p584 NATURE VOL 364 12 AUGUST 199
SIR --- Amyotrophic lateral sclerosis (ALS) is characterized by degeneration of large motor neuron nuclei in the spinal cord. Because muscle fibres are innervated by only a single motor neuron, each loss is particularly destructive. Inter neurons surrounding motor nuclei produce nitric oxide that helps modulate synaptic plasticity(1) and may indirectly contribute to motor neuron destruction in ALS by Cu,Zn superoxide dismutase (SOD). Eleven unique mutations at nine distinct sites in SOD are associated with the autosomal dominant inheritance of familial ALS (see figure)(2). Because afflicted ALS patients have one normal SOD allele, SOD activity should decrease by at most 50 per cent, which would double the steady-state concentration of superoxide. Superoxide reacts with nitric oxide at a rate of 6.7 x 10**9 M-l s-1, three times faster than with native SOD, to form the powerful oxidant peroxynitrite (ONOO-)(3). Peroxynitrite in turn reacts with SOD at ~10**5 M-l s-l to form a nitronium-like intermediate, which nitrates tyrosine residues(4).
SOD-Cu2+ + -OONO --> SOD-CuO...NO2 SOD-CuO...NO2+ + H-Tyr --> SOD-Cu2+ + HO- + NO2-Tyr
The mutations identified in ALS patients do not directly affect conserved amino acids forming the active site, but could slightly disrupt the active-site pocket to allow greater access of peroxynitrite to the copper (see figure). Chemical modification of several amino acids in the active site allows SOD to react with peroxynitrite but greatly diminishes superoxide scavenging(4). Thus, the SOD mutations may increase both peroxynitrite formation owing to reduced scavenging of superoxide and nitration by peroxynitrite of critical cellular targets.
Nitration of proteins will slowly injure motor neurons as well as other cells. However, motor neurons cannot regenerate. Motor neuron injury will accelerate as the nervous system adapts by rearranging synapses to other motor neurons, subjecting them to additional fluxes of nitric oxide. Peripheral nerve injury and chronic inflammation can induce nitric oxide synthase in spinal cord(5,6). Motor neurons may also possess a target that is unusually susceptible to nitration, such as tyrosine kinases. For example, brain-derived neurotrophic factor rescues spinal motor neurons from cell death in vivo and binds to the receptor tyrosine kinase trkB (ref.7). Altered phosphorylation also affects the abnormal assembly and distribution of neurofilaments observed in ALS.
Because native SOD also catalyses nitration by peroxynitrite, ALS in some patients with normal SOD genes may result from peroxynitrite formation due to mild trauma or inflammation near motor neurons. Therefore, caution should be exercised before attempting to treat ALS, patients with SOD until the mechanisms accounting for the dominant action of the SOD mutants are better understood.
Joseph S. Beckman
Department of Anesthesiology,
Mike Carson
Craig D. Smith
Center for Macromolecular Crystallography,
University of Alabama at Birmingham, Birmingham, Alabama 35233, USA
Wlllem H. Koppenol
Department of Chemistry.
Louisiana State University, Baton Rouge, Louisiana 70803, USA
Protein backbone of human Cu,Zn SOD is shown looking down the active site to the copper, based on the human X-ray structure(8). Mutant amino-acid substitutions are shown in red and are primarily localized on the ends of the barrel. Mutations at these diverse sites may push open the loops that form the superoxide-binding pocket and expose more of the copper for reaction with peroxynitrite.