Inactivation of chloroperoxidase (CPO) from Caldariomyces fumago by arginine was investigated. It was found that the red native CPO solution was turned into a stable green species with a concomitant shift of the Soret band from 398 to 425 nm in the presence of arginine. The green CPO lost almost all of its catalytic activity, and this inactivation was irreversible. Differential UV-vis spectrophotometry was used to examine the binding properties of arginine to CPO. The formation of CPO-arginine (1: 1) complex was highly pH-dependent. Fluorescence investigation revealed the exposure degree of prosthetic group increased. Kinetic analysis indicated that CPO has both a high affinity and specificity to arginine. This inactivation may be caused mainly by the binding of guanidinium group in arginine to the acid-base catalytic group Glu183 in CPO. The change of surrounding environment around heme induced by the interaction of heme propionates with arginine and the occupying of the sixth axial ligand position of heme iron by hydroxyl are also reasons bringing on this inactivation. (C) 2009 Elsevier Ltd. All rights reserved.