Secretion of pituitary-like peptides by the human placenta.

STRUCTURE Ascheim and Zondek (1927) first demonstrated the presence of gonadotrophic substances in the urine of pregnant women. Such urine injected into infantile mice induced premature oestrus with follicle ripening and luteinisation. hCG is a glycoprotein structurally related to the pituitary hormones TSH, FSH, and LH and comprises two subunits (Reichert et al., 1970; Pierce and Liao, 1970). The a-subunit (92 amino-acids) shows extensive structural sequence homology with FSH, LH, and TSH, and the fsubunit (145 amino-acids) is similar to that of LH apart from an extra 30 C-terminal amino-acids (Swaminathan and Bahl, 1970). The carbohydrate content comprises over 20% of the molecule (Diczfalusy and Troen, 1961) and is essential for biological activity (Whitten, 1948). Isolated aand fl-subunits possess very little intrinsic biological activity but, because of the close structural similarities, an a-subunit of hCG can be combined with the fl-subunit of LH or TSH with restoration of a high degree of biological activity appropriate to the fl-subunit (Pierce, 1971). Interestingly, the fl-subunits of all the glycoprotein hormones show a significant degree of homology, suggesting that they evolve from a common ancestral gene (Pierce. 1971; Acher, 1976) (Fig. 1).

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