16 Alpha-[125I]-beta-estradiol compared with [3H]-beta-estradiol as the tracer in estradiol receptor assays.

We assessed the potential usefulness of 16 alpha-[125I]-beta-estradiol in estradiol receptor assays as compared with a long-used [3H]-beta-estradiol dextran-coated charcoal method, measuring 472 consecutive human breast-cancer cytosols by both procedures. Six different preparations of 16 alpha-[125I]-beta-estradiol were used. Nonspecific binding in five batches was similar and comparable to [3H]-beta-estradiol. Although the sixth batch had increased nonspecific binding, this did not affect results. Dissociation constants were virtually identical (r = 0.94). Results were concordant for 98.5% of cytosols: 161 were negative and 304 positive by both methods and seven were positive by one method, negative by the other. Four were positive with the 125I procedure and undetectable with [3H]-beta-estradiol. Three were measurable by both methods, but were above the cut-off value in one and below it in the other. We find 16 alpha-[125I]-beta-estradiol to be an adequate substitute for [3H]-beta-estradiol in estradiol receptor assays.