Molecular cloning and functional expression of kidney-specific organic anionic drug transporters

大多数摄入体内的药物通过肝脏中的药物代谢酶代谢，并转化为更极化的亲水性化合物。这样形成的药物代谢物含有负电荷或正电荷，并产生更多的阴离子化合物。这些药物代谢产物需要特定的膜蛋白转运体才能排出体外，药物的排出主要有两条途径，一条是经肝脏经胆汁排出，另一条是经肾脏经尿液排出。在本研究中，我们试图克隆肾脏特异性的有机阴离子转运蛋白，并对几种异构体进行了表征，1997年，我们成功地从大鼠肾脏中分离到一种新的有机阴离子转运蛋白，并将其命名为OAT（organic anion transporter）1。此后，OAT家族的三个成员被确定。OAT 2是肝脏特异性的，具有与OAT 1不同的转运底物谱。OAT 3具有536个氨基酸残基，在肝、肾和脑中表达，分别与OAT 1和OAT 2具有49%和39%的同一性。OAT 3介导PAH、赭曲霉毒素A和硫酸雌酮的高亲和力转运。人OAT 4在肾脏和胎盘中表达，由550个氨基酸组成，与OAT 1 β 3具有38%-44%的同一性。OAT 4也具有多特异性底物识别功能，如硫酸雌酮、硫酸脱氢表雄酮和赭曲霉毒素A。虽然我们可以找到OAT家族的4种亚型，OAT 1和OAT 4，但其底物识别的分子机制仍有待进一步研究。由于OAT揭示了弱，但显着的相似性，在其结构与有机阳离子转运蛋白和肉毒碱转运蛋白，可能的分子进化，这些家庭之间应该澄清。在OAT中观察到的多特异性可能源于相对简单的底物与转运蛋白的结合，并且需要阐明两个重要结合位点（如疏水部分和离子电荷）的假设。

Most drugs ingested into the body are metabolized by drug-metabolizing enzymes in the liver, and converted to more polarized hydrophilic compounds. Thus formed drug metabolites contain either negative or positive charge, and more anionic compounds are produced. To eliminate these drug metabolites out of the body, specific membrane proteins named transporters are required.There are two major pathways for drug elimination, one of which is hepatic one through bile and another is renal one to urine. In this investigation, we tried to clone kidney-specific organic anion transporters, and to characterize several isoforms.In 1997, we could succeed to isolate a novel organic anion transporter from rat kidneys and named it OAT (organic anion transporter) 1. Thereafter, three members of OAT family have been identified. OAT2 is liver-specific having different spectrum of transport substrates from OAT1. OAT3 having 536 amino acid residue is expressed in the liver, kidney and brain showing 49% and 39% identities with OAT1 and OAT2, respectively. OAT3 mediates high affinity transport of PAH, ochratoxin A, and estron sulfate. Human OAT4 is expressed in the kidney and placenta and consists of 550 amino acids having identity at 38%〜44% with OAT1〜3. The OAT4 is also multispecific in substrate recognition, like estron sulfate, dehydroepiandrosterone sulfate, and ochratoxin A.Although we could chine 4 isoforms of OAT family, OAT1〜4, molecular mechanisms on substrate recognition remain to be carried out by further investigation. Since OATs reveal weak but significant similarity in their structure with organic cation transporters and carnitine transporters, possible molecular evolution should be clarified among these families. The multi specificity seen in OATs may be originated from relatively simple substrate binding to the transporters and the hypothesis of two important biding sites like hydrophobic moiety and ionic charge needs to be elucidated.