Nature of the Subunits of Human Chorionic Gonadotropin

Abstract

Subunits of human chorionic gonadotropin have been isolated from carboxymethyl- HCG by preparative poly aery lamide electrophoresis, by chromatography on Sephadex G-25, and from native HCG by chromatography on DEAE-Sephadex A-25. HCG-α subunit has been estimated to have a Stokes’ radius of 23.3 Å by gel filtration and a molecular weight by SDSgel electrophoresis of approximately 18,000; the HCG-β subunit has a Stokes' radius of 30.2 Å and a molecular weight of approximately 30,000. There are significant differences in amino acid composition between the subunits. Alanine, serine and valine were found as NH2-terminal amino acids of native HCG. Alanine and valine were found to be the NH2-terminal residues of the a subunit, and alanine and serine were found in the NH2-terminus of the β subunit. There appears to be a homology between the sequence of the a subunits of HCG and TSH, and between the 0 subunit of HCG and the CII subunit of LH. The biological activity of the α subunit was less than 1 % of the native hormone, while the β subunit appears to have biological activity in the order of 6% of native HCG. It is possible to recombine the subunits by incubation in 0.01M phosphate buffer, pH 7.0, for 16 hr at 37 C (Reichert et al., J Biol Chem244: 5110, 1969) and this reassociation could be demonstrated by disc gel electrophoresis and by gel filtration. After recombination, there was a major increase in activity, the material assaying at at least 66 % the activity of an equivalent amount of native HCG. (Endocrinology88: 1045, 1971)