Mutagenicity studies on coffee. The influence of different factors on the mutagenic activity in the Salmonella/mammalian microsome assay

The effects of partial thiamin deficiency and oxidative stress (i.e., glyoxal and methylglyoxal) on the levels of α-oxoaldehyde plasma protein adducts in Fischer 344 rats

We hypothesized that in marginal thiamin deficiency intracellular alpha-oxoaldehydes form macromolecular adducts that could possibly be genotoxic in colon cells; and that in the presence of oxidative stress these effects are augmented because of decreased detoxification of these aldehydes. We have demonstrated that reduced dietary thiamin in F344 rats decreased transketolase activity and increased alpha-oxoaldehyde adduct levels. The methylglyoxal protein adduct level was not affected by oral glyoxal or methylglyoxal in the animals receiving thiamin at the control levels but was markedly increased in the animals on a thiamin-reduced diet. These observations are consistent with our suggestion that the induction of aberrant crypt foci with marginally thiamin-deficient diets may be a consequence of the formation of methylglyoxal adducts.

Protein identification by peptide mass fingerprinting and peptide sequence tagging with alternating scans of nano-liquid chromatography/infrared multiphoton dissociation Fourier transform ion cyclotron resonance mass spectrometry

We have developed a method for protein identification with peptide mass fingerprinting and sequence tagging using nano liquid chromatography (LC)/Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). To achieve greater sensitivity, a nanoelectrospray (nano-ES) needle packed with reversed-phase medium was used and connected to the nano-ES ion source of the FTICR mass spectrometer. To obtain peptide sequence tag information, infrared multiphoton dissociation (IRMPD) was carried out in nano-LC/FTICR-MS analysis. The analysis involves alternating nano-ES/FTICR-MS and nano-ES/IRMPD-FTICR-MS scans during a single LC run, which provides sets of parent and fragment ion masses of the proteolytic digest. The utility of this alternating-scan nano-LC/IRMPD-FTICR-MS approach was evaluated by using bovine serum albumin as a standard protein. We applied this approach to the protein identification of rat liver diacetyl-reducing enzyme. It was demonstrated that this enzyme was correctly identified as 3-alpha-hydroxysteroid dehydrogenase by the alternating-scan nano-LC/IRMPD-FTICR-MS approach with accurate peptide mass fingerprinting and peptide sequence tagging.

Identification of human liver diacetyl reductases by nano-liquid chromatography/Fourier transform ion cyclotron resonance mass spectrometry

Several forms of diacetyl-reducing enzyme were found to exist in the human liver cytosol. Three (DAR-2, DAR-5, and DAR-7) of them were purified as a single band on SDS-PAGE by a combination of a few kinds of column chromatographies. The in-gel tryptic digests of the purified enzymes were analyzed by nano-liquid chromatography (LC)/Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS), which provided peptide masses at a ppm-level accuracy. The enzymes, DAR-2, DAR-5, and DAR-7, were identified as alcohol dehydrogenase beta subunit (ADH2), carbonyl reductase (CBR1), and aldehyde reductase (AKR1A1), respectively, by peptide mass fingerprinting. In addition, an alternating-scan acquisition of nano-LC/FT ICR mass spectra, i.e., switching of normal acquisition conditions and in-source fragmentation conditions scan by scan, provided sets of parent and fragment ion masses of many of the tryptic peptides in a single LC/MS run. The peptide sequence-tag information at the ppm-level accuracy was used to further confirm the protein identities. It was demonstrated that nano-LC/FT ICR MS can be used for rigorous protein identification at a subpicomole level as an alternative technique to nano-LC/MS/MS.

Genotoxicity of instant coffee: possible involvement of phenolic compounds

Instant coffee exhibits direct genotoxic activity in the tester strains TA 98, 100, 102, 104 and YG 1024. In the Ames tester strain TA 100, the presence of S9 mix, S100 mix, S9 mix without cofactors led to a significant decrease of the genotoxicity observed. The decrease observed in the presence of S9 mix seems to be highly correlated with the catalase content of S9 mix. The genotoxicity of instant coffee detected in strain TA 100 was dependent on the pH, with higher genotoxic effects at pH values above neutrality. Also, dependent on the pH was the ability of some phenolic molecules present in coffee promoting the degradation of deoxyribose in the presence of Fe3+/EDTA. These results suggest that apart from other molecules present in instant coffee responsible for their genotoxicity in several short term assays, phenolic molecules could also be implicated in the genotoxicity of coffee, via reactive oxygen species arising from its auto-oxidation.

