Sugar Transport into Protoplasts Isolated from Developing Soybean Cotyledons

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Abstract
The effects of metabolic inhibitors, pH and temperature on the kinetics of sucrose uptake in protoplasts isolated from developing soybean cotyledons were studied. Structural requirements for substrate recognition by the sucrose carrier were examined by observing the effects of potential alternate substrates for the saturable component on sucrose uptake. Uptake by the 3 components (saturable, sulfhydryl reagent-sensitive nonsaturable and diffusive) was calculated over a range of sucrose concentrations. The saturable component dominated uptake at external sucrose concentrations below 12 mM and was approximately equal to the nonsaturable and diffusive components at 44 and 22 mM external sucrose, respectively. The 3 uptake components showed different temperature sensitivities. Increasing external pH decreased both the linear component and the Vmax calculated for the saturable component. Increasing pH increased the calculated Km (sucrose) for the saturable component. Sucrose uptake by the saturable component was insensitive to several mono- and divalent cations. Competition for uptake of 0.5 mM sucrose by several sugars suggested that the .beta.-D-fructofuranoside bond and molecular size of sucrose were particularly important in sugar recognition by the saturable component carrier.
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