Biological Properties of Recombinant FSH Isohormones
A FSH preparation can be characterized with four essentially different assays, each having its own specific merits: (i) The immunoassay determines FSH-specific structural features and provides a relative measure for the quantity of FSH. (ii) The receptor binding assay provides information on the proper conformation for interaction with the FSH receptor. Receptor binding studies with calf testis membranes have shown that FSH isoform activity in follitropin b decreases when going from high to low pI isoforms. (iii) The in vitro bioassay measures the capability of FSH to transduce signals into target cells (the intrinsic bioactivity). The in vitro bioactivity, assessed in the rat Sertoli cell bioassay, also decreases when going from high to low pI isoforms. (iv) The in vivo bioassay provides the overall bioactivity of a FSH preparation. It is determined by the number of molecules, the plasma residence time, the receptorbinding activity and the signal transduction. Surprisingly, in contrast to the receptor binding and in vitro bioassays, the in vivo biological activity determined in rats shows an approximate 20-fold increase between isoforms with a pI value of 5.49, as compared to those with a pI of 4.27. These results indicate that the basic isohormones exhibit the highest receptor binding and signal transduction activity, whereas the acidic isohormones are the more active forms under in vivo conditions.