USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

Which protein, also known as MDR1, functions as a transmembrane efflux pump with a broad specificity for hydrophobic compounds?

1. P-Selectin

2. E-Cadherin

3. P-Glycoprotein

4. L-Selectin

The correct answer is: P-Glycoprotein

P-Glycoprotein, also referred to as MDR1 (multidrug resistance protein 1), is a crucial component of the body's defense against potentially harmful compounds. It functions by actively transporting a wide array of hydrophobic substances out of cells, which contributes to the cellular detoxification processes. This transmembrane efflux pump is particularly significant in the context of pharmacology and toxicology, as it plays key roles in determining the absorption, distribution, and resistance of various drugs within the body. P-Glycoprotein is encoded by the ABCB1 gene and is especially noted for its ability to extrude a variety of xenobiotics and endogenous compounds, thereby influencing the pharmacokinetics of numerous medications. Its broad substrate specificity means that it can interact with many hydrophobic compounds, limiting their intracellular accumulation. This feature is central to the phenomenon of multidrug resistance observed in certain cancer cells, where increased expression of P-Glycoprotein can lead to reduced effectiveness of chemotherapy. In contrast, other proteins like P-Selectin and L-Selectin have roles in leukocyte adhesion and recruitment during inflammatory responses, while E-Cadherin is primarily involved in cell-cell adhesion in epithelial tissues. Therefore, they do not