The Blood Carrier: Plasma Protein Binding and Transport Quiz

Most drug protein interactions involve weak bonds like van der Waals forces or hydrogen bonding. This allows for a constant equilibrium between the bound fraction (storage) and the free fraction (active).

Only the free or unbound drug can exit the vasculature to reach target receptors. High protein binding effectively keeps the drug locked in the plasma, limiting its immediate pharmacological effect.

With fewer binding sites available, a larger percentage of the administered dose remains in the free, unbound state. This can lead to increased drug effects or toxicity even at standard doses.

Active transport uses energy (usually ATP) to move molecules from an area of low concentration to high concentration. This is vital for the secretion of drugs into the bile or the removal of toxins from the brain.

Facilitated diffusion move molecules down their concentration gradient (like passive diffusion) but uses a protein channel or carrier to help polar molecules cross the hydrophobic lipid bilayer.

The tight junctions of the blood brain barrier prevent the passage of polar or large molecules. Only highly lipophilic drugs can dissolve through the endothelial cell membranes to enter the CNS.

The pores of the glomerular basement membrane do not allow large proteins like albumin to pass. Therefore, drugs bound to albumin are protected from glomerular filtration.

Vectorial transport involves the coordinated action of uptake and efflux transporters to move a drug across a cell layer in one consistent direction, such as from the intestinal lumen into the blood.

Albumin has multiple distinct binding sites (such as Site I and Site II). While competition can occur between drugs of similar charge and structure, acidic and basic drugs often bind to different regions of the protein.

For molecules that are too large for transporters and too polar for diffusion, the cell membrane can engulf the substance in a vesicle to bring it inside the cell, though this is less common for standard small molecule drugs.

While albumin binds most acidic and neutral drugs, basic drugs primarily bind to alpha 1 acid glycoprotein. Levels of this protein often increase during inflammation, which can alter the free fraction of the drug.

ABC transporters, such as P glycoprotein, use ATP to pump drugs out of the cell. In cancer, overexpressed ABC pumps can eject chemotherapy agents before they can act on the cell nucleus.

SLC transporters are crucial for moving drugs across membranes that are otherwise impermeable. Unlike ABC transporters, they do not utilize direct ATP hydrolysis but often rely on electrochemical gradients.

Protein binding is a saturable process. If the drug concentration is extremely high or if other substances (like bilirubin) compete for the same sites, the percentage of bound drug will change.

These transporters are found in the blood brain barrier, the placenta, and the testes. They serve a protective role by actively pumping potentially harmful xenobiotics out of these sensitive tissues.