Effect of calcium and magnesium ions on radiation-induced inactivation of plant peroxidases

Radiation-induced inactivation of soybean, horseradish, peanut, and tobacco peroxidases in the presence and in the absence of calcium and magnesium salts involves two steps which differ in the character of the effect of the metal cation. The effect of calcium cations is determined by the total charge of the enzyme molecule under experimental conditions. In general, calcium stabilizes the peroxidases, although in the case of anionic soybean and tobacco peroxidases it destabilizes the enzymes at the first inactivation step, probably due to absorption on their surface and changes in the native conformation. The effect of magnesium ions is determined by the specific features of the enzyme structure, and the data obtained allow us to suggest the existence of additional metal-binding sites in tobacco and peanut peroxidases.