Catalyzes the GSH-dependent isomerization of maleylpyruvate to fumarylpyruvate which is subsequently processed by NagK to form pyruvate and fumarate. 3-maleylpyruvate = 3-fumarylpyruvate Aromatic compound metabolism; naphthalene degradation. Homodimer. Belongs to the GST superfamily. Zeta family.