Involved in the degradation of gentisate. Catalyzes the conversion of gentisate (2,5-dihydroxybenzoate) to maleylpyruvate. Can also use other substrates such as 3-bromogentisate, 3-fluorogentisate, 3-isopropylgentisate, 3-methylgentisate and 3-ethylgentisate, but is unable to utilize gentisate that is substituted at the carbon-4 position. 2,5-dihydroxybenzoate + O2 = 3-maleylpyruvate + H(+) Partially inhibited by Na(+), Ca(2+) or Mn(2+) at concentrations of 5 mM. Presence of 5 mM of Zn(2+), Hg(2+) or EDTA results in more than 95% inhibition. Optimum pH is 7.0-8.0. Optimum temperature is 25 degrees Celsius. Homotetramer. Strictly inducible. Induced by 3-hydroxybenzoate and gentisate. Belongs to the gentisate 1,2-dioxygenase family.