

I saw your note asking for clarification and use of IC50 values. For
classical inhibitors exhibiting simple Michelis - Menton kinetics, the IC50
values are always proportional to Ki (or Kd) values and the proportionality
constant is dependent on the concentration of the substrate ( or ligand) used
in the experiment.  Here are the exact relationships between IC50 and the Ki
values for the standard mechanisms one encounters:

competitive binding:     IC50 = Ki*(1+S/Km)
Non-competitive binding: IC50 = Ki
Un-competitive binding:  IC50 = Ki*(1+Km/S)

For ligand binding studies replace S and Km with L and Kd, respectively. 
Therefore, the ratio of IC50 values should be exactly same as the ratio of
the corresponding Ki ( or Kd) values provided the IC50 values were obtained
under identical conditions. So, it is perfectly alright to deduce the
free-energy differences between two compounds from their IC50 values.

Hope, this is of some help.If you have any further questions please feel free
to write to me or contact me at 499-2457.
-Sam. 

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From: "Pazhanisamy" <[email-masked]>

Hi Ellen,
I sent you a message on this topic before. I hope you received it. I just
want to qualify my statement as follws:
"It is coreect to use the IC50 ratios in place of the Ki ratios provided both
compounds observe the same simple Michelis-Menton kinetics."
I am curious to know if you received any other responses. Thanks.
My Email address is [email-masked].

-Sam.
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