BIOLOGICAL CHARACTERIZATION OF THE NATURALLY-OCCURRING ANALOGS OF INTRAPITUITARY HUMAN FOLLICLE-STIMULATING-HORMONE

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 (pl value 7.6 - 7.1, pool I, 1.5 +/- 0.13% of total immunoactivity recovered; pl 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; pl value 4.5 - 4.1, pool IV, 54.8 +/- 4.9% of total; pl 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.