The effect of several metal ions on the activity of AtGUS(-3t)-E and AtGUS-E was investigated. The enzyme activity was determined in the reaction mixture consisting of K + (KCl), Na + (NaCl), Mg 2+ (MgCl 2 ), Mn 2+ (MnCl 2 ), Co 2+ (CoCl 2 ), Ca 2+ (-CaCl 2 ), Ni 2+ (NiSO 4 ), Cu 2+ (CuSO 4 ), and Al 3+ (AlCl 3 ) ions at final concentrations of 1 and 5 mM. The enzyme activity was subsequently determined at the optimum temperature after incubation for 30 min.