Biological and immunological properties of the International Standard for FSH 83/575: isoelectrofocusing profile and comparison with other FSH praparations.

The new international standard for FSH, IS 83/575, has been analyzed, after isoelectric focusing separation, by Sertoli cell in vitro bioassay, radioligand receptor assay and two highly specific immunometric assays. Its molecular composition was then compared with the isoelectric focusing profiles obtained from the fractionation of the reference preparation 2nd IRP 78/549 and from pools of human male and female pituitary extracts and male and female sera. The results showed that > 80% of immunoreactive and bioactive FSH in the IS 83/575 has a pI value < 4, while such very acidic material was represented much less in the other FSH preparations tested. All the immunoreactive material contained in the IS 83/575 was shown to be capable of receptor binding and bioactivity in vitro. A generally good correspondence between IEF profiles obtained by bioassay and by immunofluorimetric assay was evident in the case of IS 83/575, 2nd IRP 78/549 and pituitary extracts, although the profiles recorded by immunofluorimetric assay were rather smooth and more isoforms were detected by bioassay. A striking discrepancy between immunoreactive FSH and bioactive FSH was observed after isoelectric focusing fractionation of the serum pools, in which some bioactive material was not detected by immunofluorimetric assay and some of the immunoreactive FSH peaks were devoid of bioactivity, indicating that serum contains inhibitors of FSH action and that immunometric assays based on monoclonal antibodies may miss some bioactive FSH isoforms.