STUDIES FOR THE MECHANISM OF MULTIPLICITY IN SUBSTRATE SPECIFICITY OF TRANSPORTERS.

药物的药代动力学特征对于药物的有效治疗是非常重要的，并且它们受到许多因素的影响，包括结合蛋白、药物代谢酶和膜转运蛋白。其中，药物转运蛋白对药物的肠道吸收、组织分布、肝、肾排泄等具有重要作用。因此，什么样的药物被接受为每种转运蛋白的底物将是重要的。有机阳离子/肉毒碱转运蛋白OCTN和有机阴离子转运多肽OATP-C也接受多种生理和药物化合物作为底物。在本研究中，我们研究了这些转运蛋白的底物识别的多特异性的机制，通过关注themOCTN转运蛋白作为生理底物和阳离子化合物的多功能性， 关于我们 作为异生物物质。当OCTN转运肉毒碱时，它只表现出Na^+依赖性，而四乙铵（TEA）等有机阳离子的转运则不依赖于Na^+。利用OCTN突变蛋白的研究表明，肉毒碱和TEA部分共享OCTN 2上的底物识别位点，但它们并不完全相同。Na^+对肉毒碱与OCTN 2的亲和力有很大的影响，而TEA对Na^+的亲和力没有影响。而肉毒碱和TEA则表现出相互抑制作用，但不能用完全竞争抑制动力学来解释。肉毒碱和TEA之间转运机制的差异可以通过肉毒碱特异性羧基部分的存在和Na^+影响肉毒碱羧基部分与OCTN 2蛋白之间的相互作用来解释。这些观察结果表明，OCTN的多特异性是由于OCTN的多功能性，通过改变依赖于底物的运输机制，关于驱动力，Na^+。因此，澄清每种底物的转运蛋白的驱动力对于澄清转运蛋白的多特异性机制是重要的。在OATP的情况下，目前还没有明确证明驱动力，而OATP接受各种化合物作为底物。为了阐明OATP多特异性的机制，将来必须确定每种底物的驱动力。

Pharmacokinetic characteristics are important for the adequate drug therapy and they are affected by many factors, including binding proteins, drug metabolizing enzymes and membrane transporters. Among them, drug transporter are important for the intestinal absorption, tissue distribution, and hepatic and renal excretion of drugs. Accordingly, it will be important what kinds of drugs are accepted as substrates for each transporter. However, some drug transporters accept various compounds as substrates and it has not been clarified the mechanism for such multispecificity of drug transporterOrganic cation/carnitine transporter OCTN and organic anion transporting polypeptide OATP-C also accept various physiological and drug compounds as substrates. In the present study, we studied the mechanism for the multispecificity in the substrate recognition of those transporters by focusing on the multifunctionality of themOCTN transports carnitine as physiological substrates and cationic compounds … More as xenobiotics. When OCTN transports carnitine, it exclusively shows Na^+-dependence, while the transport of organic cation such as tetraethylammonium (TEA) is Na^+-independent. The study using mutant protein of OCTN exhibited that carnitne and TEA partially shares the substrate recognition sites on OCTN2, while they are not identical. Na^+ largely affect the affinity of carnitine to OCTN2, while TEA did not show any change in the affinity to OCTN2 by Na^+. However, carnitine and TEA exhibited mutual inhibitory effect, while they are not explained by complete competitive inhibition kinetics. The difference in the transporting mechanism between carnitine and TEA could be explained by the presence of the carboxylmoiety that is specific for carnitine and the Na^+ affect the interaction between carboxylmoiety of carnitine and OCTN2 protein. These observations suggested that multispecificity of OCTN is due to the multifunctionality of OCTN by changing the transport mechanisms depending on the substrates, regarding the driving force, Na^+. Accordingly, clarification of the driving force of the transporter for each substrate is important to clarify the mechanism of multispecificity of transporters. In the case of OATP, at present no driving force has been clearly demonstrated, while OATP accept various compounds as substrates. To clarify the mechanism for the multispecificity of OATP, it will be essential to identify the driving force for each substrate in future Less