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

Abstract

The effects of calcium ions (Ca 2+) on the stability of artichoke ( Cynara scolymus L.) peroxidase (AKPC) have been studied. The thermal stability of AKPC was improved by the addition of Ca 2+; the melting temperature increased by 20 °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. Ca 2+-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 (H 2O 2) compared to AKPC-Ca 2+. 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 Ca 2+ content. AKPC-Ca 2+ was considerably more resistant to inactivation by H 2O 2 than Ca 2+-free AKPC suggesting that the presence of Ca 2+ boosts turnover under oxidizing conditions. AKPC has been applied as an alternative to horseradish peroxidase (HRP) in glucose concentration assays; the presence of Ca 2+ or of the Ca 2+ chelating agent ethylenediaminetetraacetic acid made no difference to the final result. The possibility is discussed that addition and removal of a labile Ca 2+ from AKPC could be used to control enzyme activity both in vivo and in vitro.