We studied the effects of Ag⁺ on arginine kinase (AK) from Fenneropenaeus chinensis. Ag⁺ inactivated the activity of AK in a dose dependent manner (IC₅₀ = 15 μM). Kinetic studies showed that the inactivation of AK by Ag⁺ was reversible and occurred in a noncompetitive inhibition manner (K_i = 2.8 μM). Spectroflurorimetry results showed that Ag⁺ did not induce conspicuous tertiary structural changes in AK at the corresponding concentration ranges of inactivation studies. However, the secondary structure measured by circular dichroism was slightly changed by Ag⁺. Taken together, these data suggest that the active site of AK is flexible, with the complete loss of activity occurring prior to significant changes in overall structures. Our study provides important insight into the inhibitory mechanism of Ag⁺ on AK and increases our understanding of the influence of Ag+ on the mechanism of this metabolic enzyme.

Key words: Arginine kinase; Ag⁺; Inhibition; Kinetics.