ABSTRACT

The pituitary honnones luteinizing honnone (LH, lutropin), follicle-stimulating honnone (FSH, follitropin), and thyroid-stimulating honnone (TSH, thyrotropin), as well as the placental honnone human chorionic gonadotropin (hCG) are glycoproteins which are composed of two distinct polypeptide chains. Their alpha subunits are identical in amino acid sequence within a species, while their beta subunits possess a distinct sequence which is specific for the respective honnone. It is the beta subunit, in combination with the common alpha subunit, which detennines honnonal specificity (reviewed in Chapter I). These four honnones have carbohydrate moieties attached to the protein core. Both of the subunits are glycosylated, containing one or two asparagine-linked oligosaccharide units per subunit. The beta subunit of hCG also possesses four serine-linked oligosaccharide residues (reviewed in Chapter 2). The carbohydrate moieties very often contribute to the expression of molecular pleomorphism. Glycoprotein honnones which exhibit microheterogeneity exist as a series or family of isohonnones which may differ in molecular weight, isoelectric point (pi), circulatory half-life, receptor-binding activity and/or biological activity. It is becoming clear that subtle changes in the carbohydrate structure of glycoprotein honnones can dramatically alter their respective biological activities. Thus, qualitative as well as quantitative changes in the production of the glycoprotein honnones may occur.