Comparison of four indirect methods for fluid superoxide dismutase activities

Relatively small sample dilutions could render fluid extracellular (EC) superoxide dismutase (SOD) activity assays more subject to interfering compounds than tissue SOD assays. Highly variable relative SOD activity were obtained when comparing four indirect assays for several fluid samples (human plasma, human synovial fluid, and plasma from healthy or inflamed rats). Analysis of rat plasma fractionated with Sephadex G-150 showed that each assay (three xanthine oxidase based assays plus a modified pyrogallol assay) detected apparent SOD almost entirely at the same molecular weight as rat lung EC SOD. However, unfractionated fluid samples caused interferences with the xanthine oxidase based SOD assays, though not with the pyrogallol method. Examples of interference were stimulation of xanthine oxidase activity, color formation without xanthine oxidase, color formation despite excess Cu-Zn SOD addition, and absorbance changes with cyanide inhibition of EC SOD that were above or below blank values. In summary, relative fluid SOD values depended on the assay used, and a modified pyrogallol assay was not subject to several interferences found for three xanthine oxidase based assays of fluid SOD activity.