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Evidence is presented that alpha-fetoprotein (AFP), a serum globulin, accounts mainly, if not entirely, for the high estrogen-binding properties of uterine cytosols from immature rats. By the use of specific immunoadsorbents to AFP and by competitive assays with unlabeled steroids and pure AFP, it has been demonstrated that in hypotonic cytosols AFP is present partly as free protein with a sedimentation coefficient of about 4-5 S and partly in association with some intracellular constituent(s) to form an 8S estrogen-binding entity. The (7,8) and in adult animals bearing primary liver cancer (9). In immature rats, serum AFP ranges from about 1 mg/ml (21-day-old animals) to several hundred ,ug/ml (25-day-old animals).Numerous studies, reviewed recently (10, 11), have demonstrated the presence of high-affinity hormone binders, called "receptor" proteins, in soluble homogenates of estrogen-responsive tissues. Rat-uterine cytoplasmic extracts (cytosols) incubated with tritiated estrogens and subjected to density gradient centrifugation have revealed two major macromolecular complexes with sedimentation coefficients close to 4-5 S and 8-9 S; these are currently referred to as the 4-5S and the 8S "receptors." The 8S complex predomiAbbreviation: AFP, alpha-fetoprotein. nates in hypotonic solutions, whereas in salt concentrations above 0.2 M the 4S complex is by far the major binding entity.The relatively high levels of serum AFP in immature rats prompted us to explore the contribution of AFP to the estrogen-binding capacity of uterine homogenates. The results obtained with specific anti-AFP immunoadsorbents (12,13) provided evidence that at low salt concentrations,'AFP ac-' counts for most of the estrogen-binding capacity associated with the 4-5S macromolecular complex. It was concluded that the 4-5S entity and AFP were identical and that the presence of AFP in uterine cytosols was probably due to serum contamination.We report here further observations which strongly suggest that the other macromolecular complex, the 8S entity, is also made up of AFP in reversible association with some intracellular constituent(s). Immunologic data and competitive assays with unlabeled estrogens and anti-estrogens have also enabled us to demonstrate significant changes in antigenicity and binding specificity of native serum AFP upon its transition to the 8S form and the reversion to the original properties of native AFP after the 8S --4-5S transformation in hypertonic solutions.MATERIAL AND METHODS Animals. Female Wistar rats were used in all experiments. Amniotic fluids were obtained by puncturing, the amniotic sacs of animals on the 17th to 19th day of pregnancy. The fluids were pooled, treated with charcoal (2 mg/ml) for 20 min at 370 with mild agitation, centrifuged at 5000 rpm (4000 X g), and stored at -80°. Uteri

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Summary Serum concentrations of sex steroid‐binding protein (SBP) were measured by an immunodiffusion technique in a total of 133 samples from 70 healthy pregnant women, 18 puerperal women and 21 non‐pregnant women. In 6 individuals, trends of serum SBP were established from multiple samples taken throughout pregnancy. Maternal serum SBP levels had increased significantly above base line values as early as by 10 weeks gestation, and attained a plateau between 25 and 30 weeks, at which time mean values were about 10‐fold higher than the normal non‐pregnant range, the levels then declined slightly twoards term. Parallel measurements of dihydrotestosterone‐binding capacity of SBP showed a good correlation between SBP content and biological activity. Six abnormal pregnancies were encountered in the present study, and in five of them the molar ratios of bound hormone to SBP concentration were much lower than the normal value, suggesting a reduced binding capacity of the protein for the hormone.

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