Binding of [³h]tamoxifen in rat uterine cytosols: A comparison of swinging bucket and vertical tube rotor sucrose density gradient analysis

The interaction of the antioestrogen tamoxifen with rat uterine oestrogen receptor has been studied using swinging bucket and vertical tube rotor techniques of sucrose density gradient analysis. Tamoxifen inhibits the binding of [³H]oestradiol to the 8S oestrogen receptor protein whereas using the swinging bucket rotor technique of sucrose density gradient analysis [³H]-tamoxifen appears to bind to an oestradiol specific protein which sediments at 4-5S. In contrast, using rapid sucrose density gradient analysis with a vertical tube rotor [³H]tamoxifen shows oestrogen specific binding in the 8S region. It is suggested that conventional techniques of sucrose density gradient analysis are unsuited for the investigation of the low affinity ligand interactions with oestrogen receptor proteins since rapid dissociation of complexes can result in ligand binding to nonreceptor proteins.