Sequential changes in the protein synthetic activity of male Xenopus laevis liver following induction of egg-yolk proteins by Estradiol-17 beta.

Administration of a single injection of estradiol-17 beta to adult male Xenopus laevis induces the synthesis and secretion by the liver of the egg-yolk protein complex vitellogenin. Explants taken from livers of hormone-treated animals and maintained in organ culture continue to produce this protein at a rate which is dependent on the length of time between estrogen injection of the donor and preparation of the explants. Organ culture, therefore, can be used to quantitate the vitellogenic response and to study its time course in detail. An increase in the rate of incorporation of radioactive amino acids into secreted protein is first detectable in explants taken from animals 12 hours after estradiol injection. The response becomes increasingly pronounced as the period after hormone treatment of the donors is prolonged, and it is maximal in explants taken from frogs 12 days after estrogen administration. At this time production of vitellogenin can account for as much as 90 per cent of total protein synthesis in the liver. Labeling of intracellular protein with [14C]serine is stimulated 2-fold and incorporation into secreted protein increased 36-fold under conditions where the rate or extent of uptake of radioactive precursor into the acid-soluble pool does not change. After a lag period of 2 to 4 hours the secretion of vitellogenin in culture continues at a constant rate for up to 3 days. At least part of the stability of yolk protein synthesis in culture is attributable to long lived messenger RNA species. The major component of vitellogenin labeled wither in vivo or in culture has a molecular weight of approximately 180,000 as shown by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Material labeled with [3H]serine gives a similar distribution of radioactivity on gel electrophoresis as that labeled with [35S]methionine, with no peak corresponding to the molecular weight of phosvitin. It is suggested that phosvitin is not a primary product of translation but is synthesized as part of the large subunit of vitellogenin.