14C-labeled proteins as markers for gradient analysis of steroid-hormone receptors.

Two useful [14C]marker proteins--[14C]human serum albumin (4.6S) and [14C]glucose oxidase (7.9S)--can be simply prepared. Both may be used as molecular-mass standards in polyacrylamide gel electrophoresis as sucrose density-gradient centrifugation. The utility of these markers for estrogen receptor studies was investigated under a variety of conditions, to ensure that they do not interfere with current assay procedures. Their use as internal markers allows more samples to be analyzed per rotor, a significant factor because each centrifugation run requires 16 h and two bucket spaces for each sample assayed; improves accuracy and overall quality control by eliminating any problems resulting from variations among individual gradients; and facilitates evaluation of changes in gradient profiles, which may provide clinically and biochemically relevant information concerning the microheterogeneity of estrogen receptors.