UPTAKE OF ASCORBIC-ACID AND DEHYDROASCORBIC ACID BY CHROMAFFIN GRANULES OF THE ADRENAL-MEDULLA

Adrenal chromaffin granules incubated with [14C]ascorbate or [14C]dehydroascorbate accumulated radioactively labeled compounds. Dehydroascorbate was generated from ascorbate by including oxidized cytochrome c in the incubation mixture but thin layer chromatography revealed that dehydroascorbate was present even in the absence of cytochrome c. This observation coupled with the fact that radioactively labeled compound was accumulated much faster in the presence of cytochrome c than in its absence indicates that dehydroascorbate but not ascorbate is taken up by the chromaffin granules. Uptake of radioactivity was not saturable with ascorbate or dehydroascorbate concentration, and granules which had taken up the radioactively labeled compound lost most of the material in 12 h. In addition 1 mm adenosine 5′-triphosphate--Mg2+ + had no effect on either ascorbate or dehydroascorbate uptake. These data indicate that uptake proceeds via an energy independent mechanism. The rate of uptake of dehydroascorbate is 50-fold lower than the rate of catecholamine uptake. Ascorbate but not dehydroascorbate acts as a cofactor for the enzyme dopamine β-hydroxylase in the conversion of dopamine to norepinephrine. Dehydroascorbate taken up by the granules must therefore be reduced in order to be used by dopamine i3-hydroxylase. We suggest that the chromaffin granule electron transport system may be involved in this reduction. © 1979.