Two site Scatchard Plots

[Michael D. Baron]

In article <gayler-0509951254300001 at 198.178.220.249>, gayler at immunex.com 
(Richard Gayle) says:
>
>I routinely use the curve fitting routines from Deltagraph Pro. 
Recently,
>I have needed to plot Scatchard graphs for receptors and their ligands
>that require 2 site fits. What are the model and parameters to use to 
get
>a proper fit? TIA,
>R.B. Gayle

The model in the Ligand program developed originally by Munson and 
Rodbard was (if I remember correctly):

B=K1*F + K2*F + N*F

Where K1 and K2 are the equilibrium binding constants for the 2 receptor 
sites, N is a constant for non-specific absorption, and Bound and Free 
ligand are self-explanatory. Note that the model assumes that 
non-specific binding is a function of Free ligand, and this is not the 
same as the binding of labelled ligand at high concentrations of 
unlabelled ligand.

LIGAND did a least-squares fit to this equation, without transforming to 
Scatchard plot co-ordinates, as these are poorly defined mathematically 
(due to the same datum (Bound ligand) appearing on both axes). You should 
set Deltagraph to do the same, I guess, assuming it can do non-linear 
least-squares fitting.

Hope this helps

Michael D. Baron