Transport of 3-o-Methylglucose Into and Out of Storage Cells of Daucus carota

Abstract

The movement of labeled 3-o-methylglucose (MeG) into and out of thin carrot discs has been followed in order to gain information on sugar entry and exit mechanisms. Little or no metabolism of this derivative appears to occur in the tissue, since no products were detected either by chromatography or by analysis of respiratory CO2. The curve relating entry to external concentration deviated somewhat from a rectangular hyperbola but suggested a carrier mechanism. Glucose and MeG each competitively inhibited the uptake of the other. Ki for MeG was estimated to be 3 times the Km for its uptake. When discs incubated in MeG were transferred to H2O, MeG lost to the solution from the free space was re-absorbed against a 7-fold concentration gradient. The addition of unlabeled MeG or glucose to the medium surrounding discs which had been maintaining a ratio of internal to external MeG of 751 brought about release of stored isotope. This was probably not due to exchange diffusion stricto sensu. Efflux of previously absorbed isotopic MeG into a medium containing unlabeled MeG or glucose was temperature-sensitive. The kinetics of efflux were complex and did not suggest a simple diffusion process related to overall MeG content. However, there is evidence (including the falling rate of exit with time) that slow diffusion (or slow release from adsorption) contributed substantially to efflux. The source of this flow appeared to be neither the readily accessible free space nor the main storage compartment. Calculation indicated that the volume of this "slow diffusion compartment" might be about 1% of the total volume of the discs.