Determination of radiation equivalence of the chemical furylfuramide (AF-2) for the induction of gene conversion in diploid yeast and estimation of genetic risk to the Japanese population

Carcinogenic risk of food additive AF-2 banned in Japan: a case study on reassessment of genotoxicity

BACKGROUND

Carcinogenic risk assessment studies have been repeatedly improved and are still being debated to find a goal. Evaluation might be changed if new approaches would be applied to some chemicals which means that new approaches may change the final assessment. In this paper, the risk assessment of a chemical, in particular the proper carcinogenicity, is examined using the long-banned food additive, 2-(2-furyl)-3-(5-nitro-2-furyl)-acrylamide, AF-2, as a case study.

RESULTS

First, Ames tests were carried out using strains TA1535, TA100, TA1538, and TA98 and their nitroreductase-deficient strains YG7127, YG7128, YG7129, and YG7130. The results showed that mutagenic activity was reduced by about 50% in the nitroreductase-deficient strains, indicating that part of the mutagenic activity shown in Ames test was due to bacterial metabolism. Second, in vivo genotoxicity tests were conducted, including the one that had not been developed in 1970's. Both a micronucleus test and a gene mutation assay using transgenic mice were negative. Third, assuming it is a genotoxic carcinogen, the virtual safety dose of 550 μg/day was calculated from the TD50 in rats with a probability of 10-5.

CONCLUSION

AF-2 has been shown to be carcinogenic to rodents and has previously been indicated to be genotoxic in vitro. However, the present in vivo genotoxicity study, it was negative in the forestomach, a target organ for cancer, particularly in the gene mutation assay in transgenic mice. Considering the daily intake of AF-2 in the 1970s and its virtually safety dose, the carcinogenic risk of AF-2 could be considered acceptable.

Deactivation of furyl furamide (AF-2) by rat-liver microsomes and its implication in short-term tests for mutagenicity/carcinogenicity

The genetic activity of AF-2 in both bacteria and yeast rapidly disappeared in the presence of rat-liver microsomal fraction (S9 mix). Incubation of AF-2 with S9 mix even for 10 min at 37 degrees C was sufficient to inactivate it completely. Data available in the literature suggest that activation of AF-2 is necessary for its geno-toxic effect. The activation step may involve reduction of the nitrofuran to an amino group probably by the enzyme reductase I. Most cultured cell systems, such as bacteria, yeast, Neurospora, mammalian cells and human lymphocytes, can probably bring about this reduction. However, the rapid disappearance of the genetic activity of AF-2 in the presence of rat-liver homogenate suggests that rat-liver microsomes may further metabolize the reduction products to inactive forms. Thus, it becomes necessary to test even those chemicals that are mutagenic per se, with mammalian microsomal preparations before their mutagenic/carcinogenic potentialities can be assessed in short-term tests.

Induction of gene conversion in diploid yeast by chemicals: correlation with mutagenic action and its relevance in genotoxicity screening

More than 200 chemicals, comprising drugs, pesticides, herbicides, industrial chemicals, food additives, etc. tested by various workers for their ability to induce gene conversion in diploid yeast, have been reviewed. This review shows that a strong correlation exists between the convertogenic and mutagenic abilities of these chemicals. This conclusion confirms the earlier one by Zimmermann which was based on fewer mutagenic chemicals. Published data have further been analysed to confirm quantitatively Zimmermann's other important conclusion that gene conversion is not mutagen-specific. This is a positive advantage in using gene conversion in preliminary screening of chemicals because it reduces the chances of false negatives.