Regulation of tryptophan-niacin metabolism by hormones

The regulation of tryptophan-niacin metabolism by pituitary and adrenocortical hormones was investigated. Hypophysectomized rats fed on a niacin-free purified diet were injected with bovine somatotropin, predonine acetate or both. The urinary excretion of N-methylnicotinamide (MNA) and N-methyl-2-pyridone-5-carboxamide (2-Py) after oral administration of tryptophan was then compared before and after the hormone treatment. The amount of urinary MNA was found to be increased after the injection of somatotropin, but decreased after the predonine injection. On the other hand, no change in urinary MNA was observed in the rats administered with both hormones. The amount of 2-Py appeared to be reduced by the predonine treatment, but was not affected by somatotropin injection. The activity of liver alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase (picolinic carboxylase) was shown to be directly proportional to the ratio (oral tryptophan)/(urinary MNA). Moreover, the enzyme activity appeared to be inversely proportional to the concentration of niacin in liver. In addition, the amount of urinary MNA was suggested to be affected by the change of body weight gain. The administration of both pituitary hormone and insulin failed to normalize the reduced urinary MNA excretion in diabetic rats, suggesting that any change in pituitary hormone was not responsible for the abnormal tryptophan-niacin metabolism in diabetes.