Effect of protein quality on dietary induction of hepatic ornithine decarboxylase

Dietary induction of ornithine decarboxylase (ODC) in rat liver depended on the quality of protein in the diet. Zein did not induce ODC unless it was supplemented with the deficient amino acids, tryptophan and lysine. Similar phenomena were observed with gelatin (tryptophan and methionine) and hemoglobin (isoleucine). However, ODC was found to be significantly induced by an amino acid mixture simulating zein. The difference between amino acid diet and protein diet effects could not be explained by digestibility of zein. Unphysiologically rapid influx of amino acids appeared to induce ODC by a mechanism different from that by which dietary protein induced ODC. After ingestion of zein, the concentration of tryptophan and lysine decreased markedly in plasma and liver. However, it was confirmed that their supply from intracellular protein degradation continued even after feeding. In contrast to ODC, tyrosine aminotransferase was induced by zein as well as by casein, indicating that the requirement of tryptophan and lysine as precursor amino acids for enzyme synthesis was satisfied by reutilization of the amino acids liberated by intracellular protein degradation. Therefore, it was concluded that good quality protein was required for ODC induction mainly as a signal but not as a source of precursor amino acids.