Activities of antioxidant enzymes in mitochondria of growing and dormant sugar beet roots

In mitochondria isolated from growing (70-85 days) and dormant (stored for 8-12 weeks) sugar beet (Beta vulgaris L.) roots, activities of superoxide dismutase (SOD) and enzymes of the ascorbate-glutathione cycle were determined. The activity of SOD, the enzyme involved in superoxide detoxification, was much higher in mitochondria of the growing root, whereas activities of ascorbate peroxidase (APO) and glutathione reductase (GR), key enzymes of the ascorbate-glutathione cycle involved in the hydrogen peroxide degradation, increased substantially in mitochondria of dormant storage roots. Catalase (CAT) activity was detected in the fraction of root mitochondria purified in the sucrose density gradient, which activity was inhibited by cyanide by 85-90% and much weaker, by aminotriazol (by 30-35%). Submitochondrial localization of APO and CAT was analyzed using proteinase K. It was established that a substrate-binding APO center is localized on the external side of the inner membrane, whereas CAT is localized in the mitochondrial matrix. A possible role of mitochondria as ROS (hydrogen peroxide) acceptors in the cells of storage parenchyma of the stored root is discussed.