Assay of superoxide dismutase: cautions relevant to the use of cytochrome c, a sulfonated tetrazolium, and cyanide.

Two commonly used assays for superoxide dismutase (SOD) activity have been compared, one using cytochrome c and the other using XTT (2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) as the indicating scavenger of superoxide. The use of cyanide to selectively suppress Cu,Zn-SOD and thus to allow assay of both Cu,Zn-SOD and Mn-SOD in mixtures of the two was also explored, as was the influence of pH. The XTT assay became more sensitive at elevated pH, because the rate of the superoxide/XTT reaction declines with increasing pH. This was clearly seen with the Cu,Zn-SOD but barely with Mn-SOD because the former retains full activity from pH 5 to 10 while the latter does not. Cyanide reacted with cytochrome c, but not XTT, in a concentration- and time-dependent manner and thus diminished its reducibility by superoxide. Cytochromes endogenous to tissue fractions were reduced by the xanthine oxidase reaction and this caused a decrease in absorbance 470 nm which interfered with the XTT assay. The alkalinizing effect of cyanide salts and the problems encountered in neutralizing cyanide stock solutions are discussed. Copyright 2001 Academic Press.