VARIATION IN ASCORBIC-ACID OXIDATION ROUTES IN H2O2 AND CUPRIC ION SOLUTION AS DETERMINED BY GC MS/

Recent reports have suggested that ascorbic acid protects low-density lipoprotein from peroxide-induced oxidation, but does not protect and may actually function as a prooxidant in the presence of cupric ions. However, dehydroascorbic acid, (the first oxidation product of ascorbi c acid) has been shown to protect low-density lipoprotein from cupric ion oxidation but not peroxide-induced oxidation. We have examined the degradation of ascorbic acid, uniformly labeled [C-13(6)]ascorbic aci d, and [6,6-H-2(2)]ascorbic acid in hydrogen peroxide and cupric ion s olutions using gas chromatography/mass spectrometry to determine produ cts and routes of oxidation using different oxidant sources. We have f ound that hydrogen peroxide leads to the formation of a six-carbon pro duct with a mass increment of 32 (a double oxygen addition) relative t o ascorbic acid, consistent with the oxidation sequence of ascorbic ac id(mass 176) going to dehydroascorbic acid (mass 174) to 2,3-diketogul onic acid (mass 192) to 2,3-diketo-4,5,5,6-tetrahydroxyhexanoic acid ( mass 208). Cupric ion solutions, on the other hand, do not appear to i nduce significant amounts of 2,3-diketo-4,5,5,6-tetrahydroxyhexanoic a cid but rather lead to the formation of a threo-hexa-2,4-dienoic acid lactone (mass 174) as the major six-carbon species. These data suggest that different oxidation stresses lead to solutions containing differ ent ascorbic acid oxidation products. These ascorbic acid-derived spec ies could, in turn, interact differently with other substances in the aqueous environment, including free metal ions and low-density lipopro tein. This may help explain previous reports showing divergent protect ive effects of ascorbic acid and dehydroascorbic acid on low-density l ipoprotein when different oxidation methods are used.