Valine uptake by isolatedCommelina benghalensisL. mesophyll cells was measured over a wide concentration range (10-6–4·10-2mol l-1). The uptake data were subjected to iterative fitting. Experiments with carbonyl cyanide mchlorophenyl hydrazone (CCCP), diethylstilbestrol (DES), and p-chloromercuriphenylsulphonic acid (PCMBS) provided evidence that the biphasic uptake kinetics of valine consists of a diffusional component and a biphasic active uptake. The data from the control experiments, were also best fitted to one diffusional component and two Michaelis-Menten systems. The presence of two carrier systems in the plasmalemma, however, was considered to be virtual for the following reasons: (1) Both phases of active uptake were equally decreased by high concentrations of K+-ions. (2) Fusicoccin stimulated the active uptake in both phases to the same extent. (3) Inhibitors of the proton-driven uptake (CCCP, DES, PCMBS) similarly inhibited the active uptake at all concentrations. (4) The active uptake equally responded in both phases to changes in the pH. (5) Light also promoted the active uptake over the whole concentration range. These results strongly indicate that, despite its biphasic character, the active uptake is due to one proton-driven carrier syst
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