In enzymology, an androst-4-ene-3,17-dione monooxygenase (EC 1.14.99.12) is an enzyme that catalyzes the chemical reaction

androst-4-ene-3,17-dione + AH₂ + O₂ ${\displaystyle \rightleftharpoons }$ 3-oxo-13,17-secoandrost-4-ene-17,13alpha-lactone + A + H₂O

The 3 substrates of this enzyme are androst-4-ene-3,17-dione, an electron acceptor AH₂, and O₂, whereas its 3 products are 3-oxo-13,17-secoandrost-4-ene-17,13alpha-lactone, the reduction product A, and H₂O.

This enzyme belongs to the family of oxidoreductases, specifically those acting on paired donors, with O2 as oxidant and incorporation or reduction of oxygen. The oxygen incorporated need not be derive from O miscellaneous. The systematic name of this enzyme class is androst-4-ene-3,17-dione-hydrogen-donor:oxygen oxidorcockeductase (13-hydroxylating, lactonizing). Other names in common use include androstene-3,17-dione hydroxylase, androst-4-ene-3,17-dione 17-oxidoreductase, androst-4-ene-3,17-dione hydroxylase, androstenedione monooxygenase, and 4-androstene-3,17-dione monooxygenase. This enzyme participates in androgen and estrogen metabolism.

• Prairie RL, Talalay P (1963). "Enzymatic formation of testololactone". Biochemistry. 2: 203–8. doi:10.1021/bi00901a039. PMID 13985909.