Structure of the Human Chorionic Gonadotropin β-Subunit Fragment from Pregnancy Urine*

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Abstract
A major portion of the hCG immunoreactivity detectable in pregnancy urine is derived from a fragment of hCG.beta.. This lacks the COOH-terminal portion of hCG.beta., but retains immunoreactivity with most antibodies raised against the .beta.-subunit of hCG. To improve clinical measurements of hCG and assess the importance of such fragments in human urine, we have isolated and determined the structure of this molecule. The hCG.beta. fragment ws isolated from a partially purified commercial preparation of hCG (Organon) by gel filtration and immunoaffinity chromatography using monoclonal antibodies. It was found to consist to two polypeptide chains composed of residues .beta.-(6-40) disulfide-bridged to residues .beta.-(55-92). It also differs from the .beta.-subunit of hCG in its carbohydrate structure, lacking sialic acid and having a low but variable amount of galactose. A .beta.-fragment containing the same two NH2-terminal sequences was also isolated from a single pregnant woman''s urine. The two major polypeptides comprising the .beta.-fragment contain a total of nine half-cystine residues, raising the possibility that a free thiol may exist or that a third undetected disulfide-bridged peptide is present in the intact fragment. However, tests for the presence of a free thiol have been negative. Another intrinsic characteristic of the .beta.-fragment is the formation of a variable amount of dimer in solutions of neutral pH. .beta.-fragment will not combine with intact .alpha.-subunit. Despite the absence of regions .beta.-(1-5), .beta.-(41-54), and .beta.-(93-145), the .beta. fragment is recognized by the SB-6 antibody and most monoclonal antibodies elicited to the .beta.-subunit, thus excluding half of the amino acids of the .beta.-subunit from the epitope(s) where these antibodies bind.

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