Oxidative sensitivity of alternative functions of the glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase

The glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), is susceptible to inflammatory oxidants such as hypochlorite (OCl") and hydrogen peroxide (H2O2). Evidence [1] suggests that the monomeric form of GAPDH exhibits uracil-DNA glycosylase (UDG) activity. As oxidative damage has often been implicated as a possible cause of carcinogenesis, the sensitivity of UDG to inflammatory oxidants was investigated. An assay, modified from Sirover (1979) [2], was developed which successfully quantitated the UDG activity of both bacterial UDG and UDG extracted from a human cell line, HCT 116. UDG activity in these cells was proportional to cell number, with an upper limit of 2.5 x 10^ cells, possessing 3033U UDG/mg protein. Aliquots containing 2.5 x 10^ cells were exposed to the inflammatory oxidants, OCl" and H2O2. Exposure of HCT 116 cells to OCl" up to a concentration of 5 x lO'^M resulted in no change in UDG activity released from these cells. However, exposure to OCl" at concentrations higher than 5 x lO'^M resulted in complete loss of cellular UDG activity. In contrast, UDG activity was not inhibited at all in HCT 116 cells exposed to H2O2 up to a concentration of lO'^M. Bacterial UDG, however, was not inhibited by either OCl' up to a concentration of lO'^M, or H2O2 up to a concentration of lO'^M. As bacterial UDG displays different properties to human UDG, it may not be used as a model to investigate the mode of inactivation of human UDG by OCl".