Sucrose is the predominant form of photosynthetically reduced carbon transported in most plant species. In the experiments reported here, an active, proton-coupled sucrose transport system has been idenfffied and partially characterized in plasmalemma vesicles Isolated from mature sugar beet (Beta vulgarls L. cv Great Westem) leaves. The isolated vesicles concentrated sucrose fivefold In the presence of an imposed pH gradient (basic interior). The presence of carbonyl cyanide

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... zone, a protonophore, prevented sucrose accumulation within the vesicles. ApH-dependent sucrose transport exhibited saturation kinetics with an apparent K. of 1.20 ± 0.40 millimolar, suggesting translocation was carrier-mediated. In support of that conclusion, two protein modifiers, diethyl pyrocarbonate and p-chloromercuribenzenesulfonic acid, were found to be potent inhibitors with 50% inactivation achieved at 750 and 30 micromolar, respectively. ApH-Dependent sucrose transport was not inhibited by glucose, [tris(hydroxymethyl)methylamino]propane; CCCP, carbonyl cyanide m-chlorophenylhydrazone; DEPC, diethyl pyrocarbonate; NEM, Nethylmaleimide; PCMBS, p-chloromercuribenzenesulfonic acid; PM, plasmalemma. 1318 www.plantphysiol.org on May 1, 2019 -Published by Downloaded from