The use of homocysteine in the estimation of dehydroascorbic acid.

Two main methods are in current use for the estimation of dehydroascorbic acid. In the one, developed by Roe and his colleagues (Roe & Kuether, 1943; Roe, Mills, Oesterling & Damron, 1948), the dehydroascorbic acid is condensed with 2:4-dinitrophenylhydrazine and the product treated with sulphuric acid to give a red colour, the intensity of which is measured photoelectrically. In a second technique (Tillmans, Hirsch & Siebert, 1932; Eekelen, Emmerie, Josephy & Wolff, 1933; Bessey, 1938), dehydroascorbic acid is measured as acorbic acid after reduction with hydrogen sulphide and removal of excess of reductant. Disadvantages are associated with both methods; they are nonspecific for dehydroascorbic acid, timeand labourconsuming, and in the method involving reduction with hydrogen sulphide it may be difficult to ensure the removal ofexcess ofreductant before estimating the ascorbic acid formed with 2:6-dichlorophenolindophenol. In this paper an account is given ofanew method, based on the two findings that homocysteine rapidly reduces dehydroascorbic acid at pH 7 0, and that under certain simple conditions 2:6-dichlorophenolindophenol can be used to estimate ascorbic acid in the presence of homocysteine without interference from the latter compound.