Oestradiol receptors were determined by isoelectric focusing in cytosol from breast cancer tissue from 12 pre- and 70 postmenopausal patients. Simultaneous determination of the peripheral serum levels of oestradiol-17β was carried out in the 12 premenopausal and 57 of the postmenopausal patients. At oestradiol-17β levels below 150 pM the receptor values ranged from below 0.01 to 7.19 fmole bound [3H] oestradiol-17β/μg of DNA. At oestradiol-17β levels above 150 pM only low receptor values (<0.25 fmole bound [3H] oestradiol-17β/μg of DNA) were recorded. The results indicate an influence of the endogenous oestrogen on the receptor assay probably due to blockade of the hormone binding site. It is proposed that an endocrine evaluation of the patient should be performed together with the oestrogen receptor assay.
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