The endiol groups at the second and third carbon atoms are sensitive to oxidation and can easily convert into a diketo group, L-dehydroascorbic acid. This oxidized form of the vitamin is just as effective against scurvy as the reduced substance. Aqueous solutions of the nutrient L-dehydroascorbic acid are air-oxidized further both rapidly and irreversibly to L-diketogulonic acid and other biological inactive substances.
Research on vitamin C has been exhaustive; however, the precise biochemical function of ascorbic acid remains obscure. This limited knowledge concerning the metabolic functions of ascorbic acid has hindered efforts to develop fully satisfactory procedures to identify vitamin C deficiency or to assess nutritional status. Vitamin C is the only water-soluble vitamin for which there is no microbial assay.
Methods for determining ascorbic acid are numerous. In general, chemical analyses for the vitamin may be divided into two groups; the determination of the reduced form and the determination of the oxidized form. The former group of analyses are usually based upon the oxidation-reduction properties of ascorbic acid. These are widely used as the fundamental reactions in the measurement of vitamin C. The latter group of analyses is usually based upon the oxidation of the ascorbic acid and the subsequent formation of a hydrazone or a fluorophor. Best results are obtained if samples, especially plasma, are quickly stabilized with either trichloroacetic acid or metaphosphoric acid and immediately analyzed. Prompt stabilization is especially important in the case of plasma or serum. The greater stability of ascorbic acid in acid solution is due to the decreased tendency for hydrolysis of the lactone ring with decreasing pH. In alkaline solution the hydrolysis is fairly rapid, and such solutions lose vitamin activity in a short period of time. Also the oxidation of ascorbate in solution is catalyzed by various metal ions. If necessary, acidified samples may be stored frozen at -700. 2 1 The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense.