Control of enzyme activities in rat liver by tryptophan and its metabolites

Tryptophan administration increases the activities of tyrosine and tryptophan transaminases, tryptophan dioxygenase, histidase, serine dehydratase and phosphoenolpyruvate carboxykinase in the livers of fed rats; these increases are blocked by cycloheximide. In fasted rats, the changes are similar except that histidase is unaffected. In diabetic rat liver, only tryptophan dioxygenase and in adrenalectomized rat liver, tyrosine and tryptophan aminotransferases, tryptophan dioxygenase and phosphoenolpyruvate carboxyki- nase activities are increased after tryptophan. Low doses of both tryptamine and 5-hydroxytryptamine, in the presence of pargyline, increase the activities of tyrosine and tryptophan aminotransferases and phosphoenol- pyruvate carboxykinase; these effects are distinct from those of the parent aminoacid. The variation in responses indicates that a number of mechanisms may be involved. Tryptophan is, however, directly effective in increasing tyrosine aminotransferase activity in isolated liver cells.