Reversible folding of human chorionic gonadotropin at acid pH or upon recombination of the α and β subunits

Abstract

Gonadotropic hormones share the property of being made of two non-covalently bound subunits, the CY and /I subunits. Isolated subunits have been found to be inactive either by biological or by radioligand receptor assays [ 1,2]. Circular dichroism (CD) measurements performed by several authors 13-51 on native ovine luteinizing hormone (oLH) and its isolated subunits strongly suggest at least a partial unfolding of the polypeptide chains upon dissociation and refolding upon reassociation and subsequent recovery of the biological activity. Recently it has been found by Aloj et al. [6] that native human chorionic gonadotropin (hCG) and hLH, but not their subunits, enhance ANS fluorescence, which seems to be a sensitive probe of the associated state. However, this property is so far limited to these luteinizing hormones since, for example, native oLH exhibits only a weak binding affinity for ANS (7 X lo3 M-’ at 37°C) and a very low enhancement (ten times less than hCG) of the quantum yield of fluorescence as found by Aloj et al. [6] and by Garnier (unpublished). It was suggested by studies of the chemical reactivity towards tetranitromethane or iodine that some tyrosine residues were exposed by dissociation of the subunits of ovine, bovine and porcine LH [7-lo]. The same conclusion was reached by difference spectroscopy [4] and CD measurements of oLH 13-51. Evidence IS presented here [fiat acid dissociation of hCG also resulted in a reversible unfolding of the subunits during which tyrosine residues were pcrturb-