Assignment of the Complete Disulphide Bridge Pattern in the Human Recombinant Follitropin β -Chain

The chemical assessment of the complete disulphide bridge pattern in the βchain of human recombinant follicotropin (βFSH) was accomplished by integrating classical biochemical methodologies with mass spectrometric procedures. A proteolytic strategy consisting of a double digestion of native βFSH using the broadspecificity protease subtilisin first, followed by trypsin, was employed. The resulting peptide mixture was directly analysed by FABMS, leading to the assignment of the first three disulphide bridges. The remaining SS bridges were determined by HPLC fractionation of the proteolytic digest followed by ESMS analysis of the individual fractions. The pattern of cysteine couplings in βFSH was determined as: Cys3-Cys51, Cys17-Cys66, Cys20-Cys104, Cys28-Cys82, Cys32-Cys84 and Cys87-Cys94, confirming the arrangement inferred from the crystal structure of the homologous βCG. A subset of the SS bridge pattern comprising Cys3-Cys51, Cys28- Cys82 and Cys32-Cys84 constitutes a cysteine knot motif similar to that found in the growth factor superfamily.