Is ascorbic acid an antioxidant for the plasma membrane?

Ascorbic acid, or vitamin C, is a primary antioxidant in plasma and within cells, but it can also interact with the plasma membrane by donating electr ons to the alpha-tocopheroxyl radical and a trans-plasma membrane oxidoredu ctase activity, Ascorbate-derived reducing capacity is thus transmitted bot h into and across the plasma membrane. Recycling of alpha-tocopherol by asc orbate helps to protect membrane lipids from peroxidation. However, neither the mechanism nor function of the ascorbate-dependent oxidoreductase activ ity is known. This activity has typically been studied using extracellular ferricyanide as an electron acceptor. Whereas an NADH:ferricyanide reductas e activity is evident in open membranes, ascorbate is the preferred electro n donor within cells. The oxidoreductase may be a single membrane-spanning protein or may only partially span the membrane as part of a trans-membrane electron transport chain composed of a cytochrome or even hydrophobic anti oxidants such as alpha-tocopherol or ubiquinol-10, Further studies are need ed to elucidate the structural components, mechanism, and physiological sig nificance of this activity. Proposed functions for the oxidoreductase inclu de stimulation of cell growth, reduction of the ascorbate free radical outs ide cells, recycling of alpha-tocopherol, reduction of Lipid hydroperoxides , and reduction of ferric iron prior to iron uptake by a transferrin-indepe ndent pathway.