PROTEIN BINDING
The reversible binding of drug with non-specific and non-functional site on the body protein without showing any biological effect is called as protein binding
Drug + Albumin ⇌ Drug + Albumin complex
A drug molecule to less or more extent has a capacity to enter into specific combination with plasma protein. These molecular fractions play an important role in deciding the availability of drug for biotransformation, pharmacological site and excretion.
Depending upon whether the drug is a weak or strong acid, base or is neutral, it can bind to a single blood protein to multi proteins. The most significant protein involved in the binding of drug is albumin, which comprises more than half of blood volume. Albumin interacts with acidic or basic drugs in the plasma by Vander - Waals force, hydrophobic binding, hydrogen bonding and ionic interaction.
Acid - glycoprotein (AGP) interacts with basic entities and lipoproteins bind with both basic and neutral drugs. Protein binding values are normally given as the percentage of total plasma concentration of drug that is bound to all plasma protein
Free drug [Df] +Free protein [Pf] ⇌ Drug / protein complex (D)
The total plasma concentration of the drug is expressed as the sum of percent of free drug and the percent bound.
Total plasma concentration [D] = [D] + [D]
The drug - protein complex is large, hence nnot pass through phospholipid bilayers including capillary membranes, glomerular membrane in the nephrons and the blood brain barrier, which prolong its duration of actions. Bound drug is also less available to the enzyme involved in first pass metabolism.
After the metabolic and excretory process has cleared much of the free drug, the drug - protein complex diffuses the drug from the complex. For these reasons, drugs with high protein binding values tend to have greater half life compared to those with lower values.
Tissue Depots
The drug can be stored in the tissue depot like neutral fat, plasma protein, bone and transcellular fluid. The drug stored in this depot is in equilibrium with plasma concentration. When the therapeutic concentration falls, the drug is released form these storage depot. More lipophilic drug concentrates more in this depot. For example, barbiturate with lipophilic side chain has shorter duration of action than polar substituents. It is because tissue storage is more in lipophilic substituted barbiturate. Even the drug diffuses back to the plasma, the concentration is not sufficient to produce pharmacological response,
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