2. Protein Binding of Drug
• The phenomenon of complex formation with
proteins is called as protein binding of drug.
• Intracellular binding: when the drugs are
bound to cell protein (drug receptor). These
receptors with which drug interact to show
response are called as PRIMARY
RECEPTOR.
3. • Extracellular binding: when the drug are
bounded to extra cellular protein are called
SECONDARY RECEPTOR.
4. Kinetics of protein binding
• If ‘P’ represents protein & ‘D’ the drug, then
applying law of mass action to reversible
protein-drug binding, we can write
P + D PD ………………(i)
At the equilibrium
Ka = [PD] / [P] [D] …………….(ii)
5. [PD] = Ka [P][D] ………. …….(iii)
Where, [P] = Concentration of free protein.
[D] = Concentration of free Drug.
[PD] = Concentration of Protein drug
complex.
Ka = Association rate constant.
Kd = Dissociation rate constant.
6. Ka > Kd indicates forward reaction i.e protein-
drug binding is forward.
If PT is the total concentration of protein present,
bound & unbound form then
PT = [PD] + [P] ………………..(iv)
If, ‘r’ is the no. of moles of drug bound to total
moles of protein then.
r = [PD] / [PT ] = [PD] / [PD] + [P] ..……(v)
7. Substituting the value of [PD] from the Eq. (iii)
in Eq. (v) then we get.
r = Ka [P] [D] / Ka [P] [D] + [P]……. (vi)
r = Ka [D] / Ka [D] + 1…….……….. (vii)
This equation is for, when the protein is having 1
binding site.
If ‘N’ number of binding site are available then
r = N . Ka[D] / ka [D] + 1..……(viii)
8. The value of association constant ‘Ka’& number
of binding site ‘N’ can be obtained by plotting
equation in four different way.
10. • Scat chard plot: It is made by transforming the
equation (viii) into linear form. Thus,
r = N Ka [D] / Ka [D] + 1
r + r Ka [D] = N Ka [D]
r = N Ka [D] - r Ka [D]
Therefore,
r / [D] = N Ka – r Ka
15. Dynamic Dialysis
• It is an economical method.
• It is a quick method to establish drug-protein
binding.
• It is based on the rate of disappearance of the
drug from a dialysis bag which is proportional
to the concentration of unbound drug.
17. • Serum albumin is placed in Visking cellulose
tube.
• The drug in different concentrations is placed in
the outside vessel.
• The drug molecules enter into the bag.
• The drug protein binding is observed.
• Albumin, being macromolecules, can not pass
through the membrane to outside vessel.
• More amount of drug permeates and available in
the bag.
• If binding occurs, the drug concentration in the
bag containing proteins should be greater than its
concentration in the outside vessel.
18.
19. Method of analysis
• The general procedure is as follow:
The equation for complexation can be written as
M + n A = M A n
the stability constant can be written as
K = [M A n] / [M][A]n
20. Where,
K = equilibrium stability constant.
[M]= conc. of metal ion uncomplexed.
[A]= conc. Of ligand (A) uncomplexed.
n= number of moles.
[M A n] = conc. Of complex.
Take logarithms of both sides
Log [M A n] = log K + log[M] + n log [A].
