Human chorionic gonadotropin alpha-subunit of normal placenta: characterization of synthesis and association with beta-subunit.
The forms of human chorionic gonadotropin (hCG) alpha-subunit synthesized and released by normal placental tissue were examined in explants of first trimester placenta that had been incubated for 30 min with [35S]methionine and then incubated for 6 h in medium containing unlabeled methionine. The media and tissue extracts collected at 0, 0.5, 1, 2, 4, and 6 h after the exposure to [35S]methionine were chromatographed on Sephadex G-100 and the amounts of radioimmunoassayable alpha-subunit and immunoprecipitable 35S-labeled alpha-subunit were determined. In tissue extracts, a single form of alpha-subunit was observed at 0 h that had an apparent molecular weight smaller (Ve/Vo = 1.90) than that of a urinary hCG alpha reference preparation (Ve/Vo = 1.81). With chase times of 0.5, 1, 2, and 4 h, a second peak of alpha-subunit was detected that had a larger apparent molecular weight (Ve/Vo = 1.68), and the ratio of large to small forms increased progressively with incubation time. In contrast to that in the extracts, the 35S-labeled alpha-subunit in the culture medium consisted entirely of the large form. Large and small intracellular forms of free alpha-subunit exhibited less than 2% recombination with beta-subunit, as evidenced by gonadotropin receptor binding activity. These studies suggest that normal placental tissue synthesizes a small precursor form of free hCG alpha-subunit that is converted to a larger form prior to secretion and that the free forms of alpha-subunit do not bind to purified hCG beta-subunit.
Posillico
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Handwerger
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Tyrey
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D006063:Chorionic Gonadotropin / Q000096:biosynthesis* / Q000378:metabolism*; D018997:Chorionic Gonadotropin, beta Subunit, Human; D002850:Chromatography, Gel; D046508:Culture Techniques; D005260:Female; D015292:Glycoprotein Hormones, alpha Subunit; D006801:Humans; D007163:Immunosorbent Techniques; D007700:Kinetics; D008715:Methionine / Q000378:metabolism; D008970:Molecular Weight; D010446:Peptide Fragments / Q000096:biosynthesis* / Q000378:metabolism*; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D011498:Protein Precursors / Q000096:biosynthesis; D011863:Radioimmunoassay
D006063:Chorionic Gonadotropin; D018997:Chorionic Gonadotropin, beta Subunit, Human; D015292:Glycoprotein Hormones, alpha Subunit; D010446:Peptide Fragments; D011498:Protein Precursors; D008715:Methionine