SUBSTRATE CHARGE DEPENDENCE OF STOICHIOMETRY SHOWS MEMBRANE-POTENTIALIS THE DRIVING-FORCE FOR PROTON-PEPTIDE COTRANSPORT IN RAT RENAL-CORTEX

The proton dependence of the transport of three labelled, hydrolysis-r esistant synthetic dipeptides carrying a net charge of -1, 0 or +1 has been investigated in a brush border membrane vesicle preparation obta ined from rat renal cortex. Cross-inhibition studies are consistent wi th the transport of all peptides studied being through a single system . The extent and time course of uptake in response to an inwardly dire cted electrochemical gradient of protons differed for each peptide. Fo r the cationic peptide D-Phe-L-Lys this gradient did not stimulate the initial rate of uptake, while for the neutral dipeptide D-Phe-L-Ala a nd the anionic peptide D-Phe-L-Glu stimulation was observed. However, the effect on D-Phe-L-Glu was more marked than that on D-Phe-L-Ala and the proton activation differed for these two peptides. The calculated Hill coefficients for the two proton-dependent peptides were 1.14+/-0 .16 and 2.15+/-0.10 for D-Phe-L-Ala and D-Phe-L-GIu, respectively, pro viding evidence that the stoichiometry of proton:peptide cotransport i s different for each peptide (0:1, 1:1 and 2:1 for D-PheL-Lys, D-Phe-L -Ala and D-Phe-L-Glu respectively); studies on energetics are compatib le with this conclusion. The physiological and molecular implications of this model are discussed, as are the applicability of the conclusio ns to secondary active transport systems more generally.