Streptomycin: irreversible association with superoxide dismutases.

Streptomycin modified the electrophoretic mobility, but not the catalytic activity, of the superoxide dismutases of Streptococcus faecalis and of Escherichia coli. This effect, which was seen with crude extracts or with the purified enzymes, was due to a change in charge rather than to significant change in size. Reaction between enzymes and streptomycin, exhibited great specificity. Thus: 1. Streptomycin gave no indication of reaction with the copper-zinc superoxide dismutase of bovine erythrocytes or with the manganese enzyme from human liver. 2. It reacted with some of the bacterial superoxide dismutases examined, but not with others. 3. Brief treatment with urea, at 8.0 m, did not interfere either with the catalytic activity or the streptomycin-reactivity of the S. faecalis enzyme. 4. Removal of manganese gave an apoenzyme, whose electrophoretic mobility was identical with that of the holoenzyme. This apoenzyme did not react with streptomycin, and reincorporation of the manganese restored its ability to react. 5. A variety of proteins and enzymes, chosen at random, did not react with streptomycin. When [¹⁴C]streptomycin was used, 3.2 molecules per molecule enzyme modified were found to resist removal by exhaustive dialysis. The corresponding number after treatment with 6.0 m guanidinium chloride was 1.2 and after repeated precipitation with trichloroacetic acid was 1.0. Dihydrostreptomycin, bearing a hydroxymethyl group in place of a formyl group, was unreactive. Nevertheless, acetaldehyde (10 mm) did not interfere with the binding of streptomycin. The S. faecalis enzyme was homogeneous by the criterion of polyacrylamide gel electrophoresis, yet appeared heterogeneous in its reaction with streptomycin. Thus, 40% of the enzyme reacted rapidly, whereas the remainder reacted much more slowly.