Purification of a cyanide-sensitive superoxide dismutase from soya beans: A food-compatible enzyme preparation

A method of purifying superoxide dismutase (SOD; EC 1.15.1.1), free from lipoxygenase (EC 1.13.11.12) and catalase (EC 1.11.1.6), from ethanol‐treated soya beans is described, Preparations with the highest activity were obtained from new season's beans. Although not homogeneous by electrophoretic analysis, the preparation was 300‐fold purified. The enzyme has a molecular weight of 36 300 and a subunit molecular weight of 18 000. Its sensitivity to cyanide suggests that it is a typical cupro‐zinc SOD. Isoelectric focusing showed the presence of three separate charged species with isoelectric points of pH 4·73 (the major species), 4·55 (the minor species), and 4·37. The enzyme withstands 45 min at 65°C without loss of activity, and loses only half its activity in 30 min at 75°C at pH values between 5 and 8. Severe losses in activity at pH 4 and below are observed at 75°C. The enzyme can be stabilised for storage at 4°C in 60% (v/v) glycerol.