Summary

Superoxide radicals are involved in many physiological and pathophysiological processes. They are produced as a by-product of respiratory electron transport and cytochome P450 reactions. Activated neutrophils and macrophages can also produce a large amount during oxidative burst. Superoxide radical can react with NO at very fast rate to form peroxynitrite (ONOO-) a very powerful oxidant that has been shown to damage DNA, protein and other biological molecules. Removal of superoxide is a necessary step in cellular defense against these damages.

The enzyme superoxide dismutase (SOD) decomposes superoxide anion into hydrogen peroxide and oxygen at a high reaction rate. Since its initial discovery by McCord and Fridovich in 1969, SOD has been found to be ubiquitous in every aerobic organism from microbes to human. Four types of SOD have been identified on the basis of their metal cofactors and distribution. The copper zinc form (Cu/ZnSOD) is most common with a primary distribution in the cytoplasm of eukaryotic cells. Another copper zinc form, extra-cellular (ecSOD), has more recently been identified. This SOD is often associated with vascular tissues but can also be found in plasma, lymph, synovial fluid, cerebrospinal fluid (CSF) and elsewhere. The manganese form (MnSOD) is generally associated with the mitochondria of aerobic organisms. The iron form (FeSOD) is found predominantly in prokaryotes. The relationship between SOD and various human diseases has now been well documented and measurement of SOD activity has become commonplace in many research model systems. One unit SOD activity is defined as the amount of enzyme that will inhibit the rate of cytochrome c reduction by half under specific conditions.

The NWLSS™ SOD Activity assay method is based on monitoring the auto-oxidation rate of hematoxylin as originally described by Martin J. P., Jr et al 1987, with modifications to increase robustness and reliability. In the presence of SOD enzyme, the rate of auto oxidation is inhibited and the percentage of inhibition is linearly proportional to the amount of SOD present within a specific range. Sample SOD activity is determined by measuring ratios of auto-oxidation rates in the presence and absence of the sample and expressed as traditional McCord Fridovich "cytochrome c" units, The basic principal of the assay is shown schematically by the following equation:

Since the auto-oxidation rate for hematoxylin is not affected by cyanide, a unique feature of our assay is its ability to determine Cu/Zn SOD and MnSOD activity distinctively in a single sample, by simply adding potassium cyanide solution to inhibit Cu/ZnSOD activity but not MnSOD activity.