Influence of amino acids on rat liver phenylalanine hydroxylase activity

The influence of oral and of injected amino acids on rat liver phenylalanine hydroxylase activity has been studied in both short-term and extended experiments. Phenylalanine added to the diet in moderate amounts ordinarily causes an increase in enzyme activity, but when excess phenylalanine was included in the diet, it caused a marked decrease in the enzyme activity in rats fed ad libitum. Tryptophan or tyrosine added to the diet in the absence of added phenylalanine increased the enzyme activity in these rats. Excess tryptophan, but not tyrosine, could counteract the effect of phenylalanine on this enzyme after an 18-hr fast. There were dramatic differences in phenylalanine hydroxylase activity when various amino acids were injected. Injection of tryptophan or glycine decreased the phenylalanine hydroxylase activity in rat liver. Injections of other amino acids, notably phenylalanine and tyrosine, had no effect on the phenylalanine hydroxylase activity 5 hr after injection.