Actinomycin-sensitive Induction of Tyrosine Transaminase and Tryptophan Pyrrolase by Amino Acids and Tryptophan

Abstract

Abstract Oral administration of a mixture of amino acids to adrenalectomized rats resulted in a 4-fold rise in tyrosine transaminase activity of liver and a doubling of the tryptophan pyrrolase activity. This stimulation was completely inhibited by administration of actinomycin at the same time as the amino acid mixture. The basal level of either enzyme activity was not greatly affected by actinomycin. Tryptophan alone, administered at the dose present in the complete mixture, stimulated the activity of both enzymes by a process completely sensitive to actinomycin. Administration of larger doses of tryptophan caused greater increases in the activity of both enzymes. All of the stimulation of tyrosine transaminase activity, brought about by all doses of tryptophan, was abolished by actinomycin, but a proportion of the stimulated tryptophan pyrrolase activity brought about by the higher doses of tryptophan was insensitive to actinomycin. This portion increased with increased amounts of tryptophan. Similar results were obtained when tryptophan was administered intraperitoneally. When an amino acid mixture not containing tryptophan was administered, significant changes in tyrosine transaminase activity were elicited, but the increase in tryptophan pyrrolase activity was not statistically significant. These results show that administration of amino acids stimulates the activity of tyrosine transaminase and tryptophan pyrrolase by a process which requires RNA synthesis, and that the stimulation by tryptophan of tyrosine transaminase activity is entirely, and that of tryptophan pyrrolase partially, dependent on the synthesis of RNA. This conclusion is contrary to the generally accepted view that the stimulation of the activity of tryptophan pyrrolase by tryptophan does not require RNA synthesis and thereby differs from the hydrocortisone stimulation of the activity of the enzyme.