SUGAR-TRANSPORT AND SUGAR-METABOLIZING ENZYMES IN SUGAR-BEET STORAGE ROOTS (BETA-VULGARIS SSP ALTISSIMA)

Sugar uptake studies with isolated protoplasts from sugar beet roots indicated that glucose is transported preferentially, obviously by a H+/glucose symport mechanism. The uptake rate in conducting tissue exceeded that of storage tissue. Fructose and sucrose seemed to be transported by lower affinity. The apparent K(M) for fructose and sucrose was 7.8 and 18.6 mM respectively, whereas glucose uptake had a Michaelis constant of 1.4 mM. Competition experiments led to the assumption of two different carrier systems for glucose and fructose. While comparing activities of sucrose synthase (SS, E.C. 2.4.1.13), sucrose phosphate synthase (SPS, E.C. 2.4.1.14) and acid invertase (IV, E.C. 3.2.1.26) in different parts of sugar beet roots, in protoplasts and vacuoles, we obtained information on the localization and thus the possible physiological significance of these sucrose metabolizing enzymes: conducting tissue exhibits higher activities of SS, SPS and cell-wall bound acid IV than storage tissue. However, the activity of soluble acid IV was slightly higher in the latter. Concerning the subcellular compartmentation, SS seems to be restricted to the cytoplasm whereas more than 50 % of the total SPS activity is found in the vacuolar fraction. A working hypothesis for the unloading and storage of sucrose in sugar beets is proposed.