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Effect of Tryptophan or Phenobarbital Administration on Hepatic Polyribosomes and Protein Synthesis in Rats Force-Fed a Complete or Threonine-Devoid Diet¹

Department of Pathology, University of South Florida, College of Medicine, Tampa, Florida, 33620

Young rats were force-fed a complete or threonine-devoid diet for 3 days. Prior to killing on the fourth morning, rats of each group were treated with L-tryptophan (30 mg/100 g body weight) 2 hours before killing or with sodium phenobarbital (8 mg/100 g body weight) 14 hours before killing. Rats force-fed the threonine-devoid diet revealed increased aggregation of polyribosomes (sucrose density gradient analyses) and increased protein synthesis (in vitro [¹⁴C]leucine incorporation into protein) in the livers in comparison to that in the livers of rats force-fed the complete diet. Plasma total tryptophan levels were decreased and plasma free tryptophan as a percentage of total plasma tryptophan was increased in rats force-fed the threonine-devoid diet for 3 days. Administration of tryptophan or of phenobarbital stimulated polyribosomal aggregation and in vitro protein synthesis of the livers of rats force-fed the complete or the threonine-devoid diet. These stimulatory effects induced by tryptophan or phenobarbital were most marked in rats force-fed the threonine-devoid diet. Hepatic tryptophan pyrrolase activity was stimulated to similar degrees following the administration of tryptophan to rats force-fed the complete or the threonine-devoid diet.

KEY WORDS: • threonine-deficiency • tryptophan • in vitro protein synthesis • hepatic polyribosomes

¹ Supported by U.S. Public Health Service Research Grant AM-16530 from the National Institute of Arthritis, Metabolism and Digestive Diseases.

Manuscript received 27 August 1976.