Involved in the degradation of steroids having an A:B ring fusion in a trans configuration. Catalyzes the elimination of hydrogens located at positions 4 and 5 and the introduction of double bonds into ring A. A + a 3-oxo-5alpha-steroid = a 3-oxo-Delta(4)-steroid + AH2 5alpha-androstan-3,17-dione + A = AH2 + androst-4-ene-3,17-dione 5alpha-androst-1-ene-3,17-dione + A = AH2 + androsta-1,4-diene-3,17-dione Inhibition occurs with substrate concentrations above 25 uM. Optimum pH is 8. Optimum temperature is 40 degrees Celsius. Belongs to the FAD-dependent oxidoreductase 2 family.