Purification and biochemical characterization of a novel copper, zinc superoxide dismutase from liver of camel (Camelus dromedarius): An antioxidant enzyme with unique properties

Abstract

A novel copper, zinc superoxide dismutase (CuZnSOD) was purified to homogeneity from the liver of an animal well adapted to the stressful living conditions of the desert, the camel (Camelus dromedarius). The biochemical properties of camel liver CuZnSOD were examined. The purified enzyme had a native molecular weight of 28 kDa, as judged by gel filtration chromatography, and showed a single band at 27 kDa on SDS-PAGE, indicating that it is a monomeric protein. Optimal activity of the purified enzyme occurred at 43 °C and pH 6.0, and the activation energy was 1.42 kJ/mol. CuZnSOD activity was strongly inhibited by β-ME, DTT, H₂O₂ and SDS and slightly inhibited by EDTA, NaN₃ and PMSF. Al³⁺, Ca²⁺, Cd²⁺, Mg²⁺ and Zn²⁺ stimulated CuZnSOD activity, whereas Ba²⁺, Co²⁺, Fe²⁺ and Ni²⁺ inhibited it. The purified enzyme contained 0.010 µg of Cu and 0.69 µg of Zn per mg of protein. K_m, V_max, k_cat and k_cat/K_m values for NBT and riboflavin were 16.27 and 0.16 µM, 20.85 and 21.54 U/mg, 9.65 and 9.97 s^-1, and 0.59 and 62.33 s^-1 µM^-1, respectively. Camel liver CuZnSOD exhibited unique biochemical properties compared to those of other CuZnSODs, including lower molecular weight with a monomeric structure, higher optimum temperature, very low E_a, very low optimum pH, very low contents of Cu and Zn, and higher affinity, turnover number and catalytic efficiency for riboflavin. These unique properties of camel liver CuZnSOD might be related to the ability of this animal to inhabit stressful desert conditions.

Keywords: Biochemical characterization; Camel; Camelus dromedarius; Copper, zinc superoxide dismutase; Purification; Stressful desert conditions; Unique properties.