Muefide Aydogan & Asly Korkmaz 
Does Vitamin C Act as an Antioxidant or Prooxidant? [PDF ebook] 

Vitamin C is known to be an effective antioxidant in biological fluids and cells, and generally believed to be a beneficial component from vegetables and fruits. Vitamin C is an important dietary antioxidant which significantly decreases the adverse effects of reactive oxygen species (ROS) formed in the cell. Paradoxically, vitamin C has also been shown to oxidize the cellular components under certain in vitro experimental conditions and induce cell death in a number of different cell lines. However, the mechanism of the oxidizing feature of vitamin C is not well understood. It has been reported that vitamin C loses its effectiveness as antioxidant at high concentrations or at partial oxygen pressures