Alteration in the inducibility by dexamethasone of the activity of tyrosine aminotransferase and tryptophan oxygenase in regenerating rat liver after partial hepatectomy.

In adrenalectomized rats, the activity of tryptophan oxygenase in liver cytosol was increased 4 h after dexamethasone injection, with a peak on day 2. It then gradually decreased to the value obtained in normal animals treated with dexamethasone. In adrenalectomized, 70% hepatectomized rats, tryptophan oxygenase activity decreased significantly during the period of DNA synthesis after partial hepatectomy; it then increased to above the level existing before hepatectomy, reaching a maximum on day 5, and subsequently returned to the level before hepatectomy. These changes were well correlated with those of dexamethasone receptors at all times after adrenalectomy and partial hepatectomy. In adrenalectomized rats, tyrosine aminotransferase activity was increased 4 h after dexamethasone injection, reached a maximum on day 2, and then remained elevated. The level was increased by subsequent 70% hepatectomy, reached a maximum on day 5, and then began to decrease, but activity was still high on day 10 after hepatectomy when the receptor level had returned to the value existing before hepatectomy. When actinomycin D or cycloheximide was injected with dexamethasone, the induction of the two enzymes was completely inhibited. The activities of the two enzymes in regenerating liver of rats without adrenals and without glucocorticoid treatment were as low as those in adrenalectomized rats without glucocorticoid treatment. Thus, there was a good correlation between the dexamethasone receptor level and the induction of tryptophan oxygenase, but there was a dissociation between the receptor level and the induction of tyrosine aminotransferase after adrenalectomy and partial hepatectomy. The mechanism of this dissociation is unknown.