Labile sulfur in human Superoxide dismutase.

1. The nautre of the intense absorption band at 320 nm of the copper and zinc-containing enzyme superoxide dismutase, from human red blood cells, has been investigated. The band does not depend on the metal prosthetic groups of the enzyme, as it is still present in the apo protein. When, however, copper alone is removed from the enzyme with a treatment involving the use of cyanide, the band is also lost. Nevertheless the copper-free protein is able to recover both the enzyme activity and the native electron paramagnetic resonance spectrum as easily as the apo protein. 2. A number of other treatments are able to abolish the band. They include reaction with reducing agents such as dithiothreitol, sulfite, borohydride, exposure to denaturants such as guanidine HCl and sodium dodecyl sulfate, and exposure to pH values below pH 3 or above pH 13. 3. Four sulfur atoms per protein molecule were found to be associated to the 320-nm chromophore on the basis of quantitative determinations following reaction with cyanide or sodium borohydride. 4. A molar absorption coefficient of 1150 M-1 cm-1 was determined per each chromophoric group. In spite of this relatively high value and unusual stability, a persulfide group, R-S-SH, seems to be the most likely structure for this chromophore. 5. Bovine and equine superoxide dismutase do not show spectral or chemical evidence for such a group. This, and the recovery of activity and spectral properties of copper in the cyanide-treated human enzyme, indicate that labile sulfur is not associated with the superoxide dismutase activity of this protein.