Potent anti-tumor effects of an active site mutant of human manganese-superoxide dismutase. Evolutionary conservation of product inhibition.

Mn-SOD serves as the primary cellular defense against oxidative damage by converting superoxide radicals (O(2)(-)) to O(2) and H(2)O(2). A unique characteristic of this mitochondrial anti-oxidant enzyme is the conservation from bacteria to man of a rapidly formed product inhibited state. Using site-directed mutagenesis, we have generated an active site mutant (H30N) of human Mn-SOD, which exhibits significantly reduced product inhibition and increased enzymatic efficiency. Overexpression of the H30N enzyme causes anti-proliferative effects in vitro and anti-tumor effects in vivo. Our results provide a teleological basis for the phylogenetically invariant nature of position His-30 and the evolutionary conservation of product inhibition. These data also provide more direct intracellular evidence for the signaling role associated with H(2)O(2).