Molecular dissection of an hCG-beta epitope using single-step solid phase radioimmunoassay

Peptides and proteins have both sequence-specific (contiguous) and conformation-specific (discontiguous) epitopes. Sequence-specific epitopes are delineated by peptide approach and other robust methods like competition assays, gene expression assays, synthetic peptide library based assays etc. Available methods for delineation of conformation-specific epitopes are cumbersome (X-ray crystallography etc.), time-consuming and require costly sophisticated equipments. Hence, there is a need to develop a simple method for identification and mapping of conformation-specific epitopes.

In the single-step solid phase radioimmunoassay (SS-SPRIA), an immunochemical bridge of 'mouse IgG-anti-mouse IgG' was prepared in the polypropylene wells followed by adsorption with hCG specific monoclonal antibody (MAb) G(1)G(10).1. The extent of competitive inhibition in binding ability of (125)IhCG-beta with chemically or enzymatically modified hCG-beta to immobilized MAb G(1)G(10).1 in comparison to hCG-beta standards was utilized to identify the epitopic amino acid involved in epitope-paratope interaction.

Data clearly suggest that the epitope under investigation consisted of Arg (94, 95) and Asp (99) at the core region with a Lys (104) and a His (106) in the proximity and absence of chymotrypsin susceptible Phe or Tyr in this region.

The data of SS-SPRIA revealed the 93-100 loop of amino acid sequence, as the core region of conformation-specific epitope of hCG-beta at or near the receptor-binding region. Hence, SS-SPRIA seems to be a simple method for identification and mapping of conformation-specific epitopes.