Human Chorionic Gonadotropin α-Subunit from Cultured Choriocarcinoma (JEG) Cells: Comparison of the Subunit Secreted Free with That Prepared from Secreted Human Chorionic Gonadotropin*

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Abstract
The cultured human choriocarcinoma cell line, JEG-clone 3, secretes both biologically active hCG [human chorionic gonadotropin] (JEG-hCG) and substantial quantities of free .alpha.-subunit (JEG-.alpha.). This study is concerned with a comparative characterization of the .alpha.-subunits of 3 distinct origins: standard hCG-.alpha. obtained after dissociation of urinary (pregnancy) hCG, JEG-.alpha. freely secreted by the JEG-3 cells and JEG-hCG-.alpha. obtained after dissociation of secreted JEG-hCG. Comparison of these .alpha.-subunits was made by gel exclusion chromatography, isoelectric focusing and ability to form a radioreceptor assay-active hCG when incubated with standard hCG-.beta. subunit. The freely secreted JEG-.alpha. exhibits an apparent MW larger than that of standard hCG-.alpha. and JEG-hCG-.alpha., whose apparent MW are similar. The majority of JEG-.alpha. is represented by a component with acidic isoelectric pH (pI 4.8), and this component is virtually absent in standard hCG-.alpha. and JEG-hCG-.alpha.. When incubated with .beta.-subunit, JEG-hCG-.alpha. forms a radioreceptor assay-active hCG almost to the same extent as does standard hCG-.alpha., unlike JEG-.alpha. which is unable to do so. Thus, no major difference appear between the 2 .alpha.-subunits that were associated with a .beta.-subunit [either in urinary (pregnancy) hCG or in JEG-hCG]. In contrast, different characteristics were found in freely secreted JEG-.alpha.. Two different types of .alpha.-subunit can be obtained from the JEG-3 cells: an .alpha.-subunit which is normal and incorporated into JEG-hCG and another .alpha.-subunit which is freely secreted. Further understanding of the structural and chronological relation between these 2 different .alpha.-subunits obtained from the JEG-3 cells should prove valuable in the unraveling of hCG biosynthesis.