Transporters are membrane proteins that control the influx of essential nutrients and ions and the efflux of cellular waste, environmental toxins, and other xenobiotics. Approximately 6% of genes in the human genome encode transporters or transporter-related proteins. Drug-transporting proteins contribute to both therapeutic and adverse effects of drugs (Figure 2-1).
Two major superfamilies dominate the area of drug transporters: ATP-binding cassette (ABC) and solute carrier (SLC) transporters. Most ABC proteins are primary active transporters, which rely on adenosine triphosphate (ATP) hydrolysis to actively pump their substrates across membranes. The 49 known genes for ABC proteins are grouped into seven subclasses or families (ABCA to ABCG). Well known examples are P-glycoprotein (encoded by ABCB1) and the cystic fibrosis transmembrane regulator (CFTR, encoded by ABCC7). The SLC superfamily includes genes that encode facilitated transporters and ion-coupled secondary active transporters, 43 SLC families with -300 transporters. Many mediate drug absorption and disposition. Prominent SLC transporters include the serotonin transporter (SERT, encoded by SLC6A4) and the dopamine transporter (DAT, encoded by SLC6A3).
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