Involved in the utilization of cholesterol as the sole carbon and energy source by degrading the side chain. Primarily catalyzes the sequential oxidation of the terminal methyl of cholest-4-en-3-one into (25R)-26-hydroxycholest-4-en-3-one (alcohol), (25R)-26-oxocholest-4-en-3-one (aldehyde), to finally yield the carboxylic acid (25R)-3-oxocholest-4-en-26-oate. Also able to sequentially oxidize cholesterol itself, not only cholest-4-en-3-one. cholest-4-en-3-one + 5 H(+) + 3 O2 + 6 reduced [2Fe-2S]-[ferredoxin] = (25R)-3-oxocholest-4-en-26-oate + 4 H2O + 6 oxidized [2Fe-2S]-[ferredoxin] Steroid metabolism; cholesterol degradation. By cholesterol. Cells lacking this gene show a slight reduction in the levels of cholest-4-en-3-one-26-oate, but the concentration of 26-hydroxycholest-4-en-3-one is not measurably affected. The levels of 26-hydroxycholest-4-en-3-one and cholest-4-on-3-one-26-oate are drastically reduced in cells lacking both cyp125A3 and cyp142A2. Belongs to the cytochrome P450 family.