Purification and properties of NADPH:flavin oxidoreductase from Entamoeba histolytica.

Amebal NADPH:flavin oxidoreductase was purified to apparent homogeneity. Molecular weights of 40 000 and 38 000 were estimated by gel filtration and by sodium dodecyl sulfate polyacrylamide gel electrophoresis, respectively, indicating that the enzyme is composed of a single polypeptide chain. The enzyme does not contain firmly bound flavin. It exhibited 20-fold selectivity for NADPH over NADH. With the former donor it reduced riboflavin, galactoflavin, FMN, or FAD. Aerobically the reducing equivalents were passed from reduced flavin to oxygen to form hydrogen peroxide. Intact amebae do not produce peroxide when they respire. If the title enzyme functions to reduce flavin in the intact cells some electron carrier must intervene between reduced flavin and oxygen so that the final step produces water instead of peroxide.