Tryptic digestion of the alpha subunit of human choriogonadotropin.

Abstract

In order to further characterize chemical, physicochemical, and immunochemical properties, as well as structure-function relationships, of the common alpha subunit of human glycoprotein hormones, a tryptic core was prepared from the alpha subunit of human choriogonadotropin. The core was purified in greater than 80% yield using gel permeation and anion-exchange chromatography, and, following reduction and S-carboxymethylation, the constituent peptides were purified by gel permeation and high performance liquid chromatography. The disulfide-bridged peptides comprising the alpha core were identified as residues 1-35 and residues 52-91 by amino acid composition and amino acid carboxyl sequence analyses of the reduced, S-carboxymethylated peptides. The alpha tryptic core contained both N-asparagine carbohydrate moieties, but was devoid of residues 36-51 and the carboxyl-terminal serine at position 92. The small peptides cleaved from residues 36-51, a known potential O-glycosylation region of the alpha subunit, were purified and identified. The tryptic core retained full immunopotency relative to the intact subunit in the binding to polyclonal and monoclonal antibodies directed against the alpha subunit. The region consisting of residues 36-51 is not part of the epitope recognized by these antibodies. With antisera generated to the reduced, S-carboxymethylated subunit, peptide 1-35, but not 52-91, was immunoreactive. This finding is consistent with the known dominant antigenicity of the amino-terminal region in the reduced, S-carboxymethylated molecule. The core exhibited no appreciable interaction with the complementary beta subunit, and, not surprisingly, was unable to compete with intact hormone binding in a radioreceptor assay using rat testicular homogenates. Circular dichroic spectroscopy was used to probe gross features of tertiary structure (240-300 nm) and secondary structure (190-240 nm). The tryptic core and each of the two constituent peptides exhibited spectra above 240 nm that resembled that of the reduced, S-carboxymethylated subunit more than that of the native material, thus suggesting a significant loss of tertiary structure in the core and isolated peptides. This finding is unexpected in consideration of the full retention of immunopotency by the alpha core although consistent with failure of the core to combine with intact complementary beta subunit. The intact subunit as well as the isolated constituent peptides exhibit little if any helicity in aqueous solution. Interestingly, the reduced, S-carboxymethylated chain and peptide 52-91 displayed helicity in 80% trifluoroethanol, a helicogenic solvent.