Decreased Zinc Affinity of Amyotrophic Lateral Sclerosis‐Associated Superoxide Dismutase Mutants Leads to Enhanced Catalysis of Tyrosine Nitration by Peroxynitrite

TLDR

Mutations in Cu/Zn superoxide dismutase linked to familial ALS trigger an unknown toxic reaction that selectively kills motor neurons, and protein‑bound tyrosine nitration is a permanent modification that can impair protein function. The study investigates how more than 50 missense mutations in SOD generate a shared toxic phenotype. Neurofilament‑L, abundant in motor neurons, binds multiple zinc atoms with high affinity, acting as a zinc sink that can strip zinc from both wild‑type and mutant SOD while sparing copper, thereby promoting zinc‑deficient SOD. The mutants exhibit up to 30‑fold lower zinc affinity yet retain copper binding, and zinc loss from wild‑type SOD roughly doubles its peroxynitrite‑mediated tyrosine‑nitration activity, implying that ALS‑associated SOD toxicity may stem from enhanced protein nitration following zinc depletion.

Abstract

Abstract: Mutations to Cu/Zn superoxide dismutase (SOD) linked to familial amyotrophic lateral sclerosis (ALS) enhance an unknown toxic reaction that leads to the selective degeneration of motor neurons. However, the question of how >50 different missense mutations produce a common toxic phenotype remains perplexing. We found that the zinc affinity of four ALS‐associated SOD mutants was decreased up to 30‐fold compared to wild‐type SOD but that both mutants and wild‐type SOD retained copper with similar affinity. Neurofilament‐L (NF‐L), one of the most abundant proteins in motor neurons, bound multiple zinc atoms with sufficient affinity to potentially remove zinc from both wild‐type and mutant SOD while having a lower affinity for copper. The loss of zinc from wild‐type SOD approximately doubled its efficiency for catalyzing peroxynitrite‐mediated tyrosine nitration, suggesting that one gained function by SOD in ALS may be an indirect consequence of zinc loss. Nitration of protein‐bound tyrosines is a permanent modification that can adversely affect protein function. Thus, the toxicity of ALS‐associated SOD mutants may be related to enhanced catalysis of protein nitration subsequent to zinc loss. By acting as a high‐capacity zinc sink, NF‐L could foster the formation of zinc‐deficient SOD within motor neurons.