The Uptake of 2‐Deoxy‐dGlucose by Isolated Ricinus Cotyledons

The uptake of the non‐metabolizable glucose analogue, 2‐deoxy‐d‐glucose (DOG) has been followed in isolated cotyledons of Ricinus communis L. Within 2 min of immersion in a labelled DOG medium, DOG phosphate (DOG‐P) was detectable in the tissue. DOG uptake was relatively rapid for 30 min after which it slowed down but nevertheless continued for many hours; DOG was eventually accumulated against a concentration gradient. Internal DOG‐P concentration also rose for the first 30 min, but then reached a plateau. The relationship between uptake and external concentration was close to linear during the first hour, and subsequently curvilinear. Analysis of “wash‐out” curves indicated that after very rapid exit from the surface and free space of the tissue, the half‐time for exit of the remaining 85% of the absorbed DOG was approximately 250 min. By comparison, the half‐time for exit of sorbitol was 30 min. No DOG‐P could be detected in the wash‐out solutions. When cotyledons were transferred to labelled medium after preloading in non‐labelled DOG, the specific activity of the internal DOG pool rose faster than that of the DOG‐P. Both curves flattened out when their specific activity was less than half that of the external medium. Addition of d‐glucose to the medium depressed DOG‐P formation, but only when the incubation period was less than 45 min. Arguments are presented for concluding (a) that DOG uptake is mediated by a specific uptake mechanism; (b) that more than one internal DOG pool is involved; and (c) that the observed phosphorylation is not a necessary step in the entry process into either of the pools, whether the latter are in series or in parallel.