Requirement for an accessory factor for binding of [3H]estradiol to protein in the cytosol fraction of rat pancreas.

Supernatant fractions from rat pancreas bind approximately 300 fmol of [3H]estradiol per mg of protein when incubated with 5 nM [3H]estradiol for 1 hr at room temperature. Passage through gel filtration columns reduces binding in the eluate to approximately 1% of its initial activity. Extracts of the supernatant contain a factor that reactivates binding in gel filtrates. Addition of accessory factor to fractional eluates gives one sharp peak of activity. Since fractions that cannot be reactivated contain as much or more protein as fractions that can be reactivated, it is concluded that interaction of accessory factor and [3H]estradiol-binding protein is specific. Peptides such as antipain [(1-carboxy-2-phenylethyl)carbamoyl-L-arginyl-L-valyl-L-argininal] and, especially, N-benzoyl-L-phenylalanyl-L-valyl-L-arginine-p-nitroanilide also enhanced binding of [3H]estradiol. Accessory factor is water soluble, dialyzable, and heat stable. Although as currently purified, it contains several substances, the data suggest that at least one component of accessory factor is an oligopeptide.