INACTIVATION OF TEA LEAF HYDROPEROXIDE LYASE BY FATTY-ACID HYDROPEROXIDE

Tea leaf hydroperoxide lyase (HPO lyase) was rapidly and irreversibly inactivated by linoleic acid 13-hydroperoxide [13(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid (LA 13-HPO)]. The rate of the inactivation was directly dependent upon the concentration of hydroperoxide, and its second-order rate constant pH 5.5 and 25-degrees-C was 265 M-1 s-1. Linoleic acid 9-hydroperoxide [9(R)-hydroperoxy-10(E), 12(Z)-octadecadienoic acid], which is little catalyzed by tea leaf HPO lyase, also inactivated the enzyme, but the other organic hydroperoxides such as cumene, tert-butyl hydroperoxide, and H2O2 did not. Inactivation by LA 13-HPO was protected either by dithiothreitol or by LA 13-hydroxide, which suggests that the inactivation is caused by destruction of an essential SH group near the reaction center of the enzyme. Hydrophobic radical scavengers such as alpha-tocopherol and nordihydroguaiaretic acid effectively protected HPO lyase from the inactivation, but mannitol and diethylenetriaminepentaacetic acid showed no effect.