Metabolism of Radioactive Sugars by Tobacco leaf Disks

Destarched tobacco-leaf disks were floated on per cent. (w/v) solutions of sucrose uniformly labelled with 14C in either the glucose or fructose moiety, and on invert sugar in which one hexose only was so labelled. The experiments were carried out in an atmosphere of oxygen at 25° C. Seventy-five per cent, of the sugar lost from the external solutions was recovered as starch, sucrose, fructose, glucose, and CO2. With sucrose as the substrate, 30 per cent, of the material was recovered as CO2 and 17 per cent. each as starch, sucrose, fructose, and glucose. With invert sugar as the substrate, 30 per cent, was again recovered as CO2 only 20 per cent. as the three sugars together, and 50 per cent. as starch. Whichever hexose was initially labelled and whether the sugar was supplied as sucrose or hexose, the relative specific activities of starch and sucrose in the leaf disks and of the CO2 evolved were equal or nearly equal to that of the sugar supplied. With sucrose as the substrate the sucrose in the disks retained its asymmetry of label, and free hexoses produced were similarly asymmetrically labelled. When invert sugar was the substrate the sucrose synthesized was strongly labelled in both moieties, as also were the free hexosea. It is concluded that fructose and glucose free or combined in sucrose were equally available for starch synthesis and CO2, formation, and that there can be no question of preferential utilization of one or other hexose. Starch and CO2 must arise from a common source in which readily formed derivatives of the hexoses are rapidly equilibrated. Free hexose cannot participate directly in either sucrose or starch synthesis. Accumulation of sugar not immediately metabolized and inversion of sucrose take place at a site remote from the common pool. A scheme to accommodate the results is discussed.