Identification of hydroxyhydroquinone in coffee as a generator of reactive oxygen species that break DNA single strands

A component in instant coffee that caused DNA single strand breaks was isolated by successive ethyl acetate:ethanol extraction, silica gel column chromatography and high performance liquid chromatography using a reversed phase column. The active component was identified as hydroxyhydroquinone (HHQ). Incubation of supercoiled pBR 322 DNA with HHQ at 0.1 mM in phosphate buffer (pH 7.4) at 37 degreesC for 1 h caused single strand breaks, and reactive oxygen species, hydrogen peroxide and hydroxyl radical, were involved in DNA breaking by HHQ. Genotoxic effects of HHQ including DNA breaking activity through generation of reactive oxygen species have been well-demonstrated because the component is considered to be an important genotoxic intermediate metabolite of benzene. Occurrence of HHQ in coffee must have an important significance to consider genotoxicity of coffee.

Genetic damage induced by methylglyoxal and methylglyoxal plus X-rays in Drosophila melanogaster germinal cells

The effect of methylglyoxal (MG) and MG administered prior to X-irradiation was investigated in Drosophila melanogaster germinal cells using the sex-linked recessive lethal (s.l.r.l.), II-III autosomal translocation (AT) and X-chromosome nondisjunction (ND) tests. For the s.l.r.l. test the males were either injected with MG (0.5 M, 0.75 M or 1.7 M) or fed for 24 h (1 M) and two 24 h broods (A and B) were obtained. For the AT test the males were injected with MG 1.7 M and the same brooding scheme was followed. ND was tested in females fed on MG 1 M. The only effect observed after MG treatment was a significant increase on the yield of s.l.r.l. with MG 1.7 M. In the combined treatments MG was administered prior to irradiation with 20 Gy of X-rays and the induction of s.l.r.l. and AT was assessed. Pre-treatment with MG 0.75 M and 1.7 M enhanced the frequency of s.l.r.l. in cells sampled in brood B, consisting mainly of the rather hypoxic late spermatids. It is suggested that this radiosensitizing effect could be ascribed to a decrease in the level of glutathione due to the metabolization of MG.

32P-Postlabeling analysis of a DNA adduct, an N2-acetyl derivative of guanine, formed in vitro by methylglyoxal and hydrogen peroxide in combination

Methylglyoxal is a direct-acting mutagen in Salmonella typhimurium TA100 and its mutagenicity is markedly enhanced in the presence of hydrogen peroxide. In addition, a mixture of methylglyoxal and hydrogen peroxide reacts with 2'-deoxyguanosine to form N2-acetyl-2'-deoxyguanosine. We examined whether the guanine residues in DNA were acetylated by methylglyoxal in the presence of hydrogen peroxide using the 32P-postlabeling method. First, N2-acetyl-2'-deoxyguanosine 3'-monophosphate and N2-acetyl-2'-deoxyguanosine 3,5'-diphosphate were chemically synthesized as standard compounds for the analysis. Then calf thymus DNA (3.24 micromol) was treated with methylglyoxal (64.8 micromol) at pH 7.4 for 3 h at 37 degrees C, and subsequently with hydrogen peroxide (64.8 micromol) at 37 degrees C for 2 h. The adduct formation was analyzed using HPLC in combination with the 32P-postlabeling method under the standard conditions. N2-Acetyl-2'-deoxyguanosine was detected at levels of 2/10(6) nucleotides in double-stranded DNA and 1/10(5) nucleotides in single-stranded DNA. The estimated limit of detection by our method was 3 per 10(8) nucleotides.

The inhibitory effects of coffee on radical-mediated oxidation and mutagenicity

Hydrogen peroxide (H2O2) has been implicated as a major contributor to coffee mutagenicity and genotoxicity in vitro. We have used three assays to show the gradual formation of H2O2 in freshly prepared roasted ground coffee and in instant coffees over time reaching levels of 400-450 microM after a 1-h incubation period. Formation of H2O2 occurs through an auto-oxidation process where polyphenolics, in the presence of transition metals, reduce atmospheric oxygen. However, because of these polyphenolics, coffee also possesses in vitro antioxidant activity as shown by its capacity to inhibit lipid peroxidation in Fenton-catalysed hydroxylation reactions. The pro- and antioxidative effects of coffee are also reflected in its mutagenic and antimutagenic activity in the Ames test. Coffee is directly mutagenic in strains TA100 and TA102 due to H2O2 formation. However, coffee is also an antioxidant and antimutagen. This beverage exerts a strong protective effect against the mutagenicity and cytotoxicity induced by the oxidant t-butylhydroperoxide (t-BOOH). Thus, coffee, like many antioxidants, exhibits dual effects in vitro which are highly dependent upon parameters such as dose, atmospheric oxygen, transition metals as well as the biological and chemical endpoints used for measurement. Consequently, the data obtained on the pro- and antioxidant properties of foods and beverages from in vitro bioassays must be interpreted with caution and the results are not easily extrapolated in vivo to assess the impact on human health.

