Studies on amphibian yolk. VII. Serum phosphoprotein synthesis by vitellogenic females and estrogen-treated males of Xenopus laevis

Female toads given 1000 units of human chorionic gonadotropin ovulated within 24 h and began to synthesize a phosphoprotein which appeared, but did not accumulate, in the serum (physiological half-life, 2 days). The period of synthesis lasted about 30 days, after which mature oocytes were once again observed in the ovary. Male toads given a single dose of 0.1–1.0 mg estradiol-17β also immediately began to synthesize a phosphoprotein. The rate of maximum synthesis and the length of time for the maximum to be reached were directly proportional to the amount of estrogen administered. The serum phosphoprotein in male toads, however, had a physiological half-life of approximately 40 days. Ovariectomized females produced a serum phosphoprotein with a physiological half-life similar to that of males. We therefore concluded that under normal conditions the circulating phosphoprotein produced by the liver of the female as a response to estrogen is accumulated by the vitellogenic ovary. The serum phosphoprotein is apparently a lipophosphoprotein complex from which at least one of the yolk proteins, phosvitin, may be derived.