants of the trpE8 marker induced by 2-aminopurine (2AP) in both an excision-proficient strain (at all 2AP dose levels tested) and an excision-deficient strain (only at the lower dose levels). In the excision-deficient strain, doses of 2AP above 300 micrograms/plate were highly toxic when ethionine was also present. It was for this reason that no 2AP-induced revertants were recovered at the higher 2AP concentrations. Treatment of the trpE8 strain with methionine also enhanced the yield of 2AP-induced revertants of this marker.

Effects of ethionine on the replicational fidelity in V79 chinese hamster cells

The possibility that ethionine, the ethyl analog of methionine and potent liver carcinogen, exerts its action by blocking the methylation of DNA and thereby rendering the post-replicative methylation instructed error-avoidance system inoperative was investigated. While the results are not directly supportive for the existence of such a repair system in V79 Chinese hamster cells, effects of ethionine were found. Following the exposure of ethionine-treated cells to EMS an increase in cell killing and a decrease in mutation induction was observed. The base analog, 6-hydroxyaminopurine, was shown to be clearly mutagenic in the mammalian cells and in the presence of ethinine a drastic decrease in mutant frequency was observed. Ethionine itself did not appear mutagenic over the entire dose range tested (1-1000 micrograms/ml).

DNA polymerase of a basidiomycete fungus, Coprinus cinereus

DNA polymerase activity was studied in Coprinus cinereus, a basidiomycete fungus. Only one from of the enzyme could be demonstrated, whether by affinity or ion-exchange chromatography; this enzyme had a molecular weight of 185000 on Sephadex G-200, and was inhibited by mercaptoethanol. Coprinus, a representative of the most advanced type of the filamentous fungi, resembles other eukaryotic micro-organisms in its lack of a mammalian beta-type DNA polymerase. The properties of the polymerase are compared with those of two other fungi, and found to resemble most closely the yeast polymerase A in Mg2+ requirements and template preference.

Isolation of Neurospora crassa bradytrophs

A method was developed for the isolation of Neurospora bradytrophs. The bradytrophs (representing lesions in a number of pathways) were resistant to DL-p-fluorophenylalanine when growing in a leaky fashion but were sensitive when grown in the presence of their stimulating supplement.

Toxic and mutagenic effects of carcinogens on the mitochondria of Saccharomyces cerevisiae

Nineteen haploid yeast (Saccharomyces cerevisiae) strains were used to assess the relative growth inhibitory potencies on fermentable vs. non-fermentable media of a collection of carcinogenic and non-carcinogenic chemicals. The majority of carcinogens were distinctly more potent on the non-fermentable (glycerol) medium, where mitochrondrial function is required for growth, than on the fermentable medium, where it is not. The anti-mitochondrial selectivity indicated by these growth tests was much slighter for the non-carcinogens. Similarly most carcinogens induced the cytoplasmic petite mutation whereas the non-carcinogens did not. Five carcinogens which were tested impaired the development of cytochromes aa3 and b in glucose cultures. Six carcinogens, when tested, inhibited growth on three fermentable sugars, the utilisation of which requires mitochondrial function. Out of five carcinogens which were examined, four suppressed the surface-dependent phenomenon of fluocculence in a flocculating strain of yeast, at concentrations primarily affecting the mitochondrial system; the fifth had a similar but less pronounced effect.

The modification of induced genetic change in yeast by an amino acid analogue

Treatment of diploid yeast cultures with the amino acid analogue, para-fluorophenylalanine (PFPA), at concentrations which caused inhibition of growth, resulted in up to 5 fold increases in the frequency of mitotic gene conversion at two different heteroallelic loci. With haploid yeast cultures, growth in PFPA increased the rate of forward mutation to canavanine resistance by at least 2 fold. Growth of diploids in PFPA prior to exposure to the deaminating agent nitrous acid, the cross-linking agent mitomycin C, the alkylating chemical ethylmethanesulphonate (EMS) and UV light resulted in significant changes in the potency of these diverse mutagens to induce intragenic recombination. For all four mutagens, increased frequencies of gene convertants/viable cell were observed in those cultures which had been exposed to the amino acid analogue prior to mutagen treatment. In haploid WT yeast cells, amino acid analogue incorporation resulted in an enhanced frequency of UV induced forward mutation to canavanine resistance whilst in a DNA repair deficient rad 6 mutant this interaction between UV and PFPA was abolished. The results have been interpreted on the basis of incorporation of the analogue into enzymes involved with DNA replication with a consequent loss of fidelity of such enzymes and increased errors in base incorporation.

