Steroid Protein Interactions

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Abstract
Steroid hormones are more or less reversibly bound by serum albumin, perhaps each on a different binding site. Many variations occur with different hormones, chemical modification in the albumin, varying pH, and inhibitors. Corticosteroids are bound by plasma solutions, by the globulin, in particular. These results also vary with several factors, notably relative concentration, temperature, and the composition of the steroid. Some of these data reveal the specificity of binding sites on the protein. Attempts to determine how the cortico-steroid-binding globulin is distributed among plasma fractions revealed that one (Fraction IV4) contains protein with an affinity for cortisol considerably higher than that of albumin. The author has achieved successful electrophoretic demonstration of corticosteroid binding by serum proteins. Using the method of dialysis equilibrium and small amounts of cortisol-4-c14 it was found that pregnancy was not associated with an increase in binding, while plasma from the umbilical cord vein contained significantly more unbound cortisol. The binding activity of plasma proteins in a few cases of dysproteinemia, cirrhosis, multiple myeloma, and nephrosis was investigated; also in various body fluids (spinal, amniotic, synovial, and pleural); also in plasma from rat, dog, rabbit, guinea pig, and alligator. With large amounts of cortisol the binding power of plasma from pregnant women was considerably above normal, as was also that of patients receiving large doses of estrogens. The author discusses the binding of steroid hormones by other plasma proteins than albumin and globulin, and by tissue proteins. Interpretation of the activity of a hormone solely on the basis of total plasma concentration will lead to erroneous conclusions unless the degree of binding is considered. The unbound steroid hormones of plasma are'' probably the biologically significant moieties. The organism is protected from rapid changes in them by the presence of the bound fraction. Binding also conserves undue loss of hormone by diffusion through capillary walls and glomerular membranes.