Subunits of human sex hormone binding globulin. Interindividual variation in size

Sex hormone binding globulin (SHBG), a dimeric plasma glycoprotein with a molecular mass of about 90 kDa, was purified from healthy individuals by a rapid two-step procedure using immunoaffinity chromatography on a monoclonal antibody column followed by fast protein liquid chromatography. The individual SHBGs so isolated were pure by several criteria, and the overall yield was usually about 20% according to radioimmunoassay. The isolated SHBGs were analyzed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate which showed the SHBG isolated from most subjects to be composed of subunits of two different sizes (52 and 49 kDa) present in the approximate ratio of 10:1 (double-banded SHBG). The SHBG of the remaining subjects contained a third subunit with an estimated molecular mass of about 56 kDa (triple-banded SHBG). In this kind of SHBG, the two heavy subunits were present in approximately equal amounts, suggesting that individuals with triple-banded SHBG are heterozygotes for a genetic variant of the protein. The various subunits of SHBG were separated and individually subjected to amino-terminal amino acid sequence analysis. They all had a heterogeneous amino terminus, but since the sequences obtained seemed to be identical, the structural differences between the subunits would appear to reside in other parts of the molecules. On isoelectric focusing, both kinds of SHBG were resolved into about 10 components, all with steroid-binding activity. Differences were noted between double-banded and triple-banded SHBG, the latter having a greater abundance of acidic species. Screening of 121 healthy individuals by a procedure involving small-scale isolation of SHBG on an antibody column followed by Western blotting revealed that 20% of the subjects had triple-banded SHBG. This new variant of SHBG was found in persons of both sexes and in children as well as in adults.