Application of a mathematical model for two component receptor binding to two high affinity oestrogen binding sites in nuclei from DMBA rat mammary tumours.

Saturation binding of [3H]oestradiol has been determined using exchange conditions, on nuclei from DMBA tumours from rats treated prior to sacrifice with oestradiol and tamoxifen alone or in combination. Application of a model to the binding data enabled the amounts (C2) and apparent dissociation constants (Kdapp) of a second lower affinity binding component to be determined as well as the amount of a higher affinity site (C1) and its dissociation constant (Kd1). Kdapp did not change significantly with any pretreatment but 2 h after oestradiol (5 micrograms) and after tamoxifen alone there was a significant decrease in Kd compared with control. It is suggested that the difference in Kd of the higher affinity binding sites in control and 2 h oestradiol treated animals may be due to the loss of an essential co-factor, possibly cytosolic, when nuclei are isolated in the absence of ligand.