In enzymology, a 3,4-dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione 4,5-dioxygenase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are 3,4-dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione and O<sub>2</sub>, whereas its product is .
This enzyme belongs to the family of oxidoreductases, specifically those acting on single donors with O<sub>2</sub> as oxidant and incorporation of two atoms of oxygen into the substrate (oxygenases). The oxygen incorporated need not be derived from O<sub>2</sub>. The systematic name of this enzyme class is 3,4-dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione:oxygen 4,5-oxidoreductase (decyclizing). Other names in common use include steroid 4,5-dioxygenase, and 3-alkylcatechol 2,3-dioxygenase. It employs one cofactor, iron.