Molecular identification of a novel carnitine transporter specific to human testis. Insights into the mechanism of carnitine recognition.

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Enomoto A, Wempe MF, Tsuchida H, Shin HJ, Cha SH, Anzai N, Goto A, Sakamoto A, Niwa T, Kanai Y, Anders MW, Endou H

Molecular identification of a novel carnitine transporter specific to human testis. Insights into the mechanism of carnitine recognition.

J Biol Chem. 2002 Sep 27;277(39):36262-71. Epub 2002 Jun 27.

PubMed ID

12089149 [ View in PubMed

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Abstract

l-Carnitine is an essential component of mitochondrial fatty acid beta-oxidation and plays a pivotal role in the maturation of spermatozoa within the male reproductive tract. Epididymal plasma contains the highest levels of l-carnitine found in the human body, and initiation of sperm motility occurs in parallel to l-carnitine increase in the epididymal lumen. Using a specific carrier, epididymal epithelium secretes l-carnitine into the lumen by an active transport mechanism; however, the structure-activity relationship comprising the carnitine-permeation pathway is poorly understood. We discovered a novel carnitine transporter (CT2) specifically located in human testis. Analyzing the primary structure of CT2 revealed that it is phylogenetically located between the organic cation transporter (OCT/OCTN) and anion transporter (OAT) families. Hence, CT2 represents a novel transporter family. When expressed in Xenopus oocytes, CT2 mediates the high affinity transport of l-carnitine but does not accept mainstream OCT/OCTN cationic or OAT anionic substrates. We synthesized and tested various carnitine-related compounds and investigated the physicochemical properties of substrate recognition by semi-empirical computational chemistry. The data suggest that the quaternary ammonium cation bulkiness and relative hydrophobicity be the most important factors that trigger CT2-substrate interactions. Immunohistochemistry showed that the CT2 protein is located in the luminal membrane of epididymal epithelium and within the Sertoli cells of the testis. The identification of CT2 represents an interesting evolutionary link between OCT/OCTNs and OATs, as well as provides us with an important insight into the maturation of human spermatozoa.

DrugBank Data that Cites this Article

Drug Transporters

Drug	Transporter	Kind	Organism	Pharmacological Action	Actions
Levocarnitine	Solute carrier family 22 member 16	Protein	Humans	Unknown	Substrate	Details

Polypeptides

Name	UniProt ID
Solute carrier family 22 member 16	Q86VW1	Details