Possible occurrence of new mutagens with the DNA breaking activity in coffee

Brewed and instant coffee emitted strong chemiluminescence due to singlet oxygen and excited carbonyls, which may be originated by the Maillard reaction of sugars and amino acids but not by the reaction of polyphenolics. Instant coffee cleaved DNA giving single-strand breaks only after it was purified by high performance liquid chromatography (HPLC) or by gel filtration. Retention times of component(s) with strong DNA breaking activity in HPLC were different from those of chemiluminescence emitters, although they were coeluted on a gel filtration. The major DNA breaking component(s) must be different from chemiluminescence emitters. Active oxygen radicals participated little in DNA breaking because active oxygen radical scavengers had only a marginal effect on DNA breaking of the active gel fraction. DNA breaking by the active gel fraction was inhibited by high concentrations of inorganic salts probably because the salts stabilized the DNA double strands. The active gel fraction was mutagenic to Salmonella typhimurium TA98 without metabolic activation. The number of His+ revertant colonies/g of instant coffee powder was estimated to be 4000.

Potential genotoxic, mutagenic and antimutagenic effects of coffee: a review

Coffee and caffeine are mutagenic to bacteria and fungi, and in high concentrations they are also mutagenic to mammalian cells in culture. However, the mutagenic effects of coffee disappear when bacteria or mammalian cells are cultured in the presence of liver extracts which contain detoxifying enzymes. In vivo, coffee and caffeine are devoid of mutagenic effects. Coffee and caffeine are able to interact with many other mutagens and their effects are synergistic with X-rays, ultraviolet light and some chemical agents. Caffeine seems to potentiate rather than to induce chromosomal aberrations and also to transform sublethal damage of mutagenic agents into lethal damage. Conversely, coffee and caffeine are also able to inhibit the mutagenic effects of numerous chemicals. These antimutagenic effects depend on the time of administration of coffee as compared to the acting time of the mutagenic agent. In that case, caffeine seems to be able to restore the normal cycle of mitosis and phosphorylation in irradiated cells. Finally, the potential genotoxic and mutagenic effects of the most important constituents of coffee are reviewed. Mutagenicity of caffeine is mainly attributed to chemically reactive components such as aliphatic dicarbonyls. The latter compounds, formed during the roasting process, are mutagenic to bacteria but less to mammalian cells. Hydrogen peroxide is not very active but seems to considerably enhance mutagenic properties of methylglyoxal. Phenolic compounds are not mutagenic but rather anticarcinogenic. Benzopyrene and mutagens formed during pyrolysis are not mutagenic whereas roasting of coffee beans at high temperature generates mutagenic heterocyclic amines. In conclusion, the mutagenic potential of coffee and caffeine has been demonstrated in lower organisms, but usually at doses several orders of magnitude greater than the estimated lethal dose for caffeine in humans. Therefore, the chances of coffee and caffeine consumption in moderate to normal amounts to induce mutagenic effects in humans are almost nonexistent.

Contribution of coffee aroma constituents to the mutagenicity of coffee

About 40 coffee aroma constituents belonging to the classes of dicarbonyls, sulphur-containing compounds, furfuryls, N-heterocyclics and others were systematically evaluated in three Ames tester strains. Only aliphatic dicarbonyl compounds showed notable direct mutagenic activity, which mainly affected 'base-pair substitution' in Ames tester strains TA100 and TA102. Very weak effects were also seen with some N-heterocyclics, mainly affecting frameshift tester strain TA98 upon metabolic activation. However, it was shown that these N-heterocyclics do not contribute substantially to the mutagenicity in coffee. The hydrogen peroxide and methylglyoxal contents of coffee were determined up to 26 hr after preparation. Their concentrations tended to decrease whereas mutagenic activity decreased significantly with time in tester strains TA100 and TA102. It is concluded that several highly labile coffee constituents contribute to the bacterial mutagenicity and also that the synergism between hydrogen peroxide and methylglyoxal is not the main factor. The absence of coffee mutagenicity/carcinogenicity in rodents with these highly reactive coffee aroma compounds can be explained in part by detoxification of microsomal enzyme systems.

