Proton-coupledl-lysine uptake by renal brush border membrane vesicles from mullet (Mugil cephalus)

Summary

The uptake of the basic amino acid,l-lysine, was studied in brush border membrane vesicles isolated from the kidney of the striped mullet (Mugil cephalus). The uptake ofl-lysine was not significantly stimulated by a Na⁺ gradient and no overshoot was observed. However, when a proton gradient (pH₀-5.5; pH_i=8.3) was imposed across the membrane in the absence of Na⁺, uptake was transiently stimulated. When the proton gradient was short circuited by the proton ionophore, carbonylcyanidep-triflouromethoxyphenyl hydrazone, proton gradient-dependent uptake of lysine was inhibited. Kinetics of lysine uptake determined under equilibrium exchange conditions indicated that theV _max increased as available protons increased (2.1 nmol/min/mg protein at pH 7.5 to 3.7 nmol/min/mg at pH 5.5), whereas the apparentK _m(4.9±0.6mm) was not altered appreciably. When membrane potential (inside negative) was imposed by K⁺ diffusion via valinomycin, a similar (but smaller) stimulation of lysine uptake was observed. When the membrane potential and the proton gradient were imposed simultaneously, a much higher stimulation in lysine uptake was shown, and the uptake of lysine was approximately the sum of the components measured separately. These results indicate that the uptake mechanism for basic amino acids is different from that of neutral or acidic amino acids and that the proton-motive force can provide the driving force for the uptake ofl-lysine into the isolated brush border membrane vesicles.