REDUCTION OF EXTRACELLULAR DEHYDROASCORBIC ACID BY K562 CELLS

K562 erythroleukaemic cells produced ascorbate when incubated with deh ydroascorbic acid. The reduction depended on the number of cells and o n the concentration of dehydroascorbic acid. The observed rate consist s of a high affinity (apparent K-m 7 mu M, V-max 3.25 pmol min(-1) (10 (6) cells)(-1) and a law affinity component, which was non-saturable u p to 1 mM of DHA (rate increase of 0.1 pmol min(-1) (10(6) cells)(-1) (1 mu M of DHA(-1)). The rate was dependent on temperature and was sti mulated by glucose and inhibited by phloretin, N-ethylmaleimide, parac hloro-mercuribenzoate and thenoyltrifluoroacetone. Although uptake of DHA proceeded at a higher rate than its extracellular reduction, the g eneration of extracellular ascorbate from DHA cannot be accounted for by intracellular reduction and the release of ascorbate, since the lat ter was not linear with time and had an initial rate of approximately 3 pmol min(-1) (10(6) cells(-1)). At a concentration of DHA of 100 mu M this is 25 per cent of the observed reduction.