Studies on Enzyme System of Tryptophan Metabolism in Fish Liver

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1. Several enzymes which are involved in the metabolism of tryptophan in fish liver were investigated.2. It is known that the kynurenine pathway is the major route in tryptophan metabolism in animals. The activity of fish liver tryptophan pyrrolase which catalyzes the first step in kynurenine pathway was comparatively low, and it was not activated by hematin.3. Arylformamidase, the second enzyme in kynurenine pathway, of the rainbow trout liver was purified and its properties were compared to those of the cattle enzyme. The fish enzyme was inhibited by tryptophan. This suggests that the kynurenine pathway of fish is regulated by tryptophan, the first substrate of the pathway.4. The ratio of hydroxyanthranillate oxygenase activity to aminocarboxymuconic semialdehyde decarboxylase activity indicated that the NAD-forming pathway is exceedingly inefficient in the rainbow trout.5. Fish liver extract exhibited an apparent activity of tryptophan hydroxylase, the first enzyme in the serotonine pathway. The Km value of hydroxylase to tryptophan was lower than that of pyrrolase. It appears that the importance of the serotonine pathway exceeds that of the kynurenine pathway.