Differences in inducibility by glucocorticoids of rat liver TO and TAT

The modulation of tryptophan-oxygenase (TO) and tyrosine amino-transferase (TAT) in the rat liver after a single dose of hydrocortisone has been studied under various physiological conditions. Differences in the induction behavior of the two enzymes have been observed dependent on sex, age, amount of administered hormone, and presence or absence of the adrenals. Some of the results observed are the following: 1) TO reached its maximum induction level 3 h prior to TAT in normal male rats. This difference disappeared when adrenalectomized male rats or when female animals were tested. 2) The maximal induction values of both enzymes were 50--80% higher in adrenalectomized rats than in normal rats. This effect was independent of sex. 3) Higher doses of hydrocortisone were necessary for optimal induction of TO and TAT in normal than in adrenalectomized rats. 4) The minimum dose of hydrocortisone necessary for enzyme induction was significantly lower for TO than for TAT. 5) Actinomycin D caused a complete inhibition of the induction of TO and TAT when given simultaneously with the glucocorticoid. The inhibition was less complete the longer the interval between hormone and actinomycin D administration. The activity of TAT was suppressed to a larger extent than TO.