In enzymology, a 3,4-dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione 4,5-dioxygenase (EC 1.13.11.25) is an enzyme that catalyzes the chemical reaction

3,4-dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione + O₂ ${\displaystyle \rightleftharpoons }$ 3-hydroxy-5,9,17-trioxo-4,5:9,10-disecoandrosta-1(10),2-dien-4-oate

Thus, the two substrates of this enzyme are 3,4-dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione and O₂, whereas its product is 3-hydroxy-5,9,17-trioxo-4,5:9,10-disecoandrosta-1(10),2-dien-4-oate.

This enzyme belongs to the family of oxidoreductases, specifically those acting on single donors with O₂ as oxidant and incorporation of two atoms of oxygen into the substrate (oxygenases). The oxygen incorporated need not be derived from O₂. 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.