Effect of Dietary Protein Source on the Activity of Polysomal Ornithine Decarboxylase Messenger RNA in Rat Liver

To pursue the mechanism underlying dietary induction of ornithine decarboxylase (ODC) activity in rat liver, changes in free-polysome-associated messenger RNA (mRNA) activity for the enzyme were studied in rats fed diets containing either casein or zein. The enzyme activity increased 50- to 100-fold in 4 h after feeding a 50% casein diet. Changes in the amount of immunoreactive ODC protein roughly paralleled changes in the enzyme activity. Dietary casein was found to cause a several fold increase in polysomal mRNA activity coding for this enzyme, which preceded the increase in catalytic activity. Therefore, the induction of hepatic ODC by casein feeding is due partly to an increase in the activity of its mRNA associated with free polysomes and partly to some translational and/or posttranslational control. On the other hand, feeding the zein diet hardly increased ODC activity, immunoreactive ODC protein or free-polysomal ODC-mRNA activity. This indicates that adequate supply and balance of amino acids are required both for the increase in polysome-associated ODC-mRNA activity and for the translational and/or posttranslational mechanism(s) during the induction of hepatic ODC activity.