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MUCOSAL BIOLOGY

Transport of butyryl-L-carnitine, a potential prodrug, via the carnitine transporter OCTN2 and the amino acid transporter ATB^0,+

¹Department of Pediatrics, Medical College of Georgia, Augusta; and ²Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta, Georgia

Submitted 23 May 2007 ; accepted in final form 10 September 2007

L-Carnitine is absorbed in the intestinal tract via the carnitine transporter OCTN2 and the amino acid transporter ATB^0,+. Loss-of-function mutations in OCTN2 may be associated with inflammatory bowel disease (IBD), suggesting a role for carnitine in intestinal/colonic health. In contrast, ATB^0,+ is upregulated in bowel inflammation. Butyrate, a bacterial fermentation product, is beneficial for prevention/treatment of ulcerative colitis. Butyryl-L-carnitine (BC), a butyrate ester of carnitine, may have potential for treatment of gut inflammation, since BC would supply both butyrate and carnitine. We examined the transport of BC via ATB^0,+ to determine if this transporter could serve as a delivery system for BC. We also examined the transport of BC via OCTN2. Studies were done with cloned ATB^0,+ and OCTN2 in heterologous expression systems. BC inhibited ATB^0,+-mediated glycine transport in mammalian cells (IC₅₀, 4.6 ± 0.7 mM). In Xenopus laevis oocytes expressing human ATB^0,+, BC induced Na⁺-dependent inward currents under voltage-clamp conditions. The currents were saturable with a K_0.5 of 1.4 ± 0.1 mM. Na⁺ activation kinetics of BC-induced currents suggested involvement of two Na⁺ per transport cycle. BC also inhibited OCTN2-mediated carnitine uptake (IC₅₀, 1.5 ± 0.3 µM). Transport of BC via OCTN2 is electrogenic, as evidenced from BC-induced inward currents. These currents were Na⁺ dependent and saturable (K_0.5, 0.40 ± 0.02 µM). We conclude that ATB^0,+ is a low-affinity/high-capacity transporter for BC, whereas OCTN2 is a high-affinity/low-capacity transporter. ATB^0,+ may mediate intestinal absorption of BC when OCTN2 is defective.

butyryl-L-carnitine; OCTN2; ATB^0,+; carnitine; butyrate; Crohn's disease; necrotizing enterocolitis