The livers of rats fed 10 diets previously found to inhibit or accelerate the induction of tumors by 4-dimethylaminoazobenzene were analyzed for riboflavin, biotin, and vitamin B6 after 6 and 19 weeks on the diets. Those fed diets accelerating tumor induction had average hepatic riboflavin levels of 9 to 11 µg. per gm. The livers of the control and protected rats averaged 14 and 15 to 19 µg. per gm., respectively. The hepatic levels of vitamin B6 and biotin averaged 7.0 and 0.54 µg. per gm., respectively, and were independent of the protective nature of the diet. The livers of rats fed the same diets in the absence of 4-dimethylaminoazobenzene contained about 50 per cent more of each of these vitamins than those from dye-fed rats.

In a second series the tumor incidence and the hepatic riboflavin and vitamin B6 were determined simultaneously. In this experiment urethane decreased the level of hepatic riboflavin and increased the rate of induction of tumors slightly. Methionine inhibited tumor induction to a small extent while cystine had a more pronounced retarding effect; the level of hepatic riboflavin for both groups was higher than in the controls.

The livers of rats fed the basal diet, 0.054 per cent of 4-dimethylaminoazobenzene, or equimolar amounts of its 2-, 2'-, 3'-, or 4'-methyl derivatives were analyzed for riboflavin at various times over a 16 week period. In each case the level of hepatic riboflavin decreased throughout the experimental period, but the rate and extent of the riboflavin loss were greatest with the more carcinogenic compounds.

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