Human testosterone-binding globulin is a dimer composed of two identical protomers that are differentially...

Affinity-purified human testosterone-binding globulin (hTeBG) is composed of two subunits [mol wt (Mr), 52,200 and 48,600], as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), electrophoretic transfer, and immunochemical localization with a monoclonal antibody raised against rat androgen-binding protein. Fluorography of SDS-PAGE gels on which photoaffinity-labeled hTeBG was analyzed yielded essentially identical results. Enzymatic deglycosylation of hTeBG with neuraminidase to remove sialic acid led to the production of two subunits of 50,800 and 47,300 Mr when assessed by SDS-PAGE. Treatment of hTeBG with an optimal concentration of N-glycanase to remove Asn-linked oligosaccharides produced a single subunit of 44,100 Mr. When hTeBG was treated with neuraminidase and O-glycanase to remove O-linked oligosaccharides, three subunits were seen, two of which had Mr not clearly different from those obtained with neuraminidase treatment alone plus a subunit of 40,900 Mr. Treatment of hTeBG with a combination of all three enzymes produced a single subunit of 42,900 Mr. Chemical deglycosylation with trifluoromethane-sulfonic acid produced a single subunit with a Mr identical to that produced by treatment with all three enzymes. We concluded that this is the Mr of completely deglycosylated hTeBG. Based on this Mr, carbohydrates contribute 18% and 12% to the apparent Mr of the heavy and light subunits of hTeBG, respectively. Two-dimensional PAGE analysis of hTeBG with its oligosaccharides intact indicated that the heavy subunit was composed of seven isoelectric variants with pI values of 5.87-6.55, while the light subunit was composed of four charge variants with pI values of 6.14-6.55. Treatment of hTeBG with the enzymes resulted in a shift in the pH values to a more basic pH range, indicating that carbohydrate removal also removed charged species from the protein. The greatest cathodal shift occurred when hTeBG was treated with a combination of the three enzymes (pI 7.33-7.77) or when it was chemically deglycosylated (pI 6.37-7.02). Despite the apparent removal of all carbohydrates, the single subunit was still composed of multiple isoforms. This finding suggests that other charged species remain on the hTeBG molecule.