PREPARATION AND CHARACTERISTICS OF CORTICOSTEROID-BINDING GLOBULIN (CBG, TRANSCORTIN)

Abstract

The corticosteroid-binding globulin (CBG, transcortin) rises in the blood of the developing mammal from small concentrations in the young to adult levels which are similar in man and woman, but twice as high in the female rat as in the male. The affinity of the CBG complexes with corticosteroids is maximal at approximately pH 8; it decreases with increasing temperature. Study of the relative binding strength for different steroids clearly indicated a species specificity of the CBG molecules. This was confirmed by the isolation of CBG from serum of man, rat and rabbit, accomplished by a variety of chromatographic procedures. The pure glycoproteins, homogeneous by physicochemical criteria, proved to be distinct molecules on the basis of amino acid and carbohydrate composition and other properties. They possess one principal steroid binding site. Thermodynamic data indicate a very tight fit of the interacting components in the complexes. Removal of about 90% of corticosterone from its complex with rat CBG by gel filtration at 23° results in polymerization of the glycoprotein molecule to dimeric, tetrameric and octameric forms. Recombination of the polymeric mixture with 1 mole corticosterone per 53 000 g CBG re-forms the monomer. Possible biological significance of this reversible polymerization of a carrier protein under the regulatory control of the binding steroid hormone has to be explored.