As explained above, FSH exists in many distinct molecular forms (isohormones), with identical poly-peptide backbones but diffences in oligosaccharide structure, in particular in the degree of terminal sialylation. These isohormones can be separated by chromatofocusing or isoelectric focusing on the basis of their different isoelectric points [pI, as has been demonstrated for follotropin b (de Leeuw et al., 1996)] (Fig. 2). The typical pattern for FSH indicates an
isohormone distribution between pI values of 6 and 4. To obtain structural information at the subunit level, the two subunits were separated by RP-HPLC and treated to release the N-linked carbohydrate side-chains. Fractions with low pI values (acidic fractions) displayed a high content of tri- and tetrasialo oligosaccharides and a low content of neutral and monosialo oligosaccharides. For fractions with a high pI (basic fractions) value the reverse was found. The β-subunit carbohydrate side chains appeared to bemore heavily sialylated and branched than the α-subunit carbohydrate side chains. The low pI value isohormones of follitropin b have a high sialic acid/ galactose ratio and are rich in tri- and tetra-antennary N-linked carbohydrate side chains, as compared with the side chains of the high pI value isohormones.