Different ionic forms of estrogen receptor in rat uterus and human breast carcinoma

Estrogen receptors from rat uterus and human breast carcinoma were analyzed by diethylaminoethyl-cellulose chromatography. Cytosols which had been incubated for short periods of time demonstrated a single discrete elution peak, indicating a single ionic form, while cytosols incubated for longer periods of time generated a second ionic form of receptor. Addition of cations to cytosols also promoted the rapid appearance of this second ionic form of receptor. Either leupeptin, a protease inhibitor, or sodium molybdate prevented the appearance of this second ionic form of estrogen receptor. The estrogen receptor from rat uterine cytosol incubated without leupeptin or molybdate had a smaller apparent molecular weight than did estrogen receptor from cytosols incubated with leupeptin or molybdate. Altogether, these experiments suggested that a cation-dependent protease present in the cytosols from both tissues was degrading the estrogen receptor to a second smaller species during extended incubation times.