An analysis of rates of polypeptide chain elongation in avian liver explants following in vivo estrogen treatment. I. Determination of average rates of polypeptide chain elongation.

Average rates of polypeptide chain elongation have been determined in cockerel liver explants on 4 successive days following an in vivo injection of 17 beta-estradiol. Incorporation of [3H]leucine by the explants is linear for at least 24 h, and the rate of protein synthesis increases significantly after estrogen injection. The explants synthesize and secrete serum albumin. B-apolipoprotein, and the phosphoprotein vitellogenin at relative rates which are similar to those reported for liver in vivo. Using this system, changes in the average rates of polypeptide chain elongation have been analyzed as a temporal sequence following a single injection of 17 beta-estradiol into cockerels. For this, average ribosome half-transit times were determined by measuring the kinetics of transfer of labeled polypeptides from polysomal-bound to released polypeptides. The data revealed a dramatic effect of estradiol on the average ribosome half-transit time with a maximum increase of 4.6-fold; however, the average size of polypeptides synthesized by explants at the peak of induction increased only 15% when compared to uninduced liver explants. These findings indicate that injection of estradiol results in large changes in the actual rates at which amino acids are added to the growing nascent polypeptide chain; that is, rates of polypeptide chain elongation. Therefore, translation-level regulation of protein synthesis in cockerel livers plays a significant part in determining the magnitude of the response to hormone stimulation.