Sugar transport into protoplasts isolated from developing soybean cotyledons. II. Sucrose transport kinetics, selectivity, and modeling studies [Mathematical linear methods]

The effects of metabolic inhibitors, pH, and temperature on the kinetics of sucrose uptake protoplasts isolated from developing soybean Glycine max L. cv Wye 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 three 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 millimolar and was approximately equal to the nonsaturable and diffusive components at 44 and 22 millimolar external sucrose, respectively. The three uptake components showed different temperature sensitivities. Increasing external pH decreased both the linear component and the Vmax calculated for the saturable component. Conversely, 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 millimolar sucrose by several sugars suggested that the β-D-fructofuranoside bond and molecular size of sucrose were particularly important in sugar recognition by the saturable component carrier.