Effects of Thyroxine on the Conversion Ratio of Tryptophan to Nicotinamide in Rats

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It has been reported that thyroxine increased the activity of kynurenine aminotransferase and reduced the activity of kynureinine 3-hydroxylase, with a resulting increase in the reaction of kynurenine to kynurenic acid. By the traditional view, these changes were taken to mean that thyroxine suppressed the conversion of tryptophan to nicotinamide ; however, the effect of this hormone on the conversion ratio of tryptophan to nicotinamide has not yet been actually demonstrated. We, therefore, investigated the effect on the conversion of feeding to rats a nicotinic acid-free, 20% casein diet containing 0.002% thyroxine. The conversion ratio was calculated from the ratio of the sum of the urinary excretion of nicotinamide and its metabolites, N^1-methylnicotinamide, N^1-methyl-2-pyridone-5-carboxamide, and N^1-methyl-4-pyridone-3-carboxamide, to the tryptophan intake. The ratio was statistically higher in hyperthyroid rats than in the control rats. The present data clearly demonstrate that thyroxine increased the conversion of tryptophan to nicotinamide and that the kynurenine channel did not play a critical role in this conversion. Furthermore, the present data suggest that aminocarboxymuconate-semialdehyde decarboxylase played a critical role in this conversion.
社団法人日本農芸化学会の論文
1994-10-23