EFFECT OF DESIALYLATION OF HIGHLY PURIFIED ISOFORMS OF HUMAN LUTEINIZING-HORMONE ON THEIR BIOACTIVITY IN-VITRO, RADIORECEPTOR ACTIVITY AND IMMUNOACTIVITY

To establish whether sialic acid content is responsible for an observe d 7-8-fold variability in bioactivity in vitro of highly purified huma n pituitary luteinizing hormone (hLH) isoforms, the bioactivity in vit ro, radioreceptor activity and immunoactivity of hLH isoforms were det ermined before and after enzymatic desialylation. Three immunofluorome tric assays with different hLH specificities allowed characterization of 13-24 pituitary hLH isoform preparations of pI 7.03-8.98 in terms o f sialic acid content (1-5 sialic acid residues per LH molecule), bioa ctivity in vitro (4030-30 000 I.U. mg(-1)), radioreceptor activity (64 20- 400 I.U. mg(-1)) and hLH immunoactivity (2900-4400 to 18 300-27 30 0 I.U. mg(-1)). Significant positive correlations between sialic acid content and either immunoactivity or in vitro bioactivity were observe d, whereas radioreceptor activity showed a curvilinear response. Follo wing more than 90% removal of sialic acid, both in vitro bioactivity a nd radioreceptor activity were increased, although specific activity s till differed between isoforms; immunoactivities were unaffected. It i s concluded that the presence of the sialic acid residue(s) on hLH iso forms does partially contribute to the in vitro bioactivity and radior eceptor activity of the isoforms, but that hLH immunoactivity is indep endent of sialic acid content.