CARRIER-MEDIATED UPTAKE OF DEHYDROASCORBATE INTO HIGHER-PLANT PLASMA-MEMBRANE VESICLES SHOWS TRANS-STIMULATION

The activity of the ascorbate (Asc) carrier of purified Phaseolus plas ma membranes is demonstrated to be highly stimulated when membrane ves icles are preloaded with Asc. Asc transport is inhibited by DTT but is not affected by glutathione or ferricyanide, indicating that dehydroa scorbate (DHA) is the preferred species for uptake, Asc transport in t he loaded vesicles showed saturable kinetics with an apparent affinity constant of 24 mu M and maximal uptake rate of 94 pmol/mg/min. Additi on of DHA stimulated the efflux of Asc molecules from the loaded vesic les. Together these results suggest the presence of an Asci DHA exchan ge mechanism in higher plant: plasma membranes. (C) 1998 Federation of European Biochemical Societies.