Familial amyotrophic lateral sclerosis (FALS) has been linked to mutations in the homodimeric enzyme Cu/Zn superoxide dismutase 1 (SOD1). Assay by transient expression in primate cells of six FALS mutant enzymes revealed a continuum of enzymatic activity bounded by the enzyme carrying the mutation Gly-85 -Arg, which was inactive, and mutant enzyme G37R carrying the Gly-37 -> Arg change, which retained full specific activity but displayed a 2-fold reduction in polypeptide stability. (3,4,10,11). Initial studies of SOD1 activity in blood from six FALS cases reported 50-65% reductions in enzyme activity as a function of total protein (3,12,13), and it was suggested that FALS-linked mutations affect either enzyme activity or polypeptide stability, causing a nearly complete loss of function from the mutant allele. However, subsequent studies have reported that individuals heterozygous for a His-46 --Arg mutation, which lies in the active-site domain, possess blood enzyme activity levels that are 80%o of normal (6). One problem in interpreting these data is that others have demonstrated 2-to 3-fold variability in the SOD1 levels of circulating erythrocytes in normal individuals (11,12,14), casting doubt on the significance of blood SOD1 levels. In the present study, we examined mutant SOD1 subunits in nonerythroid cells, using lymphoblasts derived from FALS individuals and DNA transfection into primate cells, to determine the intrinsic specific activity and stability of wild-type and mutant Hu-SOD1 polypeptides. MATERIALS AND METHODSCloning and Mutagenesis of Human SOD) Genes. A cDNA copy of the human SOD) gene was obtained by reverse transcription-coupled polymerase chain reaction (PCR) amplification using primers (sense primer, GTCGACAAG-CATGGCCACGAAGGCCGTGTGC; antisense primer, CCGGTCGACAGGGAATGTIIATTGGGCGATCC) that contained terminal Sal I sites and converted sequences surrounding the start codon (underlined) from human GAGT-TATGG to mouse CAAGCAIQG, enhancing the consensus sequence for translation initiation (13). The PCR product was blunt-ligated into the Sma I site of Bluescript II KS(-) and was verified by sequencing. Mutations were engineered by oligonucleotide-directed mutagenesis by using a kit purchased from Amersham. The entire open reading frame of each mutant was confirmed by sequencing.Analysis of SOD1 Enzyme Activity. Wild-type and mutant SOD) genes were cloned into the vector pEF-BOS (15) and transfected transiently into COS-1 cells (16). After 48 h, cells were lysed by a freeze/thaw cycle in water, which was then adjusted to contain 0.125 M Tris chloride (pH 6.8), 20%o (vol/vol) glycerol, 0.025% bromophenol blue, and 0.1% Nonidet P-40. After centrifugation at 10,000 x g for 5 min, the supernatant was separated by electrophoresis on 7.5% polyacrylamide gels, and SOD1 activities were determined as described (17). The intensities of the SOD1 activity signals seen in our assay gels fall within a linear range bounded by 0.0625 ug and 2 pg of purified human erythrocyte SOD1.Calculation of SOD1 En...

Mutations in superoxide dismutase 1 (SOD1) have been linked to familial amyotrophic lateral sclerosis, a dominantly inherited motor neuron disorder of midlife. Because SOD1 is a homodimeric enzyme, dimerization of mutant and wild-type SOD1 subunits could dominantly alter the activity, stability, or localization of wild-type SOD1 subunits. To explore these possibilities, we used transient and stable gene transfection to express high levels of either of two mutant human SOD1 subunits in the presence of limited levels of wild-type mouse and/or human SOD1 subunits. Although both mutant subunits displayed diminished half-lives and free radical scavenging activities, their presence caused no change in the half-life or activity of wild-type SOD1 subunits. Our data indicate that mutant subunits do not dominantly affect the function of wild-type SOD1 subunits. These findings, together with observations that many mutant SOD1 subunits retain significant stability and activity, suggest that motor neuron damage in familial amyotrophic lateral sclerosis is caused by the acquisition of injurious properties by mutant SOD1 subunits.