1. The intestinal H+-coupled peptide transporter PepT1, displays a broad substrate specificity and accepts most charged and neutral di- and tripeptides. To study the proton-to-peptide stoichiometry and the dependence of the kinetic parameters on extracellular pH (pH(o)), rabbit PepT1 was expressed in Xenopus laevis oocytes and used for uptake studies of radiolabelled neutral and charged dipeptides, voltage-damp analysis and intracellular pH measurements. 2. PepT1 did not display the substrate-gated anion conductances that have been found to be characteristic of members of the Na+- and H+-coupled high-affinity glutamate transporter family. In conjunction with previous data on the ion dependence of PepT1, it can therefore be concluded that peptide-evoked charge fluxes of PepT1 are entirely due to H+ movement. 3. Neutral, acidic and basic dipeptides induced intracellular acidification. The rate of acidification, the initial rates of the uptake of radiolabelled peptides and the associated charge fluxes gave proton-substrate coupling ratios of 1:1, 2:1 and 1:1 for neutral, acidic and basic dipeptides, respectively. 4. Maximal transport of the neutral and charged dipeptides Gly-Leu, Gly-Glu, Gly-Lys and Ala-Lys occurred at pH, 5.5, 5.2, 6.2 and 5.8, respectively. The I(max) values were relatively pH(o) independent but the apparent affinity (K(m)(app)) values for these peptides were shown to be highly pH(o) dependent. 5. Our data show that at physiological pH (pH(o) 5.5-6.0) PepT1 prefers neutral and acidic peptides. The shift in transport maximum for the acidic peptide Gly-Glu to a lower pH value suggests that acidic dipeptides are transported in the protonated form. The shift in the transport maxima of the basic dipeptides to higher pH values may involve titration of a side-chain on the transporter molecule (e.g. protonation of a histidine group). These considerations have led us to propose a model for coupled transport of neutral, acidic and basic dipeptides.
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