SIMILARITIES IN THE METABOLISM OF ALLOXAN AND DEHYDROASCORBATE IN HUMAN ERYTHROCYTES

The beta-cell toxin alloxan is reduced within cells to dialuric acid, which may then decompose to release damaging reactive oxygen species. We tested whether such redox cycling of alloxan occurs in the human er ythrocyte, a cell with stronger antioxidant defenses than beta-cells. Erythrocytes incubated with increasing concentrations of alloxan progr essively accumulated dialuric acid, as measured directly by HPLC with electrochemical detection. At concentrations up to 2 mM, alloxan decre ased cellular GSH slightly, but did not affect erythrocyte contents of ascorbate or alpha-tocopherol. Intracellular H2O2 generation, measure d as inhibition of endogenous catalase activity in the presence of 3-a mino-1,2,4-triazole (aminotriazole), was decreased by alloxan. Despite its failure to induce significant oxidant stress in erythrocytes, 2 m M of alloxan doubled the activity of the hexose monophosphate pathway (HMP). This likely reflected consumption of reducing equivalents durin g reduction of alloxan to dialuric acid. Alloxan pretreatment enhanced the ability of erythrocytes to reduce extracellular ferricyanide whil e protecting alpha-tocopherol in the cell membrane from oxidation by f erricyanide. Ninhydrin, a hydrophobic derivative of alloxan, shelved s imilar effects, but caused progressive GSH depletion and cell lysis at concentrations above 50 mu M. The ability of alloxan to enhance ferri cyanide reduction and to spare alpha-tocopherol suggests that dialuric acid or other reducing species within the cells can protect or recycl e alpha-tocopherol and donate electrons to a transmembrane transfer pr ocess. This behavior resembles that observed for the dehydroascorbate (DHA)/ascorbate pair, and leads to the unexpected conclusion that allo xan increases the reducing capacity of the erythrocyte. (C) 1998 Elsev ier Science Inc.