Conformation of the $\alpha$-subunit of glycoprotein hormones: a study using polyclonal and monoclonal antibodies

The conformation of the common $\alpha$-subunit of human glycoprotein hormones, luteinizing hormone (hLH), follicle-stimulating hormone (hFSH), thyroid-stimulating hormone (hTSH) and chorionic gonadotropin (hCG) was probed using a highly specific polyclonal antiserum against the $\alpha$-subunit of hCG and several monoclonal antibodies (MAbs) produced against hCG which recognized the $\alpha$-subunit in free and combined form. The $\alpha$-subunit was found to be conformationally altered (compared to its conformation in the isolated state) when it was in combination with various $\beta$-subunits as indicated by shifts in the displacement curves of binding of $[^{125}I]$hCG $\alpha$ to the polyclonal antiserum. The extent of the change was dependent on the $\beta$-subunit present with minimum change being observed with hLH$\beta$, intermediate with hCG$\beta$ and maximum change with hFSH and TSH $\beta$-subunits. However, the affinity constants of this antiserum for all four hormones were nearly similar. Further, it was also found that binding of any one of the glycoprotein hormones to this antibody could be completely inhibited by any other hormone suggesting that the conformation of the $\alpha$-subunit in all the four hormones is probably very similar. This was further investigated using five hCG MAbs capable of recognizing the $\alpha$-subunit, but with different epitope specificities. All these MAbs could recognize all the four hormones suggesting the presence of the epitopes in these proteins. These epitopes were conformation specific since the MAbs did not bind reduced and carboxymethylated $\alpha$-subunit. Displacement analysis using $[^{125}I]$hCG as the tracer showed that two epitopes have nearly the same conformation in all the four hormones, while two were partially modified depending on the $\beta$-subunit present. Based on these results, it is concluded that the $\alpha$-subunit of glycoprotein hormones has nearly the same conformation, though subtle differences do exist