Yokogawa K, Miya K, Tamai I, Higashi Y, Nomura M, Miyamoto K, Tsuji A
Department of Pharmacology and Pharmaceutics, Graduate School of Natural Science and Technology, Kanazawa University, Japan.
J Pharm Pharmacol. 1999 Aug;51(8):935-40. doi: 10.1211/0022357991773195.
The recently cloned organic cation transporter, OCTN2, isolated as a homologue of OCTN1, has been shown to be of physiological importance in the renal tubular reabsorption of filtered L-carnitine as a high-affinity Na+ carnitine transporter in man. Although the mutation of the OCTN2 gene has been proved to be directly related to primary carnitine deficiency, there is little information about the L-carnitine transport system in the liver. In this study, the characteristics of L-carnitine transport into hepatocytes were studied by use of cultured human hepatoma HLF cells, which expressed OCTN2 mRNA to a greater extent than OCTN1 mRNA. The uptake of L-carnitine into HLF cells was saturable and the Eadie-Hofstee plot showed two distinct components. The apparent Michaelis constant and the maximum transport rate were 6.59+/-1.85 microM (mean+/-s.d.) and 78.5+/-21.4 pmol/5 min/10(6) cells, respectively, for high-affinity uptake, and 590+/-134 microM and 1507+/-142 pmol/5 min/10(6) cells, respectively, for low-affinity uptake. The high affinity L-carnitine transporter was significantly inhibited by metabolic inhibitors (sodium azide, dinitrophenol, iodoacetic acid) and at low temperature (4 degrees C). Uptake of [3H]L-carnitine also required the presence of Na+ ions in the external medium. The uptake activity was highest at pH 7.4, and was significantly lower at acidic or basic pH. L-Carnitine analogues (D-carnitine, L-acetylcarnitine and gamma-butyrobetaine) strongly inhibited uptake of [3H] L-carnitine, whereas beta-alanine, glycine, choline, acetylcholine and an organic anion and cation had little or no inhibitory effect. In conclusion, L-carnitine is absorbed by hepatocytes from man by an active carrier-mediated transport system which is Na+-, energy- and pH-dependent and has properties very similar to those of the carnitine transporter OCTN2.
最近克隆出的有机阳离子转运体OCTN2，是作为OCTN1的同系物分离得到的，已被证明在人体肾小管对滤过型左旋肉碱的重吸收中具有生理重要性，它是一种高亲和力的Na⁺-肉碱转运体。虽然已证实OCTN2基因突变与原发性肉碱缺乏直接相关，但关于肝脏中左旋肉碱转运系统的信息却很少。在本研究中，利用培养的人肝癌HLF细胞研究了左旋肉碱转运进入肝细胞的特性，该细胞中OCTN2 mRNA的表达程度高于OCTN1 mRNA。左旋肉碱进入HLF细胞的摄取是可饱和的，伊迪-霍夫斯蒂图显示出两个不同的组分。对于高亲和力摄取，表观米氏常数和最大转运速率分别为6.59±1.85 μM（平均值±标准差）和78.5±21.4 pmol/5 min/10⁶个细胞，而对于低亲和力摄取，分别为590±134 μM和1507±142 pmol/5 min/10⁶个细胞。高亲和力的左旋肉碱转运体受到代谢抑制剂（叠氮化钠、二硝基苯酚、碘乙酸）以及在低温（4℃）下的显著抑制。[³H]左旋肉碱的摄取还需要细胞外培养基中存在Na⁺离子。摄取活性在pH 7.4时最高，在酸性或碱性pH下显著降低。左旋肉碱类似物（D-肉碱、L-乙酰肉碱和γ-丁甜菜碱）强烈抑制[³H]左旋肉碱的摄取，而β-丙氨酸、甘氨酸、胆碱、乙酰胆碱以及一种有机阴离子和阳离子几乎没有抑制作用。总之，人肝细胞通过一种由载体介导的主动转运系统吸收左旋肉碱，该系统依赖于Na⁺、能量和pH，其特性与肉碱转运体OCTN2非常相似。
