Kinetics of gonadotrophins in the mare

Isoelectric focussing of crude extracts of equine pituitaries was used to obtain fractions containing FSH and LH. By comparison with FSH, LH was distributed over a similar but wider pH range indicating more marked polymorphism as determined from their isoelectric point (pI). Molecules with more sialic acid showed lower pI consistent with the concept that sialic acid is the major factor in determining pI and polymorphism in FSH and LH. Appropriate fractions were labelled with 125I, purified further and used in kinetic studies. FSH and LH molecules of similar pI had similar kinetics; however, LH molecules of high pI disappeared from plasma more rapidly. This is attributed to the role of sialic acid in preventing hormone degradation by non-target tissues, thus increasing the half-life and therefore the biological potency of the hormone. Since the form in which gonadotrophins circulate is not known, data are presented using 2 forms of LH and one of FSH. While this provides information from which most kinetic parameters may be determined, meaningful production rates cannot be calculated until the circulating form is identified. Other experiments on gonadotrophin kinetics are reviewed critically in the light of these findings.