The effect of monovalent cations on the two enzymes was similar: the 1 and 5 mM K + and 1 mM Na + exhibited no obviously affecting effects on the activity of AtGUS(-3t)-E and AtGUS-E, while the 5 mM Na + inhibited the enzymatic activity. The divalent cations Mg 2+ and Mn 2+ discretely promoted the activities of AtGUS-E and AtGUS(-3t)-E. With increasing concentration of Co 2+ , AtGUS-E was firstly activated and then inhibited while AtGUS(-3t)-E showed an inverse effect. Ca 2+ and Ni 2+ also exhibited opposite effects on the two enzymes. Ca 2+ at 5 mM final concentration enhanced the activity of AtGUS-E by 107% but inhibited AtGUS(-3t)-E activity by 61%. In the presence of 5 mM Ni 2+ buffer, AtGUS(-3t)-E was increased by 78%, whereas AtGUS-E was decreased by 40%. Cu 2+ distinctively inhibited the enzyme activity, while Al 3+ showed activation at 1 mM concentration and inhabitation at 5 mM concentration to both enzymes. These results reveal that K + , Na + , Mg 2+ , Mn 2+ , Cu 2+ , and Al 3+ exhibited nearly similar effects on the activity of AtGUS-E and AtGUS(-3t)-E; however, Co 2+ , Ca 2+ , and Ni 2+ , showed opposite effects on both enzymes, respectively. The data reported here have been taken from three replicate samples from three independent experiments.