Catalyzes the oxidation and isomerization of cholesterol to cholestenone (4-cholesten-3-one), which is an initial step in the cholesterol degradation process. cholesterol + O2 = cholest-5-en-3-one + H2O2 cholest-5-en-3-one = cholest-4-en-3-one Steroid metabolism; cholesterol degradation. Predicted to be exported by the Tat system. The position of the signal peptide cleavage has been experimentally proven. Belongs to the GMC oxidoreductase family.