Gonadotrophin heterogeneity and its role in farm animal reproduction

This review describes the structural features of the gonadotrophins in terms of both the protein and carbohydrate components. The heterogeneity of these hormones is emphasised, with details of how the molecular structures of the various carbohydrate moieties of the gonadotrophins give rise to a heterogenous population of isoforms of each hormone. A potential pathway for the biosynthesis of gonadotrophins bearing asparagine-linked (Asn-linked) oligosaccharides is described. This includes the synthesis of an oligosaccharide core structure, which may act as a key intermediate, from which many of the oligosaccharide structures found in the gonadotrophins may be synthesised. The potential functional roles of the oligosaccharide residues, including their influence on the bioactivity of these hormones, are discussed. In general, the more acidic isoforms of a gonadotrophin have a greater half-life in circulation, but a decreased bioactivity, than the less acidic isoforms. Various factors influencing the isoform profiles of follicle-stimulating hormone (FSH) and luteinising hormone are also discussed. Oestradiol concentrations are important in stimulating the synthesis of bioactive isoforms of gonadotrophins. An increase in bioactive isoforms of gonadotrophins occurs during puberty; and an increase in bioactive FSH occurs during pregnancy in humans. The quality of FSH isoforms is also known to change during the follicular phase of the rat oestrous cycle and during the human menstrual cycle.