Copper-zinc superoxide dismutase (Cu,Zn SOD) has been extracted, purified and characterized from Radix lethospermi seed (RLS), a kind of Chinese traditional medicine. Before extraction, the lipid was removed by super critical fluid extraction (SCF). Partial protein fractionation in the crude extract was affected by using 50-75% (NH(4))(2)SO(2). Subsequently, superoxide dismutase was fractionated by column chromatographies on DEAE-52, Sephadex G-200 and DEAE-52 again. Pure Cu,Zn SOD had a specific activity of 4843 U/mg protein and was purified 267.2-fold, with a yield of 23.55%. The purified enzyme has a molecular weight of about 30,500+/-100 and is composed of two non-covalently joined equal subunits. Purity was confirmed by Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), HPLC and mass spectroscopy. Amino acid content has been investigated. The enzyme was found to remain stable in the pH range 6.0-9.0 at 25 degrees C and up to 45 degrees C at pH 7.8 for a 30 min incubation period. RLS Cu,Zn SOD appeared to have significant thermal stability lower than other Cu,Zn SODs, as revealed by irreversible heat inactivation at 60 degrees C. The enzyme was not inhibited by DTT, NaN(3) and beta-mercaptoethanol, but was inhibited by cyanide and hydrogen peroxide. Finally, in the presence of 2 mM ethylendiamine tetra acetic acid (EDTA) and sodium dodecyl sulphate (SDS), the enzyme showed approximately 18 and 34% activity loss.