

Dear Ellen:

According to Cheng and Prusoff IC50 values can be converted to Ki values also
under 'competitive' conditions. They say

Ki = IC50/(1 + [L]/Kd)

L stands for the concentration of the marker in your assay (usually a radio-
ligand), and Kd for its (the radioligand's) affinity towards the protein
(often a receptor). Beware that Ki relates to the affinity of the 'cold'
(i.e. non-radiolabelled) ligand, and Kd to the affinity of the marker. 
Now to your question: the comparison of two IC50 values. 
>From the above it is evident that if the experimental conditions are the same
(i.e. same radioligand concentration, same receptor preparation, etc) IC50
values are directly comparable to Ki values. You should check with the 
pharmacologists/biochemists whether this is indeed the case.
I give you one reference of our research group that uses Ki and IC50 values
in modeling studies, in which we deal with free energies, etc.:
Van der Wenden et al, Eur J Pharmacol - Mol Pharm Sect 206 (1991) 315 - 323.
Hope this helps,

Ad IJzerman
Leiden/Amsterdam center for drug research
PO Box 9502
2300RA Leiden
The Netherlands
[email-masked]

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