Evidence of a small molecule in mouse Leydig cell tumors which inhibits the conversion of estrogen receptor from 4S to 5S.

The activation process of cytosol estrogen receptor (RE) from mouse Leydig cell tumors was investigated. When RE from tumors which were affected in their growth by estrogen was heated at 25 C for 30 min in high ionic strength buffer it was converted from native 4.0S to a transformed (5.3S) state. Dialysis of the cytosol from these tumors at 0-4 C in either the presence or absence of estradiol also resulted in conversion to 5.3S when estradiol was added before the sample was applied to the sucrose density gradient. When the dialyzed cytosol was applied to a sucrose density gradient without estradiol, the peak of estrogen binding capacity tested by adding E2 to the fractions from the gradient, occurred at approximately 4S. The dialysate of cytosol had an inhibitory effect on activation by warming. Dialysis of cytosol in the presence of estradiol enhanced the translocation of estradiol-receptor complex to nuclei. Molecular weight (approximately 128,000) and frictional coefficient (f/f0 1.62) of the estrogen-receptor complex dialyzed in the presence of high salt concentration were identical with those of warmed RE complex, but quite different from the native RE (approximately 73,000 daltons, frictional coefficient 1.48). This evidence indicates the presence in the cytosol of one or more small molecules which, when noncovalently bound to RE, inhibit the conversion of the native cytosol RE to the transformed state R'E2RE, having an additional protein unit.