Photolysis of Cholesterol during Biological Experiments.

1. When dilute aqueous emulsions of radioactive cholesterol are exposed to daylight, extensive photolysis may take place. This results chiefly in the formation of substances more polar than cholesterol, some of which are probably acidic. Substances less polar than cholesterol are formed to a smaller extent. Photolysis is greatest when the emulsions are strongly acidic or strongly alkaline. 2. Photolysis takes place rapidly if radioactive cholesterol stored in the dry state, either on glass or on filter paper, is exposed to daylight. 3. If precautions are taken to minimize exposure to daylight during analysis and storage of the samples, the amount of non-enzymic alteration of cholesterol in biological experiments may be negligible.