The 105000 g supernatant fraction (soluble proteins) of homogenates of human uterine tissue, essentially myometrium, and peripheral blood serum, were assayed for progesterone-and cortisol-binding activity by batchwise use of Sephadex G-25 in test tubes (not gel filtration) under equilibrium conditions; the results were expressed empirically as the reciprocal of the protein concentration, 1/P, at 50% steroid binding. The steroid-binding activity of the uterine soluble proteins was due apparently to a single component closely resembling corticosteroid-binding globulin (CBG) with respect to equilibrium association constants, steroid ligand specificity, and elution behaviour on hydroxylapatite chromatography and Sephadex G-100 filtration. Therefore, the mean 1/P values for steroid-binding activity were converted to the apparent CBG content (nmol/g total soluble protein). Comparison of the mean ratio (about 0\m=.\2) of the respective total binding activity, uterus (g): serum (ml), with the mean ratio (0\m=.\03) of the total iron content, uterus:serum, indicated that the high CBG content of the uterus could not be attributed to retention of blood; the apparent CBG levels in skeletal muscle (pectoral) were very low. The mean values for the apparent CBG content of uterine soluble protein were almost as high as those for the CBG content of serum protein; the mean value for the apparent CBG content of uterine soluble protein was slightly higher for pregnant women (mostly in early pregnancy) than that for non-pregnant women with various gynaecological complications. On polyacrylamide gel electrophoresis, soluble protein from the uterus resembled that of serum in the areas corresponding to serum albumin and transferrin; the electrophoretic patterns were distinctly different, however, in the area between the origin and the transferrin band.It is suggested that CBG may accumulate in interstitial fluid of the myometrium, thereby regulating the local concentration of bound and unbound progesterone and cortisol, and influencing hormone action on the respective target cells, muscle and fibroblast. A more extensive study of levels of progesterone-and cortisol-binding activity in serum was carried out also. It was concluded that the progesterone-binding activity of human serum, as in the case of human myometrium, is due largely to CBG.

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