CA2- A POSSIBLE PHYSIOLOGICAL MECHANISM OF CONTROL( ACTIVATION OF WHEAT PEROXIDASE )

Peroxidation of substrates such as ascorbic acid, pyrogallol, or ferul ic acid, as well as indole acetic acid oxidation catalyzed by wheat ge rm peroxidase (WGP)(2) C-2, were found to be activated by Ca2+. This a ctivation is independent of the stabilizing effect of structural Ca2reported for peroxidases, Steady state kinetics of ferulic acid oxidat ion catalyzed by WGP Ca showed an increase in the rate of compound I f ormation and of compound II decomposition in the presence of the ion, evidenced as an increase in rate constants k(1), from 8.9 x 10(5) to 4 .5 x 10(6) M(-1) cm(-1), and k(3), from 4.4 x 10(5) to 1.1 x 10(6) M(- 1) cm(-1). The dissociation constant K-d, for the cyanide derivative o f the enzyme showed a marked decrease from 220 to 34 mu M in the prese nce of Ca2+, thus implying an effect of the ion in the H2O2 binding st ep, In the presence of Ca2+, a conformational change in the protein wa s revealed by tryptophan fluorescence, providing a basis for the activ ation mechanism, Other peroxidases such as horseradish peroxidase and WGP C-3 were not activated by Ca2+, The results suggest the existence of a physiological mechanism of control of peroxidase isozymes activit y mediated by Ca2+. (C) 1996 Academic Press, Inc.