Immunological properties of the beta-subunit of human chorionic gonadotropin. I. Effect of chemical and enzymatic modifications.

The beta-subunit of hCG (hCG beta) displays immunological cross-reactivity with human LH (hLH). A detailed study was undertaken to investigate the effect of chemical and enzymatic modifications on the immunological behavior of hCG beta, particularly on its cross-reactivity with hLH, with a view to obtaining a highly hCG-specific antigen. hCG beta was modified in both the carbohydrate and protein parts of the molecule. The carbohydrate part was modified enzymatically by sequential cleavage of monosaccharides by specific glycosidases, and the protein portion was modified chemically in the amino and carboxyl groups and in the cystinyl, tyrosyl, histidyl, and arginyl residues. These derivatives were immunologically evaluated by RIAs; their hCG beta activities were measured in the [125I]hCG beta-anti-hCG beta system, and the hLH cross-reactivity in the [125I]hLH-anti-hLH system. The present studies have led to the following conclusions. 1) The carbohydrate does not play a significant role in the immunological activity of hCG beta. 2) The antigenic determinants of hCG beta reside primarily in the polypeptide chain and are conformational rather than sequential in nature. 3) hCG beta has two types of antigenic determinants, those which are unique to hCG and those which are common to both hCG and hLH. 4) Finally, it is possible to destroy preferentially one or the other type of determinants. The controlled reduction and alkylation of hCG beta yielded derivatives which retain significant immunological activity in the hCG beta system, but have no or reduced cross-reactivity in the hLH system. These derivatives are much more specific antigens than hCG beta and, therefore, are of potential importance for the development of a specific RIA for hCG and possibly for use as contraceptive agents.