Enhancement of norepinephrine biosynthesis by ascorbic acid in cultured bovine chromaffin cells

Ascorbic acid donates electrons to dopamine β-monooxygenase during the hydroxylation of dopamine to norepinephrine in vitro. However, the possible role of ascorbic acid in norepinephrine biosynthesis in vivo has not been defined. We therefore investigated the effect of newly accumulated ascorbic acid on catecholamine biosynthesis in cultured bovine adrenal chromaffin cells. Cells supplemented for 3 h with ascorbic acid accumulated 9-fold more ascorbic acid than found in control cells. Under these conditions, the cells loaded with ascorbate were found to double the rate of norepinephrine biosynthesis from [¹⁴C]tyrosine compared to control. By contrast, the amounts present of [¹⁴C]3,4-dihydroxyphenylalanine and [¹⁴C]dopamine synthesized from [¹⁴C]tyrosine were unaffected by the preloading of ascorbic acid. Ascorbate preloaded cells incubated with [³H]dopamine also showed a similar increase in the rate of norepinephrine formation, without any change in dopamine transport into the cells. Thus, these data were consistent with ascorbate action at the dopamine β-monooxygenase step. In order to determine if ascorbate could interact directly with dopamine β-monooxygenase localized within chromaffin granules, we studied whether isolated chromaffin granules could accumulate ascorbic acid. Ascorbic acid was not transported into chromaffin granules by an uptake or exchange process, despite coincident [³H]dopamine uptake which was Mg-ATP dependent. These data indicate that ascorbic acid does augment norepinephrine biosynthesis in intact chromaffin cells, but by a mechanism that might enhance the rate of dopamine hydroxylation indirectly.