Mutational synergism between p-fluorophenylalanine and UV in Coprinus lagopus

Previous studies have shown that the amino acid analogue p-fluorophenylalanine (PFP) is mutagenic to Coprinus lagopus due to its incorporation into proteins [32]. Spontaneous mutations, PFP and UV mutagenesis and PFP/UV synergism have been studied in a UV resistant strain and in two complementing UV sensitive mutant strains. By comparison to the UV resistant strain, one UV sensitive strain shows normal spontaneous mutations, 1.4% PFP-induced mutations and 50-fold UV mutagenesis. The second UV sensitive strain has 19-fold spontaneous mutation frequency, 8% PFP induced mutations and slightly elevated UV mutagenesis. In all 3 strains the PFP/UV synergism is comparable (4--5 times the arithmetic expected). The results indicate that PFP mutagenesis is due to the incorporation of PFP into enzymes normally functioning in the organism but which also participate in UV repair mechanisms. A model is proposed for UV repair which is based on a PFP sensitive excision repair system of at least two enzymes, and alternative "error proof" pathway which is not suscetible to PFP and an "error prone" pathway which is responsible for UV mutagenesis and is susceptible to PFP as shown by the PFP/UV synergism. Because PFP is given before UV treatment, this implies a UV inducible cofactor and a PFP sensitive enzyme which only functions after UV activation.

Detection of carcinogens as mutagens in the Salmonella/microsome test: assay of 300 chemicals: discussion

About 300 carcinogens and non-carcinogens of a wide variety of chemical types have been tested for mutagenicity in the simple Salmonella/microsome test. The test uses bacteria as sensitive indicators of DNA damage, and mammalian liver extracts for metabolic conversion of carcinogens to their active mutagenic forms. There is a high correlation between carcinogenicity and mutagenicity: 90% (157/175) of the carcinogens were mutagenic in the test, including almost all of the known human carcinogens that were tested. Despite the severe limitations inherent in defining non-carcinogenicity, few "non-carcinogens" showed any degree of mutagenicity [McCann et al. (1975) Proc. Nat. Acad. Sci. USA 72, 5135-5139]. In the present paper, carcinogens negative in the test andapparent false positives are discussed. We also discuss evidence that chemical carcinogens and radiation, likely to initiate most human cancer and genetic defects do so by damage to DNA. The Salmonella test can play a central role in a program of prevention: to identify mutagenic chemicals in the environment (all indications are there are many) and to aid in the development of non-mutagenic products to prevent future human exposure.

A rapid rise in cell variants during the senescence of populations of human fibroblasts

An attempt has been made to measure the frequency of mutations throughout the lifespan of human fibroblast strain MRC-5. A novel procedure has been used which involves staining individual cells for high levels of glucose-6-phosphate dehydrogenase. Evidence is presented that this phenotype is due to mutation. The frequency of variants was scored from passage 16 until the final phase of senescence (passage 60). There is an exponential increase of stained cells throughout this period. The results are in agreement with the general error theory, which proposes that aging is due to a breakdown in the fidelity of information transfer between macromolecules.

Effects of p-fluorophenylalanine on the induction of mutations in bacteriophage T4. I. 5-Bromouracil mutagenesis

The amino acid analogue rho-fluorophenylalanine (PFPA) was found to have no mutagenic activity in the gamma system of bacteriophage T4. However, under standard conditions for 5-bromouracil (5-BU) mutagenesis, PFPA depressed the induced frequencies for both forward and reverse mutations. When the folate antagonist sulphanilamide (SU) was omitted from the mutangenic treatment medium or when it was replaced by Trimethoprim (TM), another folate antagonist, this depressive effect was abolished. It was proposed that PFPA alleviated the inhibitory action of SU.