Antioxidant defense enzymes in cell vacuoles of red beet roots

Abstract

The vacuolar fraction isolated from red beet (Beta vulgaris L.) taproots was shown to contain the cyanide-sensitive Cu,Zn-activated superoxide dismutase (SOD; EC 1.15.1.1). The enzyme was represented by three isoforms located in the aqueous phase (in the vacuolar sap) without association to the membrane. Effective operation of SOD in plant cells, especially of its H2O2-sensitive molecular forms, is known to depend on peroxide-utilizing enzymes; this study revealed the existence of phenol-dependent peroxidase (EC 1.11.1.7) in the plant vacuoles. It was shown that the vacuolar peroxidase of red beet roots has a high affinity to benzidines and exhibits optimal activity at low pH (pH range 4–6 depending on substrate species). This peroxidase was represented by numerous molecular forms of acidic and basic nature. The isoenzyme composition of peroxidase in storage roots was highly labile: it depended on the duration of dormant period and comprised from 10 to 17 isoforms. The peroxidase isoforms were located both in the aqueous phase (vacuolar sap) and in the membrane, being weakly associated with the tonoplast. The presence of SOD and peroxidase in the vacuolar sap indicates the existence in vacuoles of an antioxidant defense system that protects vacuolar molecular structures against the impact of superoxide radicals and excessive amounts of H2O2.