Steroid-Protein Interactions, XII. Distribution of Progesterone and Corticosteroid Hormones Among Serum Proteins

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Abstract
For an assessment of the distribution of a steroid hormone among different binding proteins in the circulating blood it is necessary to know the concentrations of binding sites and the association constants for the steroid-protein complexes involved. Human serum was cleared of endogenous steroid hormones by gel filtration at 45[degree]C, and the concentration of high affinity-binding sites for cortisol, corticosterone, and progesterone determined by equilibrium dialysis. They were found to be approximately the same for the three steroid hormones studied, i. e., on the average 7. 2 x 10-7 M, corroborating the assumption that these three steroids interact with the same binding sites. The association constants of the various complexes at 4[degree] and 37[degree] were also determined from the dialysis data. The number of binding sites for progesterone per albumin molecule was found to be 2. The association constant increased 2. 5 times after delipidation of the highly purified crystalline albumin. Using formulas based on the law of mass action, the distribution of progesterone, cortisol, and corticosterone between transcortin and serum albumin was calculated, for the assumption that only one of the steroids was present. New formulas were developed to assess the distribution of several steroid hormones when present simultaneously. A limiting condition was that the steroids compete for the same binding sites. The distribution of progesterone, cortisol, and corticosterone among transcortin, albumin and orosomucoid was calculated for pregnancy serum.