Estévez et al.(1) focused on a potential mechanism through which dominantly inherited mutation in superoxide dismutase 1 (SOD1), an abundant, ubiquitously expressed antioxidant protein, triggers the selective death of motor neurons in amyotrophic lateral sclerosis (ALS). Each subunit of SOD1 binds one zinc and one copper atom. Dismutation of the superoxide radical to H2O2 or O2 requires enzyme-bound copper, which alternates between reduced (Cu1+) and oxidized (Cu2+) forms during two asymmetric catalysis steps. Evidence from transgenic mice showed that the mutants confer disease independently of the level of SOD1 activity (2–6), findings widely interpreted to indicate that the mutations acquire one or more toxic properties. Several previous proposals have focused on potential sources of toxicity linked to aberrant copper-mediated catalysis (7–9), and all mutants examined do bind the copper in vivo (10).

Estévez et al.(1) proposed that, relative to wild-type SOD1, mutant subunits fail to bind or retain the zinc atom, thereby allowing rapid reduction of mutant SOD1 to the Cu1+ form by abundant intracellular reductants. The reduced SOD1 mutant would then run the normal catalytic step backwards, converting oxygen to superoxide; the superoxide so produced would combine with freely diffusing nitric oxide (NO), producing peroxynitrite, which would promote intracellular damage including protein nitration. The primary evidence supporting this view was that introduction by liposome fusion of purified, zincdepleted SOD1 provoked rapid death of cultured motor neurons. Toxicity required both zinc depletion and bound copper. This evidence, although persuasive in vitro, may have little relevance to the in vivo pathway of motor neuron death. Zinc-deplet-