Wenzel U, Gebert I, Weintraut H, Weber W M, Clauss W, Daniel H
Institute of Nutritional Sciences, University of Giessen, FRG.
J Pharmacol Exp Ther. 1996 May;277(2):831-9.
To investigate whether multiple peptide transporters mediate absorption of beta-lactams carrying different charges at physiological pH, we used the human intestinal cell line Caco-2 and Xenopus laevis oocytes expressing the cloned rabbit intestinal peptide transporter PepT1. Characteristics of transport of the anionic cefixime and the zwitterionic cefadroxil were assessed by 1) flux studies using radiolabeled compounds, by 2) measuring changes in pHin in cells and oocytes as a consequence of substrate-mediated proton influx and 3) by applying the two-electrode voltage clamp technique to assess the electrophysiological phenomena associated with beta-lactam transport in oocytes expressing PepT1. Both beta-lactams were rapidly taken up into Caco-2 cells and oocytes expressing PepT1 by a pH-dependent and saturable transport pathway. Mutual inhibition suggested that acidic and zwitterionic compounds may share a common transporter. Cefixime and cefadroxil caused a significant decline in intracellular pH as a consequence of proton coupled substrate influx. Uptake of cefixime and cefadroxil via PepT1 expressed in oocytes was electrogenic indicating that transport of both beta-lactams is associated with movement of net positive charge. The more acidic pH required for rheogenic cefixime uptake in both cell systems, when compared to cefadroxil uptake in both cell systems, when compared to cefadroxil uptake, and the concomitant faster intracellular acidification indicates that cefixime most likely is taken up only in its nonionized form with an additional proton being cotransported. This is supported by the observation that cefixime uptake at different pH correlated significantly with the percentage of the nonionized species being present. From our studies we conclude that a single peptide transport system can mediate electrogenic uptake of the neutral form of beta-lactam antibiotics into intestinal epithelial cells.
为研究在生理pH值下多种肽转运体是否介导携带不同电荷的β-内酰胺类药物的吸收，我们使用了人肠上皮细胞系Caco-2和表达克隆的兔肠肽转运体PepT1的非洲爪蟾卵母细胞。通过以下方法评估阴离子型头孢克肟和两性离子型头孢羟氨苄的转运特征：1）使用放射性标记化合物进行通量研究；2）测量细胞和卵母细胞中由于底物介导的质子内流导致的pHin变化；3）应用双电极电压钳技术评估在表达PepT1的卵母细胞中与β-内酰胺类药物转运相关联的电生理现象。两种β-内酰胺类药物均通过pH依赖性和可饱和的转运途径迅速被摄取到Caco-2细胞和表达PepT1的卵母细胞中。相互抑制表明酸性和两性离子化合物可能共用一个共同的转运体。由于质子偶联的底物内流，头孢克肟和头孢羟氨苄导致细胞内pH显著下降。通过卵母细胞中表达的PepT1摄取头孢克肟和头孢羟氨苄是生电的，表明两种β-内酰胺类药物的转运都与净正电荷的移动有关。与头孢羟氨苄摄取相比，在两种细胞系统中摄取生电的头孢克肟所需的酸性pH更高，同时细胞内酸化更快，这表明头孢克肟最有可能仅以其非离子化形式被摄取，并伴有一个额外的质子共转运。这一观点得到以下观察结果的支持：不同pH下头孢克肟的摄取与存在的非离子化形式的百分比显著相关。从我们的研究中我们得出结论，单一的肽转运系统可以介导β-内酰胺类抗生素中性形式的生电摄取进入肠上皮细胞。
