Response to "Comment on Specific binding of hydroxylated polychlorinated biphenyl metabolites and other substances to bovine calf uterine estrogen receptor: structure-binding relationships [Kramer and Giesy, Sci Total Environ 1999;233:141-61]".

We thank Dr. Rayne for the comments on our publication. It should be stated at the outset that none of the comments Dr. Rayne makes challenges the validity of the experimental data indicating that 25 of the 28hydroxylated PCB's tested bind to the calf uterine estrogen receptor withmeasurable affinity. The objectives as set out in our publication were achieved. Regarding the applicability of modeling the protonated form, the literature tells us that this is the form of 17β-estradiol that specifically binds to the estrogen receptor with the phenolic proton forming a key hydrogen bond (Tannenbaum et al., 1998). Thus, themolecular properties of the phenolic forms of the hydroxylated PCB's were modeled and examined for relationships with binding affinity. It is true that the anionic form may dominate in bulk solution for a subset of these molecules. However, receptor binding occurs between one receptor and one ligand and that reaction does not occur in bulk solution (it is a desolvating process). The binding pocket of the receptor is hydrophobic. So, bulk properties thatmay dominate in solution do not necessarily dominate at the level of the individual receptor and ligand binding reactions. Thus, it is appropriate to model the properties of the protonated form for purposes of examining relationships of molecular properties with receptor binding. The molecular modeling of the phenolic structure reflected the ionizability of the oxygen atom as measured by the OCHARGE parameter. So, that parameter does contain information on the potential ionization state of the oxygen atom. The binding data also incorporates the ionization state that was actually achieved in the buffer system used. The ionization potential, which is related to pKa, was incorporated in the QSBR and was discussed in the publication. The bulk solution equilibrium influences the frequency of encounters between protonated ligand and receptor. However, the affinity with which the protonated ligand binds to the receptor is not affected by the bulk solution equilibrium since this is an entirely separate chemical interaction (Eq. (1)).