Effects of calcium on the thermal stability, stability in organic solvents and resistance to hydrogen peroxide of artichoke (Cynara scolymus L.) peroxidase:: A potential method of enzyme control

The effects of calcium ions (Ca2+) on the stability of artichoke (Cynara scolymus L.) peroxidase (AKPC) have been studied. The thermal stability of AKPC was improved by the addition of Ca2+; the melting temperature increased by 20 degrees C and the deactivation energy by 26 kJ mol(-1). AKPC was stable in a selection of organic solvents but was less active with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) than under aqueous conditions. Ca2+-free AKPC retained more activity in the presence of organic solvents due to its better maintenance of the rate of compound I formation with hydrogen peroxide (H2O2) compared to AKPC-Ca2+. AKPC retained at least 75% activity over 24 h in the pH range 3.0-10.5 and about 50% over 1 month at pH 7.0 or 5.5, irrespective of the Ca2+ content. AKPC-Ca2+ was considerably more resistant to inactivation by H2O2 than Ca2+-free AKPC suggesting that the presence of Ca2+ boosts turnover under oxidizing conditions. AKPC has been applied as an alternative to horseradish peroxidase (HRP) in glucose concentration assays; the presence of Ca2+ or of the Ca2+ chelating agent ethylenediaminetetraacetic acid made no difference to the final result. The possibility is discussed that addition and removal of a labile Ca2+ from AKPC could be used to control enzyme activity both in vivo and in vitro. (c) 2006 Elsevier B.V. All rights reserved.