Glucocorticoid control of the development of tryptophan oxygenase in the young rat

Tryptophan oxygenase activity follows a characteristic developmental pattern in the young rat, being absent until about 2 weeks of life, then attaining adult levels by the 3rd week. We have investigated the factors which control this process and determined that the increase in enzymatic activity results from increased glucocorticoid release between the 14th and 21st days. This conclusion was based on our observation that adrenalectomy completely arrests the development of the enzymatic activity. Twenty-one-day-old animals which were adrenalectomized on the 10th day, and thus possess no demonstrable enzymatic activity, could respond to cortisol treatment with an elevation of tryptophan oxygenase activity. Administration of either cycloheximide or actinomycin D completely inhibited this response, suggesting that both protein synthesis and RNA synthesis were requisite for the development of the enzyme. Immunochemical studies revealed that the enzyme found in the young animal was identical with that extracted from the adult rat; furthermore, the increase in activity observed in the developing rat is the result of increased enzyme protein levels. Direct measurement of the synthetic and degradative rates showed the accumulation of enzyme protein to depend upon increased synthetic activity, and not upon decreased enzyme degradation.