Study of the causes of direct-acting mutagenicity in coffee and tea using the Ara test in Salmonella typhimurium

The mutagenic activities of 6 of the chemicals identified in coffee solutions were assayed with the Salmonella Ara test, under experimental conditions optimized for coffee mutagenicity. Caffeine was the only non-mutagenic compound. Among the other 5 chemicals, hydrogen peroxide was the strongest mutagen and chlorogenic acid the weakest; methylglyoxal, glyoxal and caffeic acid exhibited intermediate mutagenicities. The minimal mutagenic doses of these components correlated negatively with their relative concentrations in coffee. It was concluded that chlorogenic acid, caffeic acid, glyoxal and methylglyoxal cannot contribute alone to the mutagenicity of coffee in the Ara test, since their minimal mutagenic concentrations were much higher than their respective levels in the coffee samples assayed. By contrast, 40-60% of the mutagenic activity in coffee and also in tea could be attributed to their H2O2 contents. Catalase abolished more than 95% of the mutagenic activity of coffee, as detected by the Ara test. A similar sensitivity to catalase has been reported by other authors in relation to the coffee mutagenicity identified by the Salmonella His test. Nevertheless, the results presented in this paper suggest that the Ara forward and the His reverse mutation tests are sensitive to the mutagenicity of different constituents in coffee solutions. We propose that the His test, sensitive at high coffee doses, mainly recognizes the mutagenicity of methylglyoxal, whilst the Ara test, sensitive at low coffee doses, mainly detects the mutagenic activity of hydrogen peroxide. The data reported also suggest that the direct-acting mutagenicity(ies) detected by the Ara test in tea solutions is (are) based on similar, if not identical, mechanisms.

Increased sister chromatid exchange associated with smoking and coffee consumption

Sister chromatid exchange (SCE) is a very sensitive cytogenetic assay for detecting exposure to chemical mutagens and carcinogens. One application of SCE is the monitoring of populations believed to be exposed to such agents. We have, however, relatively little knowledge about common lifestyle factors that may influence SCE and therefore complicate any study designed to examine the effects of exposure to genotoxins. In this study, we assessed the effect of cigarette smoking and coffee consumption on SCE. Smoking was associated with an increase of approximately 2 SCEs per cell and a decrease in cell proliferation. A positive linear relationship between SCE and coffee consumption was also observed. This effect was similar for smokers and nonsmokers. Additionally, the folic acid content of cell culture medium seemed to affect neither SCE nor cell proliferation.

Coffee is highly mutagenic in the L-arabinose resistance test in Salmonella typhimurium

The present study shows that the L-arabinose resistance test in Salmonella typhimurium detects coffee as a strong mutagen in the absence of mammalian microsomal activation. The response of the Ara forward mutation assay was 8.5 times higher than that of TA104, which is the most sensitive to coffee of the tester strains of the Ames test. Both the mutagenesis protocol (preincubation test) and the additional genetic characteristics of the bacterial tester strain (excision repair deficiency, normal lipopolysaccharide barrier, and the presence of plasmid pKM101) were critical factors in the optimal induction by coffee of forward mutations to L-arabinose resistance. All ten samples of roasted coffee analyzed with the Ara assay were highly mutagenic: one cup of coffee (150 ml) was calculated to induce 3-4 X 10(6) AraR mutants. In contrast, coffee prepared from unroasted beans (green coffee) had no mutagenic activity. Regular- and sugar-roasted coffees showed similar mutagenicities, but the specific mutagenic activity of instant coffees (1559 AraR mutants/mg) was almost 2 times that of noninstant ones (834 AraR mutants/mg). The Ara assay allowed the direct testing of coffee, although it was demonstrated that lyophilization has no effect on the mutagenicity of this beverage. Like roasted coffee, roasted barley induced a large number of AraR mutants per mg (227), though its specific mutagenic activity was approximately 4 and 7 times lower than that of noninstant and instant coffees, respectively.

Implication of hydrogen peroxide in the mutagenicity of coffee

A cup of instant coffee (150 ml) of normal strength (15 mg/ml) was found to contain about 500 and 750 micrograms of hydrogen peroxide soon after its preparation at 37 degrees C and 80 degrees C, respectively, but the concentration of hydrogen peroxide in the coffee increased with time for up to 24 h after its preparation. Thus coffee contains a hydrogen peroxide generating system. As extracts of green coffee beans were found to have very low capacity to generate hydrogen peroxide, this generating system is produced by roasting coffee beans. Hydrogen peroxide itself was only weakly mutagenic to Salmonella typhimurium TA100, but in the presence of methylglyoxal, which is also present as a mutagenic component in coffee, hydrogen peroxide showed strong mutagenicity. Hydrogen peroxide and methylglyoxal seem to be responsible for most of the mutagenicity of instant coffee.