To investigate how vitamin B₆ (B₆) deficiency affects the whole metabolism of tryptophan-niacin, rats were fed for 19 days with each of the following four kinds of diets ; a complete 20% casein diet (control diet), the control diet without B₆, the control diet without nicotinic acid, and the control diet without nicotinic acid and B₆, and the urinary excretion of such tryptophan metabolites as kynurenic acid, xanthurenic acid, nicotinamide, N¹-methylnicotinamide, N¹-methyl-2-pyridone-5-carboxamide, and N¹-methyl-4-pyridone-3-carboxamide each and the enzyme activities involved in tryptophan-niacin pathway were measured. The urinary excretion of kynurenic acid decreased while that of xanthurenic acid increased drastically in the two B₆-deficient groups, when compared with the B₆-containing groups. These results indicate that the rats fed with the B₆-free diets were in the vitamin-deticient state. The conversion ratio was calculated from the ratio of the urinary excretion of sum of nicotinamide, N¹-methylnicotinamide, N¹-methyl-2-pyridone-5-carboxamide, and N¹-methyl-4-pyridone-3-carboxamide, to the Trp intake. The ratio was statistically lower in the B₆-free diet than in the B₆-containing diet under the niacin-free conditions.

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