Isoelectric focusing of steroid hormone receptors in slabs of polyacrylamide gel.

Isoelectric focusing in 1-2 mm thick slabs of polyacrylamide gel is a versatile and specific method for quantitation and characterization of estrogen, mineralocorticoid, progestin, and glucocorticoid receptors. This method has been used for routine measurements of estrogen receptor in human breast cancer in Stockholm since 1976. It is also used for quantitation of estrogen receptor in fine needle biopsies from breast cancer tissue. Isoelectric focusing in gel slabs allows rapid separation (1.5 hr) of receptors from other non-receptor proteins that bind steroid hormones, such as transcortin and sex hormone-binding globulin. The tritium-labeled steroid-receptor complex is localized by slicing the gel followed by radioactivity analysis in a liquid scintillation counter. The isoelectric point of the tritium peak is determined with a surface electrode. The isoelectric point serves as a qualitative control. Thus, only one analysis is needed for each receptor quantitation, provided that a saturating concentration of tritiated ligand is used for receptor binding. This means that a minimal amount of tissue is needed for one receptor analysis with isoelectric focusing.