Qualification of estrogen binding variables in tissues of the immature rat

Serum albumin, alphafetoprotein, and specific receptor binding of estradiol have been shown for the uterus, pituitary, hypothalamus, and brain cortex of the immature rat. Serum proteins were measured by immunologic methods and, following sucrose density gradient fractionation, could be definitively separated from the specific tissue estrogen receptors. The mass of alphafetoprotein in soluble tissue extracts paralleled the amounts of specific antigen detected in the serum. The choice of analytic method for designating a population of specific tissue estrogen receptor, which is free of nonreceptor steroid binding variables must therefore consider the extent of contamination by serum proteins. Excess radioinert steroid is contraindicated in receptor assays where low levels of specific receptor obtain.