Law of Mass Action and Fractional Occupancy

for a toxicant (ligand) interacting with a single (or homogeneous population) of receptor(s), mass-action principles require  

For any given system, there is a tendency for ligand and receptor to remain associated (the association rate constant k,m) and for the ligand-receptor complex to 

From basic principles of mass action, we know the relationship between these rate constants and the association (KA) and dissociation (KD) equilibrium constants  

we can rearrange Eq. (19.2) to deline the equilibrium dissociation constant KD  

From this, it is clear that the KD is the concentration of ligand that, at equilibrium, will cause binding to half the receptors. The K is the equilibrium dissociation constant, whereas the koii is the dissociation rate constant. A summary of these dehni-tions (and their chemical units) is shown in Table 19.1. 

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