Involved in the 5-nitroanthranilic acid (5NAA) degradation. Catalyzes the hydrolysis of the lactone to produce maleylpyruvate biodegradation of 5-nitroanthranilate (Probable). 2-oxo-3-(5-oxofuran-2-ylidene)propanoate + H2O = 3-maleylpyruvate + H(+) Belongs to the dienelactone hydrolase family.