In vivo effect of dietary factors on the molecular action of aflatoxin B1: role of vitamin A on the catalytic activity of liver fractions

Aflatoxin-albumin adducts and correlation with decreased serum levels of vitamins A and E in an adult Ghanaian population

A study of aflatoxin (AF) exposure and the levels of vitamins A and E was carried out with a group of 507 Ghanaian participants. AFB(1)-albumin adducts (AFB-AA) were measured by radioimmunoassay and vitamins A and E were measured by high-performance liquid chromatography (HPLC). The average level of serum AFB-AA was 0.94 +/- 0.64 (range = 0.1-4.44) pmol mg(-1) albumin. Mean levels of vitamins A and E were 1.32 +/- 0.48 (range = 0.41-4.85) micromol l(-1) and 15.68 +/- 4.12 (range = 6.35-30.40) micromol l(-1), respectively. A significantly negative correlation was found between serum AFB-AA and vitamin A levels (r = -0.110, p = 0.013). An even stronger, significant negative, correlation was found between serum AFB-AA and vitamin E levels (r = -0.149, p < 0.001). Serum AFB-AA levels were statistically higher (median = 0.985 pmol mg(-1) albumin) in subjects who had low levels of both vitamins A and E as compared with the levels (median = 0.741 pmol mg(-1) albumin) subjects who had high vitamins A and E levels (p(trend) = 0.001). To verify these findings, blood samples were again collected from 165 of the 507 people 3 months after the initial collection. Significantly negative correlations were confirmed between levels of serum AFB-AA and both vitamins A (r = -0.232, p = 0.003) and E (r = -0.178, p = 0.023). Again, high serum AFB-AA concentrations (median = 1.578 pmol mg(-1) albumin) were found in subjects with low levels of vitamins A and E compared with the concentrations (median = 1.381 pmol mg(-1) albumin) in subjects with high levels of vitamins A and E (p(trend) = 0.002). These data show that AF exposure was associated with decreased levels of serum vitamins A and E in high-risk human populations, which may significantly influence the incidence of AF-related adverse health effects.

Antimutagenic profile of antioxidant vitamins in drosophila mulation test

OBJECTIVE

To assess the antimutagenicity of antioxidant vitamins (vitamins A, C, and E) as expressed by their efficacy to lower aflatoxin-induced mutations.

METHODS

The Muller-5 method for mutation detection was used to assay the frequency of X-chromosome linked recessive lethal mutations (XRLMs) in Drosophila. Larvae were exposed to dietary concentration of aflatoxins and/or the human therapeutic doses of any of the three antioxidant vitamins. Absence of normal eyed males among M2 progeny gave an indication of mutation induction.

RESULTS

Aflatoxin supplimentation significantly increased the incidence of XRLMs in Drosophila. Mutation frequency was also raised a little above the control level in case of vitamin treatment. However, notable mitigation in mutation frequency was registered when aflatoxin-treated larvae were concomitantly fed with any of the three antioxidant vitamins.

CONCLUSION

Aflatoxin exposure can enhance the frequency of gene mutation in Drosophila which is significantly lowered by each of the three antioxidant vitamins. The degree of amelioration produced by them is almost identical. This mitigation is based on the scavenging/trapping by antioxidant vitamins of DNA-reactive products (metabolites and radicals) emanating from aflatoxin metabolism.

Effect of retinol on ochratoxin-produced genotoxicity in mice

Ochratoxin (1 microgram/kg body weight/day), when administered orally daily to albino Swiss mice for 14 continuous days, increased the incidence of abnormalities in mitotic and meiotic metaphase chromosomes, and the gross morphology of the sperm head; the sperm count per unit volume of caput epididymal suspension also decreased. These genotoxic effects were substantially reduced by concurrent oral administration of retinol at double the clinically therapeutic dose. It is possible that the electrophilic metabolites of ochratoxin form adducts with DNA or produce replacement-type mutations. Retinol may achieve its antigenotoxic effect by means of blocks and shunts in the ochratoxin metabolic pathway. Vitamin A-mediated cellular repair and scavenging of the mutagenic radicals can also take place. The vitamin itself has some genotoxic potential.

Effect of vitamin A dietary intake on in vitro and in vivo activation of aflatoxin B1

The mechanism by which vitamin A prevents or delays in chemical carcinogenesis remains unclear. In the present study, we assess the suggestive role of vitamin A in the initiation phase of carcinogenesis. We have conducted a dose-effect relationship between vitamin A dietary intake and aflatoxin B1 (AFB1) genotoxicity measured both in vitro and in vivo. Thus AFB1-induced mutagenesis in Salmonella typhimurium TA98 was investigated and compared to AFB1-induced single-strand breaks (SSBs) in DNA of rat hepatocytes. Rats were fed ad libitum with diet containing 0, 5, 50 or 500 IU of retinyl palmitate for 8 weeks. The AFB1-treated rats were injected i.p. with 1 mg/kg body weight. In the Ames test conditions TA98 back-reversion was negatively correlated with the log of vitamin A concentration in liver S9 fractions from experimental groups. However, the activities of metabolizing enzymes which specifically activate or deactivate AFB1 were found to be significantly decreased in vitamin A-deficient animals and weakly modified in vitamin A-supplemented animals. For in vivo experiments, the DNA elution rate of both AFB1-treated and untreated rats was increased in vitamin A deficiency condition (+79% and +17% respectively) and was reduced with the higher vitamin A dietary level (-44% and -53% respectively). DNA damage measured in vivo showed a significant positive correlation with mutagenic activity measured in the Ames test. These results confirm that the vitamin A status of animals can influence AFB1 genotoxic activity in vitro and indicate that this phenomenon also occurs in vivo. Thus a similar mechanism may be considered for the protective action of vitamin A both in vitro and in vivo. However, this mechanism is unlikely to involve modulation of the microsomal enzyme system responsible for AFB1 metabolism. Therefore a protective mechanism at the cytosolic or nuclear levels may be suggested.

In vivo effect of dietary factors on the molecular action of aflatoxin B1: role of riboflavin on the catalytic activity of liver fractions

Weanling rats were kept on a synthetic riboflavin-free diet for 4 weeks, and subsequently on the same diet but supplemented with riboflavin for 2 weeks. The ability of liver microsomes to catalyze reactions of aflatoxin B1 (AFB1) leading to its activation and DNA adduct formation was measured after each period of experimental feeding. A decrease in both activities was evident during riboflavin deficiency, and this could be restored after normal supply of the vitamin. The decrease was attributed to a fall in the endogenous flavin content, specifically the coenzyme flavin adenine dinucleotide which forms an integral part of the microsomal monooxygenase that catalyzes the activation reactions. The vitamin and its coenzymes, however, inhibit the microsomal enzyme activity when added in excess in the in vitro system. It is envisaged that riboflavin may play a role in regulating the carcinogenic activity of AFB.