Association/dissociation of gonadotropin subunits involves disulfide bridge disruption which is influenced by carbohydrate moiety

The association and dissociation rates of pituitary porcine luteinizing hormone (pLH) and equine LH (eLH) at oxidizing potential were slow and those of equine choriogonadotropin (eCG) were even much slower. At reducing potential mimicking endoplasmic reticulum condition, association of pLH subunits was observed in less than 5 min instead of 24 h at oxidizing potential. At neutral pH and 37 degrees C, DTNB and 2-nitro-5-thiocyanobenzoic acid (NTCB) were found to react with two cysteine residues (i.e., one S-S bridge) in pLH. The temperature dependence of the NTCB reaction on pLH was found to be similar to that of the dissociation of the hormone (Tm approximately 75 degrees C). The tight correlation between the reaction of two cysteines and dissociation of the subunits of pLH and eLH strongly suggests that transient opening of one fragile disulfide bridge is required for heterodimer assembly. Moreover, the absence of cysteine reaction with eCG indicates that its bulky carbohydrate chains exert a negative influence on the opening of this bridge leading to considerably diminished association-dissociation rates of its subunits.