Ion (Organic Acid and Base) Transporters and Homeostasis: A Review with Reference to its Usefulness as Biomarkers

摘要

Organisms are continuously exposed to a great variety of xenobiotics via food and environment. Evolution has equipped the body with a plethora of protecting systems to defend itself against the potentially harmful effects of these compounds. One of the important defense mechanisms include the active extrusion of xenobiotics by commonly shared transport proteins, mainly located in kidney, liver and intestine. An attempt has been made to understand the structural and functional properties of organic anion transporters (OAT1, OAT2, OAT3) and organic cation transporters (OCTN1, OCTN2, OCT1, OCT2, OCT3). These transporters share a predicted 12-transmembrane domain (TMD) structure with a large extracellular loop between TMD1 and TMD2, carrying potential N-glycosylation sites. Conserved amino acid motifs revealed a relationship to the sugar transporter family within the major facilitator superfamily. Following heterologous expression, most OATs transported the model anion p-aminohippurate (PAH). OAT1, but not OAT2, exhibited PAH-alpha-ketoglutarate exchange. OCT1–3 transported the model cations tetraethylammonium (TEA), N(1)-methylnicotinamide, and 1-methyl-4-phenylpyridinium.