Demonstration of heavy and light protomers of human testosterone-estradiol-binding globulin

Abstract

Human testosterone-estradiol-binding globulin (hTeBG) was purified from pregnancy serum by sequential ammonium sulfate precipitation, affinity chromatography, and hydroxylapatite chromatography. An overall purification of 2800-fold was achieved with a 27% total yield. Apparent homogeneity of the final product was shown by polyacrylamide gel electrophoresis with or without sodium dodecyl sulfate (SDS). The equilibrium dissociation constant ( K d ) at 4°C for 5α-dihydrotestosterone (DHT) was estimated to be 1.94 ± 0.95× 10 −9M. Analysis of the purified protein revealed micro-heterogeneity with regard to size on polyacrylamide gel in the presence of SDS and to charge on isoelectric focusing gels. The apparent molecular weight of native hTeBG determined by gradient gel electrophoresis was 115,000. SDS-polyacrylamide gel electrophoresis indicated that hTeBG is comprised of two molecular weight components of 53,000 and 46,000, which are designated as heavy (hTeBG H) and light (hTeBG L) protomers, respectively. Photolysis of purified hTeBG with [1,2- 3H]17β-hydroxy-4,6-androstadien-3-one ([ 3H]Δ 6-testosterone) resulted in specific labeling of its binding sites. Analysis of photolabeled products by SDS-polyacrylamide gel electrophoresis revealed two radioactive products with electrophoretic mobilities identical to those of the hTeBG H and hTeBG L. The ratio of hTeBG H to hTeBG L was about 10:1. The H and the L protomers were separated and examined by peptide mapping using protease V 8 and chymotrypsin. Comparison of the fragmentation patterns produced by these proteases revealed that hTeBG H and hTeBG L components were nearly identical. Removal of sialic acid or carbohydrate residues from hTeBG did not affect the presence of two molecular components. Isoelectric focusing of native hTeBG demonstrated three isoelectric variants with pIs at 4.75, 4.85 and 4.90. After treatment with neuraminidase and other glycosidases, only two isoelectric species were observed with more alkaline pIs. Although purified hTeBG appeared heterogeneous with regard to size and charge, it was remarkably homogeneous in its ability to absorb to Concanavalin A-Sepharose. We conclude that hTeBG, like the androgen binding proteins of the rabbit and rat, is a dimer whose monomer exhibits two protomeric forms.