title: Molecular identification of a novel carnitine transporter specific to human testis. Insights into the mechanism of carnitine recognition.
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.
authors.affiliation: Department of Pharmacology and Toxicology, Kyorin University, School of Medicine, 6-20-2 Shinkawa, Mitaka, Tokyo 181-8611, Japan.
authors.forename: Atsushi
authors.initials: A
authors.lastname: Enomoto
authors.forename: Michael F
authors.initials: MF
authors.lastname: Wempe
authors.forename: Hiroki
authors.initials: H
authors.lastname: Tsuchida
authors.forename: Ho Jung
authors.initials: HJ
authors.lastname: Shin
authors.forename: Seok Ho
authors.initials: SH
authors.lastname: Cha
authors.forename: Naohiko
authors.initials: N
authors.lastname: Anzai
authors.forename: Akiteru
authors.initials: A
authors.lastname: Goto
authors.forename: Atsuhiko
authors.initials: A
authors.lastname: Sakamoto
authors.forename: Toshimitsu
authors.initials: T
authors.lastname: Niwa
authors.forename: Yoshikatsu
authors.initials: Y
authors.lastname: Kanai
authors.forename: M W
authors.initials: MW
authors.lastname: Anders
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Endou
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D015152:Blotting, Northern; D002331:Carnitine / Q000378:metabolism / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004822:Epididymis / Q000378:metabolism; D005091:Exons; D005260:Female; D015723:Gene Library; D006801:Humans; D007150:Immunohistochemistry; D007438:Introns; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000096:biosynthesis* / Q000737:chemistry* / Q000235:genetics; D008956:Models, Chemical; D008969:Molecular Sequence Data; D009865:Oocytes / Q000378:metabolism; D029362:Organic Anion Transporters, Sodium-Independent / Q000378:metabolism; D027701:Organic Cation Transport Proteins; D010802:Phylogeny; D017386:Sequence Homology, Amino Acid; D012984:Software; D000074058:Solute Carrier Family 22 Member 5; D000070590:Solute Carrier Proteins; D013094:Spermatozoa / Q000502:physiology; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D027981:Symporters; D013737:Testis / Q000378:metabolism*; D014018:Tissue Distribution; D014981:Xenopus
chemical_list: D002352:Carrier Proteins; D018076:DNA, Complementary; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D029362:Organic Anion Transporters, Sodium-Independent; D027701:Organic Cation Transport Proteins; C467737:SLC22A16 protein, human; C109807:SLC22A4 protein, human; C492020:SLC22A5 protein, human; C000705567:Slc22a4 protein, rat; D000074058:Solute Carrier Family 22 Member 5; D000070590:Solute Carrier Proteins; D027981:Symporters; D002331:Carnitine