Increased rates of polypeptide chain elongation in placental explants from human diabetics

Average rates of polypeptide chain elongation were determined in placental explants of first trimester and term placentas from both normal and diabetic human pregnancies. Average ribosome half-transit times were determined by measuring the kinetics of transfer of labeled polypeptides from polysomal-bound to released polypeptides. The average half-transit time decreases from 75 sec per ribosome in first trimester explants to 56 sec per ribosome in term placentas. The average polypeptide molecular weights synthesized by explants from first trimester and in term are 49,300 and 49,600, respectively, which are not significantly different. The average elongation rates for first trimester and term placental explants are 172 and 231 amino acids per minute per ribosome, respectively, which are significantly different. Moreover, the average polypeptide molecular weight synthesized by term placentas from diabetic pregnancies is 48,200, while the average ribosome half-transit time is 40 sec. Thus, ribosomes from explants of term placenta from diabetics move along the average message at a much higher speed than do ribosomes in normal term tissue. The assembly rate of amino acid into polypeptide in explant of placenta of diabetic mothers is 314 amino acids per minute, which is significantly faster than 231 amino acids per minute in normal term tissue. These findings indicate that during placental development and in diabetic pregnancy there is a large change in the actual rates at which amino acids are added to the nascent polypeptide chain--i.e., the rates in polypeptide chain elongation. Therefore, translation-level regulation of protein synthesis in placenta plays a significant part in the magnitude of the response to developmental and other physiological stimulations.