Ascorbic acid and dehydroascorbic acid measurements in human plasma and serum

We investigated whether circulating ascorbic acid in humans is protein bound or free and whether ascorbic acid exists in its reduced form alone as ascorbic acid or in its reduced and oxidized forms as ascorbic acid and dehydroascorbic acid, respectively. Ascorbic acid and dehydroascorbic acid were determined by using HPLC with coulometric electrochemical detection, and protein binding was determined by centrifugal ultrafiltration. Ascorbic acid was free in plasma and serum of normal, healthy volunteers, 10 men and 10 women. Ascorbic acid was detectable only in its reduced form. However, dehydroascorbic acid could be made to appear in samples processed under oxidizing conditions. Because circulating ascorbic acid is free and is detected only as reduced vitamin, ascorbic acid may be available without intermediates for peripheral utilization. Dehydroascorbic acid may not be present in plasma and serum of normal humans unless assay conditions permit ascorbic acid oxidation.