Biochemical analyses of proteolytic nicking of the human glycoprotein hormone alpha-subunit and its effect on conformational epitopes.

Conformational features of two epitopes on the glycoprotein hormone alpha-subunit were investigated using two antihuman FSH (anti-hFSH) monoclonal antibodies (mAbs) 3A and 5F that recognize different epitopes and are specific for alpha-subunit. These mAbs were used to investigate whether the conformation of these epitopes was different in heterodimeric hFSH, hTSH, hLH, or hCG. Any differences in the mass of hormone in each preparation were accounted for by sodium dodecyl sulfate-polyacrylamide gel electrophoresis/Western blot analysis of all hormone preparations used in this study. Rabbit anti-hFSH alpha-(11-27) antipeptide antisera and 125I]protein-G were used in the Western blot analysis. Radioactivity associated with each band was determined and used to normalize the mass of alpha-subunit in each reference preparation used in the displacement assays. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was also performed in order to examine the integrity of each of the hormone reference preparations. hTSH alpha and, to a lesser extent, hLH alpha preparations contained an internal nick in the polypeptide chain. RIA analysis performed using heterodimeric glycoprotein hormones as competitors revealed that an average 100-fold difference in the ED50 values for hFSH compared to the other glycoprotein hormones was seen with mAb 3A. Therefore, the conformation of 3A epitope appeared to be different in hFSH than in hTSH, hLH, or hCG. In comparison, the epitope recognized by mAb 5F only had an average 7-fold difference in reactivity (ED50 values) for hFSH compared to hTSH, hLH, and hCG. Likewise, competition assays using the respective alpha-subunits and mAb 5F revealed a pattern of competition similar to that observed with heterodimers, with an average 4-fold difference in the ED50 values for hFSH alpha compared to those for hTSH alpha, hLH alpha, and hCG alpha. Therefore, the conformation of the 5F epitope appears unaffected by association of alpha-subunit with beta-subunit. Accordingly, any differences in the conformation of the four alpha-subunits, as demonstrated by these small differences in ED50 values, appear to be inherent to each alpha-subunit. In fact, the 5F epitope appears to be quite rigid, since nicked alpha-subunit preparations could compete with 125I]hFSH for binding to 5F with comparable potency to non-nicked alpha-subunits. These findings support the concept that epitopes on heterodimeric hFSH alpha may have different conformational features. Some are specific for heterodimeric hFSH alpha, and we refer to these as conformationally active (flexible). Others are common to the four human glycoprotein hormone alpha-subunits, suggesting that they are conformationally constrained (rigid).