Molecular identification of a novel carnitine transporter specific to human testis - Insights into the mechanism of carnitine recognition
Molecular identification of a novel carnitine transporter specific to human testis - Insights into the mechanism of carnitine recognition
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DOI：
10.1074/jbc.m203883200
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发表时间：
2002-09-27
影响因子：
4.8
通讯作者：
Endou, H
中科院分区：
文献类型：
--
作者：
Enomoto, A;Wempe, MF;Endou, H
L肉碱是线粒体脂肪酸β氧化的重要组成部分，在男性生殖道内精子成熟过程中起着关键作用。附睾血浆中含有人体中发现的最高水平的L肉碱，精子运动的启动与L肉碱在附睾腔中的增加平行发生。附睾上皮利用一种特定的载体，通过一种主动的转运机制将L-肉碱分泌到管腔内；然而，构成肉碱-渗透途径的结构-活性关系尚不清楚。我们发现了一种新的肉碱转运蛋白(CT2)，该转运蛋白位于人类睾丸中。对CT2一级结构的分析表明，它位于有机阳离子转运蛋白(OCT/OCTN)和阴离子转运蛋白(OAT)家族之间。因此，CT2代表了一个新的转运蛋白家族。CT2在非洲爪哇卵母细胞中表达时，可介导L肉碱的高亲和力转运，但不能接受主流的OCT/OCTN阳离子或燕麦阴离子底物。我们合成并测试了各种肉碱相关化合物，并用半经验计算化学方法研究了底物识别的物理化学性质。这些数据表明，季铵盐阳离子的膨胀性和相对疏水性是引发CT2-底物相互作用的最重要因素。免疫组织化学结果显示，CT2蛋白定位于附睾上皮腔膜和睾丸支持细胞内。CT2的发现代表了OCT/OCTNS和燕麦之间有趣的进化联系，也为我们提供了对人类精子成熟的重要洞察力。
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 epididyinal 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.