In 1993, it was first reported that a mutation to the gene coding for Cu,Zn superoxide dismutase (SOD1) was associated with a form of familial amyotrophic lateral sclerosis (ALS). Since that time, more than 70 single amino acid mutations to SOD1 have been found to cause ALS in humans. Mice transgenic for any one of several of the human SOD1 mutants develop progressive and ultimately lethal paralysis in a time-dependent manner reminiscent of human ALS. Studies in transgenic mice clearly indicate that SOD1 is toxic via a gained function because the mutant produces disease even in the presence of marked increases in total SOD1 enzyme activity. Despite the overwhelming evidence for a gained toxic function, the exact nature of that function has remained elusive. Equally puzzling has been the fact that SOD1 mutants are toxic only to motor neurons even though they are expressed in all cell types. Based on published results, in vitro characterizations of SOD1 mutants, studies in cultured motor neurons, and preliminary data from transgenic mice, we have formulated a hypothesis which may explain how all ALS-associated SOD1 mutations can be toxic via a common mechanism and why toxicity is manifested only in motor neurons. HYPOTHESIS: We are proposing that zinc-deficient (copper-containing) SOD1 is the common toxic phenotype of ALS-associated SOD1 mutants, that zinc-deficient SOD1 is injurious via its ability to utilize ascorbate, oxygen, and nitric oxide to catalyze the formation of the cytotoxin peroxynitrite, and that neurofilament proteins--which avidly bind zinc and are very abundant in motor neurons--contribute to the formation of zinc-deficient SOD1 preferentially in motor neurons. This study proposes: Specific Aim 1) to measure zinc-deficient SOD1 in the most widely used animal model of ALS (G93A transgenic mice) and determine the factors responsible for its accumulation, Specific Aim 2) to determine the conditions which lead to SOD1-mediated oxidant generation and its potential relationship to toxic protein aggregation, and Specific Aim 3) to evaluate the in vivo efficacy of two classes of compounds which protect cultured motor neurons from the toxic effects of zinc-deficient SOD1 and which enhance survival in G93A mice.