After refeeding a fat-free diet to fasted rats, the time courses of transcriptional rates, mRNA concentrations, and enzyme induction of hepatic acetyl-CoA carboxylase and fatty acid synthase were compared between 1.5- and 18-month-old rats. In the old rats, the levels were mostly 40–70of those in the young animals. Moreover, the peaks of the levels tended to be delayed in the old animals. The transcriptional rates were increased within only 1 h after the refeeding in the young animals, but not until 6 h in the old. The mRNA concentrations reached maximum at 16 h in the young rats, but at 24 h in the old. In the old rats, the incorporation of 3Hleucine into the enzyme proteins was also decreased roughly in proportion to the enzyme induction. The mRNA concentrations in the liver polysomes were roughly proportional to the total mRNAs. Thus, the translational activities did not appear to be altered by aging. It is suggested that the age-dependent decreases of acetyl-CoA carboxylase and fatty acid synthase induction can be mainly ascribed to the transcriptional step
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