The ascorbic acid paradox

Ascorbic acid (AA) is a common culture medium and dietary supplement. While AA is most commonly known for its antioxidant properties, it is also known to function as a pro-oxidant under select conditions. However, the complexity and often unknown composition of biological culture systems makes prediction of AA behaviour in supplemented cultures challenging. The frequent observation of outcomes inconsistent with antioxidant behaviour suggests that AA may be playing a pro-oxidant role more often than appreciated. In this work we explored the intracellular and extracellular impact of AA supplementation on KG1a myeloid leukaemia cells over a 24-h culture period following the addition of the AA supplement. At 24 h we found that supplementation of AA up to 250 μM resulted in intracellular antioxidant behaviour. However, when these same cultures were evaluated at 2 or 4 h we observed pro-oxidant activity at the higher AA concentrations indicating that the outcome was very much time and dose dependent. In contrast, pro-oxidant activity was never observed in the extracellular medium. Paradoxically, and to our knowledge not previously reported, we observed that intracellular pro-oxidant activity and extracellular antioxidant activity could occur simultaneously. These results indicate that the precise activity of AA supplementation varies as a function of dose, time and cellular location. Further, these results demonstrate how in the absence of careful culture characterization the true impact of AA on cultures could be underappreciated.

Research highlights
► In vitro ascorbic acid can function both as an antioxidant and pro-oxidant.
► Ascorbic acid supplementation resulting in antioxidant activity at 24 h can cause pro-oxidant activity at 2–4 h.
► Ascorbic acid supplementation can result in pro-oxidant activity intracellularly and antioxidant activity extracellularly simultaneously.
► The activity of ascorbic acid is a function of dose, time and intracellular and extracellular location.