The effects of denaturant and temperature on the circular dichroic spectrum of ovine lutropin-alpha

Undissociable chemically cross-linked and single-chain gonadotropins

Undissociable gonadotropins can be obtained either by chemical cross-linking of the natural heterodimeric hormones or by expressing recombinant single-chain molecules through the fusion of their α and β polypeptide sequences. These undissociable hormones are not more active than their natural heterodimeric counterparts indicating that the β-subunit seatbelt embracing the α-subunit ensures the αβ heterodimer stability in physiological conditions. The main interests of single-chain gonadotropins are that 1/only one single plasmid is required to produce an active recombinant hormone, 2/the two subunits' domains are constantly present in equal amounts and 3/they remain in close proximity even at low concentration for forming the hormone bioactive 3D structure. These undissociable gonadotropins have been shown to exhibit excellent stability and activity but they have not yet been commercialized probably because of immunogenicity risks and cost of production. Nevertheless, they might be used as a basis for the development of chemically simpler and cheaper ligands of LH and FSH receptors.

Circular dichroic spectroscopy of Arg46-nicked ovine lutropin alpha and derived fragments

The alpha subunit of ovine lutropin can be nicked with the endoproteinase Arg-C to give a single cleavage of the Arg46-Ser47 peptide bond. Following reduction by sulfitolysis, the N-terminal (residues 1-46) and C-terminal (residues 47-96) fragments can be separated and then recombined and reoxidized to yield a reconstituted nicked alpha that binds to the beta subunit but exhibits only 2-3% of the receptor-binding potency of intact lutropin. We have investigated nicked alpha, the two separated fragments, and reconstituted nicked alpha by circular dichroic spectroscopy and compared the spectra with those of intact alpha and reduced, reoxidized intact alpha. Between 200 and 225 nm the spectra of the two intact preparations are similar, as are the spectra of the two nicked preparations. However, the extremum negative ellipticities of the nicked preparations are substantially less than those of the intact preparations between 210 and 220 nm, indicating a loss in secondary structure accompanying cleavage of the Arg46-Ser47 bond. The sum of the spectra of the two fragments is significantly different from that of reconstituted nicked alpha, showing that the secondary structures in the isolated fragments are quite different from that of the reconstituted nicked protein. Reduced receptor binding by lutropin preparations containing a nicked alpha subunit may be attributable in part to the loss of secondary structure, probably helicity.