A Streptococcus mutans superoxide dismutase that is active with either manganese or iron as a cofactor

The superoxide dismutase produced by Streptococcus mutans OMZ176 during aerobic growth in a chemically defined medium (modified FMC) that was treated with Chelex 100 (to lower trace metal contamination) and supplemented with high purity manganese was purified (162-fold) by heat treatment, ammonium sulfate precipitation, and chromatofocusing chromatography. The superoxide dismutase produced during aerobic growth in the same medium, but without manganese and supplemented with high purity iron, was similarly purified (220-fold). The molecular masses of each holoenzyme were approximately 43,000 with a subunit mass of 20,700, indicating that the enzymes were dimers of two equally sized subunits. The superoxide dismutase from manganese-grown cells was a manganese enzyme (MnSOD) containing 1.2 atoms of manganese and 0.25 atoms of iron/subunit. The superoxide dismutase from iron-grown cells was an iron enzyme (FeSOD) containing 0.07 atoms of manganese and 0.78 atoms of iron/subunit. The amino acid compositions of the MnSOD and the FeSOD were virtually identical, and their amino-terminal sequences were identical through the first 22 amino acids. Dialysis of the FeSOD with o-phenanthroline and sodium ascorbate generated aposuperoxide dismutase with 94% loss of activity; subsequent dialysis of apoenzyme with either manganese sulfate or ferrous sulfate reconstituted activity (recoveries of 37 and 30%, respectively). Electrophoretic determination of cytoplasmic radioiron distribution indicated that (during aerobic growth) manganese prevented insertion of iron into superoxide dismutase, although the iron levels of at least two other cytoplasmic fractions were not altered by manganese. Therefore, S. mutans used the same aposuperoxide dismutase to form either FeSOD or MnSOD, depending upon which metal was available in the culture medium. Such "cambialistic" enzymes (those capable of making a cofactor substitution) may represent a previously unrecognized family of superoxide dismutases.