The hypothesis that dogs are more sensitive to PG than humans due to PG-mediated glutathione depletion was also studied. The results show that glutathione levels were induced by PG in human but not in dog cells, suggesting that GSH-mediated detoxification of the reactive quinone of PG may be less effective in dog RPTEC than human RPTEC (Figure ). Recent studies indicate that while dog and human glutathione-s-transferase (GST) show substantial similarity some differences in activity towards some substrates such as isothiocyanates exist with dog GST being less active than human . High concentrations of PG and GA induced a reduction of GSH levels in both dog and human cells likely due to decreased cell numbers at these high concentrations of PG and GA.