Molecular adaptation to photo-oxidative stresses in plants.

点击翻译按钮获取中文摘要

1). Ascorbate peroxidase Two isozymes of ascorbate peroxidase were found in tea leaves. One of them was purified to homogeneity. The enzymatic and molecular properties of this isozyme were characterized in comparison with the other isozyme of ascorbate peroxidase and with guaiacol peroxidases. Ascorbate peroxidases showed differences from guaiacol peroxidase with respect to the specificity of the electron donor; sensitivity to ascorbate removal; sensitivity to thiol reagents; and amino acid composition and amino acid sequence of the amino terminal region. Incubation of ascorbate peroxidase and hydrogen peroxide with p-aminophenol, p-cresol, hydroxyurea, or hydroxyamine results in the inactivation of the enzyme. The radicals of p-aminophenol and hydroxyurea were produced by ascorbate peroxidase as detected by their ESR signals. These signals disappeared upon addition of ascorbate, and the signal characteristic of monodehydroascorbate was found. Thus, ascorbate peroxidase was inactivated by the radicals of p-aminophenol, p-cresol, hydroxyurea and hydroxyamine which are produced by the perocidase reaction, and it is protected from inactivation by ascorbate via the scavenging of the radicals. Thus, these compounds are suicide inhibitors for ascorbatre peroxidase.2). Superoxide dismutase Rice leaves and seed embryo contain four isozymes of CuZn-superoxide dismutase (SOD) and two isozymes of Mn-SOD. CuZn-SOD I is a major enzyme in leaves, but not in embryos or etiolated seedlings. CuZn-SODs II, III and IV were found in the embryos but were also found as minor isozymes in leaves. CuZn-SOD I, II and IV were purified to homogeneity from rice leaves. CuZn-SOD IV was found to be a monomer and the others were dimeric enzymes. CuZn-SOD I which is expressed mainly in leaves is immunologically distinct from the other CuZn-SODs which are mainly expressed in non-photosynthetic tissues.