PHARM/PCS V5 · Procedure 22 Drug Decay and Binding
Scatchard Plot
Get the dissociation constant K and the total binding-site concentration from equilibrium binding data, with honest asymmetric limits on K.
What it computes
Plots bound/free ratio vs. bound concentration for drug-protein binding. Linear regression gives slope = −1/K and intercept = (P)t/K, from which the dissociation constant K and total binding-site concentration (P)t are computed, with asymmetric confidence limits on K.
Inputs and outputs
You provide
- Bound concentration and the bound/free ratio, one pair per line
- A confidence level
PHARM/PCS reports
- Slope and intercept of the Scatchard line
- K, the dissociation constant, with asymmetric confidence limits
- (P)t, the total binding-site concentration
- The Scatchard plot with the fitted line
When to use it
For equilibrium binding data (radioligand or protein binding) when you want the dissociation constant and the total number of binding sites from a single linearized plot.
Reading the results
A straight Scatchard line implies a single class of independent sites; a concave-curved plot points to two site classes or cooperativity, and a single K then oversimplifies. Because bound appears on both axes, ordinary regression error assumptions are strained, so the asymmetric confidence limits on K are the honest expression of that; confirm with a direct fit when a real decision hinges on K.
Why the limits are asymmetric
K is one divided by the absolute slope, and the reciprocal of a normally distributed quantity is not itself normal, so symmetric limits would misstate the uncertainty. The procedure implements the book's Equations 22.2 and 22.3 and reports the asymmetric interval instead.
This page describes the procedure. Calculations run inside the licensed PHARM/PCS V5 application. PHARM/PCS is a trademark of MCS Scientific LLC.