Nutritional Disturbances of Protein Metabolism in the Liver

Nutritional disturbances of protein metabolism in the liver are reviewed in relation to feeding experimental animals the following diets: a) purified diets deficient in amino acids; b) amino acid mixtures or single amino acids; c) protein-free (amino acid-free) diets; or d) hypertonic or hypotonic solutions. The effects of tube-feeding the diets or dietary components for days, hours, or minutes on hepatic polyribosomes and protein synthesis are described. Force-feeding a purified diet free of single essential amino acids induces within days morphologic changes resembling those that occur in humans with kwashiorkor, a world-wide nutritional deficiency disease in children. In this kwashiorkor-like model, hepatic protein synthesis and polyribosomal aggregation are increased. Administration of a complete amino acid mixture or tryptophan alone, but no other single amino acid, produces a rapid stimulation (within minutes) of hepatic protein synthesis and polyribosomal aggregation in animals that had been fasted, fed, or treated with hepatotoxic agents. A single tube-feeding of a protein-free (amino acid-free) diet induces within hours an increase in hepatic protein synthesis in fasted animals. Administration of hypertonic solutions rapidly (within minutes) inhibits, while administration of hypotonic solutions rapidly increases, hepatic protein systhesis. These experimental findings are reviewed in terms of how alterations in regulatory controls of hepatic protein synthesis may be influenced by nutritional disturbances. Such information may be of importance in designing and utilizing nutritional approaches in the therapy of liver diseases.