Chromatofocusing fails to separate hFSH Isoforms on the basis of glycan structure

Follicle-stimulating hormone (FSH) glycosylation is regulated by feedback from the gonads, resulting in an array of glycans associated with FSH preparations derived from pools of pituitary or urine extracts. FSH glycosylation varies due to inhibition of FSH beta N-glycosylation, elaboration of 1-4 branches possessed by mature N-glycans, and the number and linkage of terminal sialic acid residues. To characterize FSH glycosylation, FSH isoforms in pituitary gland extracts and a variety of physiological fluids are commonly separated by chromatofocusing. Variations in the ratios of immunological and biological activities in the resulting FSH isoform preparations are generally attributed to changes in glycosylation, which are most often defined in terms of sialic acid content. Using Western blotting to assess human FSH beta glycosylation inhibition revealed 30-47% nonglycosylated hFSH beta associated with four of six hFSH isoform preparations derived by chromatofocusing. Glycopeptide mass spectrometry assessment of glycan branching in these isoforms extensively characterized two N-glycosylation sites, one at alpha Asn(52), the critical glycan for FSH function, and the other at beta Asn(24). With two to four N-glycans per FSH molecule, many combinations of charges distributed over these sites can provide the same isoelectric point. Indeed, several glycans were common to all isoform fractions that were analyzed. There was no trend showing predominantly monoantennary glycans associated with the high-pI fractions, nor were predominantly tri- and tetra-antennary glycans associated with low-pI fractions. Thus, differences in receptor binding activity could not be associated with any specific glycan type or location in the hormone. FSH aggregation was associated with reduced receptor binding activity but did not affect immunological activity. However, as gel filtration indicated sufficient heterodimer was present in each isoform preparation to generate complete inhibition curves, the near total loss of receptor binding activity in several preparations could not be explained by aggregation alone, and the mechanism remains unknown.