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In 1959 we described elevated plasma concentrations of transcortin during pregnancy and after the administration of estrogens (1, 2). Since then, a number of investigations utilizing a variety of methodologies to ascertain the binding of cortisol to transcortin have confirmed our observations (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 ascertain 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 approximately 2.5 mg per 100 ml in normal subjects and 7.5 mg per 100 ml in estrogen-treated subjects (7). These and other values for normal , 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 theoretical physicochemical approaches utilized. The methods used in our laboratory now obviate these shortcomings and yield values similar to those obtained by other workers. The present study was undertaken to determine whether the elevated transcortin levels following estrogen therapy are due 1) to increased synthesis of the protein or 2) to decreased turnover and, hence, prolonged half-life of transcortin. This study has been made feasible through the isolation by Seal and Doe (9, 10) of a highly purified and well-characterized preparation of transcortin, making possible the injection of labeled transcortin free from any significant protein contaminants. Another important advance that encouraged us to undertake the study has been the feasibility of utilizing labeled proteins in the study of their half-lives and the demonstration that the introduction of a small number of iodine atoms into a protein appears not to change its biological behavior (11-15). Materials and Methods The transcortin used in this study was isolated according to the method of Seal and Doe (9, 10).1 The protein was iodinated according to the technique of Pressman and Eisen (16) as modified by Day, Planinsek, Korngold, and …

Transcortin: A Corticosteroid-binding Protein of Plasma. VII. Half-Life in Normal and Estrogen-treated Subjects *