Hypo-glycosylated hFSH21/18 (possesses FSHβ21 and FSH18 bands) was isolated from hLH preparations by immunoaffinity chromatography followed by gel filtration. Fully-glycosylated hFSH24 was prepared by combining the fully-glycosylated FSHβ24 variant with hCGα and isolating the heterodimer. The hFSH21/18 glycoform preparation was significantly smaller than the hFSH24 preparation and possessed 60% oligomannose glycans, which is unusual for hFSH. Hypo-glycosylated hFSH21/18 was 9- to 26-fold more active than fully-glycosylated hFSH24 in FSH radioligand assays. Significantly greater binding of 125I-hFSH21/18 tracer than hFSH24 tracer was observed in all competitive binding assays. In addition, higher binding of hFSH21/18 was noted in association and saturation binding assays, in which twice as much hFSH21/18 was bound as hFSH24. This suggests that more ligand binding sites are available to hFSH21/18 in FSHR than to hFSH24. Hypo-glycosylated hFSH21/18 also bound rat FSHRs more rapidly, exhibiting almost no lag in binding, whereas hFSH24 specific binding proceeded very slowly for almost the first hour of incubation.

Follicle stimulating hormone (FSH) is one of the important hormones that regulate gonadal functions. This hormone is glycosylated, and the glycans greatly influence the biological properties. In the present study the negatively charged glycopeptides of equine and human pituitary follicle stimulating hormone (eFSH and hFSH) have been characterized in a glycosylation site-specific manner using FT-ICR-MS and Edman sequencing. The characteristic pattern of glycan distribution at each glycosylation site has been deduced and compared between horse and human FSH preparations. The data suggest that site-specific differences exist between glycoforms of human and equine FSH. For instance, except for one site in the beta subunit (Asn7) of hFSH all other sites in both species have sulfated glycoforms. Also, glycoforms at Asn52 of hFSH are all complex type, whereas in eFSH, both complex and hybrid structures exist at this site. There is also a higher percentage of sulfated glycans in the latter site compared to the former. This is the first study that characterizes the glycans from this hormone in a glycosylation site-specific manner, and these data can be used to begin correlative studies between glycosylation structure and hormone function.

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Summary Previously, our laboratory demonstrated the existence of a β-subunit glycosylation-deficient human FSH glycoform, hFSH21. A third variant, hFSH18, has recently been detected in FSH glycoforms isolated from purified pituitary hLH preparations. Human FSH21 abundance in individual female pituitaries progressively decreased with increasing age. Hypo-glycosylated glycoform preparations are significantly more active than fully-glycosylated hFSH preparations. The purpose of this study was to produce, purify and chemically characterize both glycoform variants expressed by a mammalian cell line. Recombinant hFSH was expressed in a stable GH3 cell line and isolated from serum-free cell culture medium by sequential, hydrophobic and immunoaffinity chromatography. FSH glycoform fractions were separated by Superdex 75 gel-filtration. Western blot analysis revealed the presence of both hFSH18 and hFSH21 glycoforms in the low molecular weight fraction, however, their electrophoretic mobilities differed from those associated with the corresponding pituitary hFSH variants. Edman degradation of FSH21/18 -derived β-subunit before and after peptide-N-glycanase F digestion confirmed that it possessed a mixture of both mono-glycosylated FSHβ subunits, as both Asn7 and Asn24 were partially glycosylated. FSH receptor-binding assays confirmed our previous observations that hFSH21/18 exhibits greater receptor-binding affinity and occupies more FSH binding sites when compared to fully-glycosylated hFSH24. Thus, the age-related reduction in hypo-glycosylated hFSH significantly reduces circulating levels of FSH biological activity that may further compromise reproductive function. Taken together, the ability to express and isolate recombinant hFSH glycoforms opens the way to study functional differences between them both in vivo and in vitro.