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Protein Binding

Most drugs are bound to proteins when they circulate in the body. This makes sense when considering general solubility rules of chemistry. If a drug is simple, eg, an elemental salt such as lithium, it will easily dissolve into the aqueous environment of the blood and widely distribute through the circulation. However most drugs are complex organic molecules with variable solubility in water. For drugs that are not highly soluble in an aqueous environment, some carrier protein will be needed to solubilize that drug into blood. Serum contains a myriad of proteins. Although these proteins themselves are water soluble (since they are found dissolved in blood) they often contain lipophilic regions on them, which can bind non-polar drugs and thus carry these drugs through the circulation.

  • Albumin is a major drug-binding protein in serum.
  • Albumin is an alkaline protein, so acidic and neutral drugs primarily bind to it.
  • If albumin binding sites become saturated, acidic and neutral drugs can bind to lipoproteins.
  • Alkaline drugs tend to bind to globulins, particularly to a globulin called alpha-1 acid glycoprotein.
  • Only free, unbound drugs are able to bind protein drug receptors on cells and have therapeutic effects.

An equilibrium exists in the systemic circulation between a free and protein-bound drug and between a free and receptor-bound drug. This is illustrated in the image.