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OCTN2

Molecular structure and tissue distribution
The carnitine and organic cation transporter OCTN2 (SLC22A5) has a relatively ubiquitous distribution and is expressed in epithelial cells, muscle cells, glial cells, macrophages, lymphocytes and sperm. In humans the highest expression of hOCTN2 was observed in liver, kidney, skeletal muscle, heart and placenta. Expression of OCTN2 protein has been localized in brush-border membrane of renal proximal tubule cells, in small intestinal enterocytes, in respiratory epithelia, alveola epithelia, luminal membrane of alveolar mammary gland and was also detected in some cancer cell lines. The human OCTN2 protein consists of 557 amino acids and possesses 88% similarity to OCTN1.

Interaction of hOCTN2 with endogenous compounds and drugs
Physiologically OCTN2 mediates the active uptake of L-carnitine in small intestinal enterocytes and the renal reabsorption of L-carnitine. Human OCTN2 facilitate the uptake of L-carnitine into skeletal muscle, adipocyte, colonic epithelial cells, neurons and facilitate the transport of L-carnitine as well as acetyl-carnitine across the blood-retinal barrier. OCTN2 is also expressed in brain capillary endothelial cells and is involved in the transport of organic cations in blood brain barrier.

Functional characterization of transport in cells overexpressing hOCTN2 revealed that hOCTN2 is a Na+-dependent, high affinity transporter for L-carnitine, acetyl-L-carnitine and the zwitterionic β-lactam antibiotic cephaloridine. Cationic substrates of hOCTN2 are TEA, choline, verapamil, pyrilamine, spironolactone, oxaliplatin and the anticholinergic drugs ipratropium and tiotropium. Inhibition of hOCTN2 mediated uptake of carnitine has been demonstrated for cimetidine, clonidine, procainamide, actinomycin D, quinine, emetine, cefsulodine, nicotine and MPP.

Figure 1 Kinetic of the interaction of L-Carnitin with hOCTN2 in HEK293 cells Figure 2 Inhibitory effect of various inhibitors (100 µM) on OCTN2-mediated L-carnitin uptake

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