Pituitary gonadotrophins (follicle-stimulating hormone, FSH; luteinLzing hormone, LH) exist in different molecular forms within the anterior pituitary gland and serum of several non-mammalian and mammalian species, including man. The number and relative abundance of each gonadotrophin species will depend on the specific technique utilized for their isolation, the tissue source and the physiological status of the donor. Intracellular FSH and LH from glands of rodents (hamsters and rats) and primates exhibit charge heterogeneity and therefore may be separated into several forms or iso-hormones by isoelectric focusing (IEF). These FSH and LH species differ from each other not only in their isoelectric point (pI) but also in their relative abundance, receptor binding activity, biological activity and plasma half-life. Almost all gonadotrophin species isolated from pituitary extracts have also been detected in vitro and in vivo as secreted forms. Less bask rodent LH and FSH forms exhibit low receptor binding and in-vitro biological activities; a similar trend is found in LH and FSH species isolated from glands of monkeys and humans. However, these relatively acidic isohormones have longer circulatory half-lives and higher in-vivo biological activities than less negatively charged forms. The overall pattern of charge heterogeneity of gonadotrophins varies according to the specific endocrine status of the donor. Sex steroid hormones (mainly oestrogens) and gonadotrophin-releasing hormone seem to act in concert at the pituitary level to influence the physicochemical and functional characteristics of gonadotrophins and therefore their biological expression at the target cell. The effects of these factors appear to be mediated through the incorporation of specific carbohydrate residues and/or degree of terminal sugar sulphation at co-post-translational levels. The first result of these complex interactions between the gonad and the hypothalamic-pituitary unit is the production and secretion of various types of gonadotrophin molecules in proportions according with the physiological requirements of the subject at a given time, to perform specific actions upon gonadal maturation and/or function.

In the present studies we analysed the main physicochemical and biological properties of the several isoforms of human pituitary follicle-stimulating hormone (hFSH). Extracts of total anterior pituitary glycoproteins from adult donors were submitted to chromatofocusing and several forms of immunoactive hFSH with isoelectric points (pI) ranging from 7.6 to 3.8 were identified. An additional isoform was detected after passing through the chromatofocusing column a 1.0 M NaCl solution (salt peak). Each hFSH isoform or pool of neighbouring isoforms (pI value 7.6–7.1, pool I, 1.5 ± 0.13% of total immunoactivity recovered; pI value 5.9–5.3, pool II, 8.9 ± 1.6% of total; pI value 5.0–4.7, pool III, 14.4 ± 1.4% of total; pI value 4.5–4.1, pool IV, 54.8 ± 4.9% of total; pI value 3.9–3.8, pool V, 3.67 ± 0.9% of total; salt peak, pool VI, 16.8 ± 4.8% of total) eluted as single hFSH peaks after Sephadex G-100 exclusion chromatography (apparent M_r 60000). Even though hFSH present within each pool was recognized by a receptor preparation, the receptor-binding activity expressed as the radioreceptor assay:radioimmunoassay (RRA/RIA) activity ratio varied with the pI value of the particular hFSH isoform tested; starting from a pI value of 5.9, the receptor-binding activity of hFSH decreased from 4.25 ± 0.28 to 1.17 ± 0.14, as the pI value of the corresponding isoform declined. A similar trend was observed when the potency of each isoform was assessed by an in vitro bioassay. hFSH with pI value 7.6–7.1 exhibited a receptor-binding activity intermediate between those of hFSH isoforms V and VI and an in vitro biopotency similar to that presented by hFSH isoform IV. Isoforms I and VI exhibited the shortest plasma clearance rates (0.102 ± 0.01 and 0.104 ± 0.01 ml/min respectively) and longest mean residence times (221.6 ± 24.5 and 150.9 ± 12.6 min) and half-lives (147.2 ± 18.3 and 101.2 ± 8.7 min); pools II to V showed plasma clearance rates, mean residence times and half-lives ranging from 0.789 ± 0.02 to 0.254 ± 0.02 ml/min, 44.8 ± 5.2 to 102.4 ± 6.5 min and 35.4 ± 3.4 to 69.1 ± 4.0 min respectively. Excluding isoform I, there were highly significant (P < 0.01) correlations between these kinetic parameters and the mean pI value of the isoforms (r = 0.837, r = 0.784 and r = 0.801, respectively). These results demonstrate that the human anterior pituitary gland contains a significant number of hFSH isoforms which may be separated by their electrical properties. For the majority of the intrapituitary hFSH isoforms isolated by chromatofocusing, there is a close relationship between their physicochemical biological properties. The characteristic biological features of each isoform, as defined by these studies, might contribute to understanding the significance of the changes in relative distribution of circulating hFSH isoforms occurring during various physiological conditions.