Purification and characterization of a glutathione peroxidase from the Aloe vera plant

Extracts from the parenchymous leaf-gel of the Aloe vera plant (Aloe barbadensis Miller) were shown to contain glutathione peroxidase (GSHPx) activity. The activity was purified to homogeneity by ion exchange and gel filtration (FPLC) chromatography in the presence of 0.5 mM glutathione. The native enzyme has an apparent molecular weight of 62 kD as determined by gel filtration. In the presence of sodium dodecylsulfate (SDS), the molecular weight was estimated to be about 16 kD as determined by polyacrylamide-gel electrophoresis (SDS-PAGE). The native enzyme is proposed to be constituted of four identical subunits; it also contains one atom of selenium per subunit, as found with most glutathione peroxidases from animal sources. The Km values were determined to be 3.2 mM for glutathione and 0.26 mM for the hydroperoxide substrate, cumene hydroperoxide. The enzyme is competitively inhibited by N, S, bis-FMOC glutathione (Ki = 0.32 mM), a potent inhibitor of glyoxalase II. Inhibitors of glyoxalase I (e.g. S-octylglutathione) have no effect on the peroxidase activity.