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may in uence the reactions of the cyclobutanones not evident from their experiments on the puri ed compounds and that further knowledge is needed about the kinetics and metabolism of these compounds in the living organism. Knoll et al. (58) explored the relative sensitivities of human colon cells, representing different stages of tumor development and healthy colon tissues, to 2-DCB. HT29clone19A cells, LY97 adenoma cells, and primary human epithelial cells were exposed to 2-DCB. It was observed that the cyclobutanone was cytotoxic in a time- and dose-dependent manner in LT97 adenoma cells and in freshly isolated primary cells but not in the human colon tumor cell line. An associated induction of DNA damage by 2-DCB was noted in the LT97 cells and in freshly isolated colonocytes, while no strand breaks were detectable in HT29clone19A cells. When LT97 adenoma cells were incubated on a long-term basis with lower concentrations of 2-DCB, cytogenic effects were observed. Knoll and coworkers thus concluded that 2-DCB was genotoxic in healthy human colon epithelial cells and in cells representing preneoplastic colon adenoma. They could not, however, speculate if the cell-speci c effects of the compound were as a result of differences in cellular uptake, metabolism, DNA repair, or other pathways. It was also reported that 2-DCB induces chromosomal aberrations in a human colon adenoma, which are associated with human cancer. However, the authors note that the doses needed to cause the genetic alterations probably by far exceed the normal exposure situation. They went on to state that the amount of 2-DCB found in a 100-g beef burger equates to 3.3 g or 14 nmol (59), is some 50,000-fold lower than effective concentrations in vitro, and thus possibly too low to have a signi cant impact on human health (58). Gadgil and Smith (60) determined the mutagenic potential of 2-DCB using the Ames assay and compared the acute toxicity of 2-DCB with the food additive cylohexanone and t-2-nonenal using the Microtox assay. Cyclohexanone and 2-nonenal are both carbonyl compounds like 2-DCB. The results of this experimental work suggested that 2-DCB is not mutagenic to the Salmonella strains tested and that its acute toxicity is between that of cyclohexanone and 2-nonenal. The work indicated that 2-DCB is similar in toxicity to cyclohexanone, which has generally recognized as safe (GRAS) status in the United States, and was 10 times less toxic than t-2-nonenal, a normal food constituent of cooked ground beef, and an approved food additive (GRAS status avorant). In their 2004 Scienti c Status Summary on Irradiation and Food Safety for the Institute of Food Technologists (IFT), Smith and Pillai (61) concluded that such results indicate that 2-DCB has very low toxicity thereby not warranting concern. Further studies by Gadgil and Smith (62) followed on from the work by Horvatovich et al. (51), as mentioned previously, examined the fate of 2-DCB in rats after consumption by looking at its recovery from the feces and adipose tissue of rats and if any urinary metabolites could be identi ed. From the ndings of this work, it was observed that between 3% and 11% of the total
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amount of 2-DCB given to the rats was recovered from the feces while approximately 0.33% was recovered from adipose tissue. These results indicated that most of the 2-DCB is metabolized and excreted or stored in tissues other than adipose. The most recent study carried out on the cyto- and genotoxicity potential of 2-alkylcyclobutanones of varying chain length in different cell lines was conducted by Hartwig et al. (63). Studies undertaken included the (i) impact of the cyclobutanones on the growth of S. typhimurium TA97 strain; (ii) the mutagenic potential of the cyclobutanones using the Ames test; (iii) in vitro experiments to study the exposure of the human colon tumor cells HT 29 stem, HT 29 clone 19A to cyclobutanones; (iv) use of different test systems to investigate cytotoxicity (trypan blue exclusion test to assess membrane integrity; tetrazolium salt reduction assays applying both MTT and WST-1; assessment of cytotoxicity in logarithmically growing cell by colony forming ability); (v) genotoxicity of cyclobutanones in human colon tumor cells as determined by the comet assay in the absence or presence of Fpg protein; and (vi) measurement of DNA strand breaks and Fpg-sensitive sites in human cells measured by alkaline unwinding. Results from this extensive work indicated that the cyclobutanones have cytotoxic properties both in bacteria and human cells. However, the authors pointed out that the cytotoxic effects vary with the nature of the actual compound and the cells investigated. As a common trend, which was more pronounced in bacteria, they observed that the shorter the chain of the cyclobutanone the higher the toxic effect. No mutagenic potential was detected for any cyclobutanone using the Ames test, which is in agreement with previously published results for 2-DCB (54, 61). For 2-DCB, both the comet assay and alkaline unwinding revealed an increase in Fpg sensitivities after a 24-h incubation period but not after 30 min as determined by the comet assay. The result for short-term incubation differs to that found previously where a lack of genotoxicity was found (47). The data derived from the alkaline unwinding experiments demonstrated a genotoxic potential of all compounds investigated with the intensity of DNA damage being dependent on the length of the fatty acid side chain, the degree of unsaturation, and the cell line applied. Hartwig et al. (63) concluded that, taken together, the results indicate a genotoxic potential of puri ed cyclobutanones in mammalian cells and seem to contradict the outcome of the Raltech studies (38) where, as noted earlier, no genotoxicity was observed with radiation-sterilized chicken meat. However, as for other work carried out previously, the authors state that the effects of the cyclobutanones should be elucidated in more detail with complementary studies being needed to clarify mechanisms of action and an adequate risk assessment for human exposure undertaken. The European Commission s SCF reviewed the safety of irradiated food in 1986, 1992, 1998, and 2003, and on all occasions agreed that the irradiation of food is safe up to an overall average dose of 10 kGy. In its 2003 report (46), it referred to the toxicity studies of the cyclobutanones by Burnouf et al. (49) and stated that the genotoxicity of these compounds could not be considered
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