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Sugar Transport in Immature Internodal Tissue of Sugarcane: II. Mechanism of Sucrose Transport.

The mechanism by which sucrose is transported into the inner spaces of immature internodal parenchyma tissue of sugarcane (Saccharum officinarum L. var. H 49-5) was studied in short term experiments (15 to 300 seconds). Transport of sucrose, glucose, and fructose was each characterized by a V(max) of 1.3 mumoles/gram fresh weight.2 hours, and each of these three sugars mutually and competitively inhibited transport of the other two. When (14)C-glucose was supplied exogenously, (14)C-glucose 6-phosphate and (14)C-glucose were the first labeled compounds to appear in the tissue; no (14)C-sucrose was detected until after 60-second incubation. After 15-second incubation in (14)C-sucrose, all intracellular radioactivity was in glucose, fructose, glucose 6-phosphate, and fructose 6-phosphate; trace amounts of (14)C-sucrose were found after 30 seconds and after 5 minutes, 71% of the intracellular radioactivity was in sucrose. Although it was possible that sucrose was transported intact into the inner space and then immediately hydrolyzed, it was shown that the rate of hydrolysis under these conditions was too low to account for the rate of hexose accumulation. Pretreatment of the tissue with rabbit anti-invertase antiserum eliminated sucrose transport, but had no effect on glucose transport. Since the antibodies did not penetrate the plasmalemma, it was concluded that sucrose was hydrolyzed by an invertase in the free space prior to transport. The glucose and fructose moieties, or their phosphorylated derivatives, were then transported into the inner space and sucrose was resynthesized. No evidence for the involvement of sucrose phosphate in transport was found in these experiments.