DEHYDROASCORBATE AND ASCORBATE TRANSPORT IN RAT-LIVER MICROSOMAL VESICLES

Ascorbate and dehydroascorbate transport was investigated in rat liver microsomal vesicles using radiolabeled compounds and a rapid filtrati on method, The uptake of both compounds was time-and temperature-depen dent, and saturable. Ascorbate uptake did not reach complete equilibri um, it had low affinity and high capacity, Ascorbate influx could not be inhibited by glucose, dehydroascorbate, or glucose transport inhibi tors (phloretin, cytochalasin B) but it was reduced by the anion trans port inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and by the alkylating agent N-ethylmaleimide, Ascorbate uptake could be stim ulated by ferric iron and could be diminished by reducing agents (dith iothreitol, reduced glutathione). In contrast, dehydroascorbate uptake exceeded the level of passive equilibrium, it had high affinity and l ow capacity, Glucose cis inhibited and trans stimulated the uptake. Gl ucose transport inhibitors were also effective. The presence of intrav esicular reducing compounds increased, while extravesicular reducing e nvironment decreased dehydroascorbate influx. Our results suggest that dehydroascorbate transport is preferred in hepatic endoplasmic reticu lum and it is mediated by a GLUT-type transporter, The intravesicular reduction of dehydroascorbate leads to the accumulation of ascorbate a nd contributes to the low intraluminal reduced/oxidized glutathione ra tio.