Energization of the Sugar Transport Mechanism in the Plasmalemma of Isolated Mesophyll Protoplasts

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Abstract
The mechanism of 3-O-methyl-D-glucose transport through the plasmalemma was investigated in protoplasts isolated from the mesophyll of Pisum sativum L. cv. Dan. Analysis of the fluxes after 50 min of uptake showed that the gradual decrease in slope of the net uptake curve with time was not due to any decline in uptake capacity; it represented the approach to flux equilibrium of a small compartment of the protoplast, probably the cytoplasm. The energy of activation for initial flux into this compartment was 20 kcal/mol between 17 and 27.degree. C. Very high discrimination was shown with regard to sugar isomers. Light strongly promoted flux (by a factor of 2.5 in the case of methyl glucose). Initial flux showed sharply contrasting inhibitor sensitivity in the light and the dark. Light uptake was sensitive to the proton conductor carbonyl cyanide m-chlorophenylhydrazone (CCCP), but stable for at least the first 10 min to the ATPase inhibitors quercetin, rutin, and diethylstilbestrol, as well as to arsenate. Dark uptake, on the other hand, was stable to CCCP but was immediately depressed by quercetin, rutin, diethylstilbestrol and arsenate. Protoplasts which received a light pretreatment before incubation in the dark took up methyl glucose at the accelerated light rate for the first 7 min. Moreover, the light pretreatment sensitized subsequent initial dark uptake to CCCP, and conferred on it the stability to ATPase inhibitors and arsenate characteristic of light uptake. After about 7 min the characteristic inhibitor responses of dark uptake were resumed. More than 1 mode of energy-coupling for sugar transport may operate in these protoplasts.
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