Studies on riboflavin metabolism in the rat. I. Urinary and faecal excretion after oral administration of riboflavin-5'-phosphate

Some interactions of light, riboflavin, and aflatoxin B1 in vivo and in vitro

In previous studies, artificial sunlight and riboflavin synergistically increased acute aflatoxin toxicity in rats. Three new experiments were designed to provide information on the interaction of riboflavin, aflatoxin, and light. In a study of carcinogenesis, rats received low levels of aflatoxin 5 days/wk for 3 wk; 30 min after each dosing, half of them were irradiated for 2 hr. In some, levels of glucose-6-phosphatase and acid phosphatase were determined 5 days after completion of treatment. Remaining rats were killed at 30 or 53 wk. All underwent complete necropsies and histopathologic examination. In the second experiment, rats were dosed with riboflavin and divided into four groups: no further treatment; aflatoxin (LD50); irradiation (1-2 hr); or aflatoxin plus irradiation. Blood riboflavin levels were determined at intervals following these treatments. In the third experiment, the chemical reactions of irradated aflatoxin and/or riboflavin were studied by uv spectroscopy and TLC. The 53-wk study showed clearly that light decreased the incidence of aflatoxin-induced cancer. The other results may provide an explanation. Aflatoxin caused blood riboflavin levels to decrease-an effect enhanced by irradiation, suggesting that photosensitized riboflavin and aflatoxin form a complex. This interpretation gains support from studies in vitro that showed that riboflavin quenched aflatoxin photodegradation, perhaps by complexing with aflatoxin. Thus, low, carcinogenic doses of aflatoxin may complex with endogenous, photosensitized riboflavin, inhibiting its degradation into carcinogenic metabolites.