Studies on in vitro synthesis and secretion of human chorionic gonadotropin and its subunits.

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Abstract
Human chorionic tissues were culitvated in vitro in the presence of 3H-proline in order to study the synthesis of hCG [human chorionic gonadotropin] and its subunits by the [human] placenta. After terminating the culture, tissue extracts and media were individually gel-filtrated on Sephadex G-100. The eluted fractions were radioimmunoassayed for hCG, hCG.alpha. and hCG.beta. and were measured for 3H-radioactivity. Label incorporation was determined by immunoprecipitation. Elution profiles of tissue extracts showed the existence of large immunologic forms of hCG, hCG.alpha., and hCG.beta. emerging near the void volume. The amounts of these large immunologic species in chorionic tissue gradually decreased during the course of cultivation. 3H-proline was almost exclusively incorporated into the large immunologic forms of hCG, hCG.alpha. and hCG.beta. within the chorionic tissue during the 5 h exposure. A great quantity of hCG.alpha. was found in the media after the 3 day culture, while the amount of hCG.beta. found in the media was minute. After a 15 min pulse, the 3H-radioactivity peak within the chorionic tissue appeared in tha void volume, coincidental with hCG immunoreactivity. During the chase period, there was a shift of the 3H-radioactivity peak associated with hCG immunoreactivity from the void volume to the more retarded area. In the media until after the 60 min chase, no labeled hCG and its subunits appeared. Within the media after the 3 h chase, the hCG peak associated with 3H-radioactivity was more retarded on Sephadex G-100 than that within the tissue extract after the 15 min pulse. The large immunologic forms of hCG, hCG.alpha. and hCG.beta. are probably synthesized as the earliest detectable biosynthetic forms and that they may then be converted to small molecule species. In the culture of molar trophoblastic tissues after a 15 min pulse, considerable amounts of hCG and its subunits accopanied by high 3H-radioactivity were already secreted into the media. Protein synthesis by molar trophoblastic tissue is probably markedly enhanced as compared with that by normal chorionic tissue, and immunoreactive materials synthesized in molar trophoblastic tissue may be secreted more readily than those synthesized in normal chorionic tissue.

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