Effectsof Thyroxine on the ConversionRatioof Tryptophan to Nicotinamide in Rats

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 ef kynurenine to kynurenic acid. By the traditional view, these changes were taken to mean that thyroxine suppressed the conyersion of tryptophan to nicotinamide; however, the effect of this hormone on the conversion ratio of tryptophan to nicotinamide has not yet been actually demenstrated. We, therefore, inyestigated the effect on the conversion of feeding to rats a nicotinic acid-free, 20% casein diet containing O.O02% thyroxine. The conyersion ratio was calclllated from the ratio of the sum of the urinary excretion of nicotinamide and its metabolites, Ni-methy]nicotinamide, Ni-methyl-2-pyridone-5-carboxamide, and AJi-methyl-4-pyridone-3carboxamide, 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 tryptephan to nicotinamide and that the kynurenine channel did not play a critical role in this conyersion. Furthermore, the present data suggest that aminocarboxymuconate-semialdehyde decarboxylase played a critical role in this conversion.