Transcortin: A Corticosteroid-Binding Protein of Plasma. Iii. The Effects of Various Steroids *†

In 1959 we described elevated plasma concen-trations of transcortin during pregnancy and after the administration of estrogens (1, 2). Since then, a number of investigations utilizing a va-riety of methodologies to ascertain the binding of cortisol to transcortin have confirmed our ob-servations (3-8). The different techniques used and the diverse arithmetic approaches employed all indicate that the plasma concentration of trans-cortin rises about threefold during pregnancy or after estrogen therapy. These findings (3-8) are in essential agreement with our original, less sophisticated observations and calculations. An important contribution has been the isolation by Seal and Doe (9, 10) of a highly purified plasma transcortin preparation, making it possible to as-certain the amino acid composition of the protein and many of its physical properties. They have shown that transcortin has one binding site for cortisol and a molecular weight of approximately 45,000 (9, 10). More recently, these workers have demonstrated, by means of isolation studies, that the plasma transcortin concentration is ap-proximately 2.5 mg per 100 ml in normal sub-jects and 7.5 mg per 100 ml in estrogen-treated subjects (7). These and other values for nor-mal, estrogen-treated, and pregnant subjects (3-8) are higher than our original calculations (1). Our low values can be attributed to the crudeness of the early techniques and the less critical theo-retical physicochemical approaches utilized. The methods used in our laboratory now obviate these shortcomings and yield values similar to those ob-tained by other workers.