Catalyzes 2 different reactions between oxygen and the acireductone 1,2-dihydroxy-3-keto-5-methylthiopentene (DHK-MTPene) depending upon the metal bound in the active site. Fe-containing acireductone dioxygenase (Fe-ARD) produces formate and 2-keto-4-methylthiobutyrate (KMTB), the alpha-ketoacid precursor of methionine in the methionine recycle pathway. Ni-containing acireductone dioxygenase (Ni-ARD) produces methylthiopropionate, carbon monoxide and formate, and does not lie on the methionine recycle pathway. 1,2-dihydroxy-5-(methylsulfanyl)pent-1-en-3-one + O2 = 4-methylsulfanyl-2-oxobutanoate + formate + 2 H(+) 1,2-dihydroxy-5-(methylsulfanyl)pent-1-en-3-one + O2 = 3-(methylsulfanyl)propanoate + CO + formate + 2 H(+) Binds either 1 Fe or Ni cation per monomer. Iron-binding promotes an acireductone dioxygenase reaction producing 2-keto-4-methylthiobutyrate, while nickel-binding promotes an acireductone dioxygenase reaction producing 3-(methylsulfanyl)propanoate. Amino-acid biosynthesis; L-methionine biosynthesis via salvage pathway; L-methionine from S-methyl-5-thio-alpha-D-ribose 1-phosphate: step 5/6. Belongs to the acireductone dioxygenase (ARD) family.