Kinetics of 25-hydroperoxy-cholesterol formation during photo-oxidation of crystalline cholesterol

25-hydroxycholesterol (25-OH), a side-chain product of cholesterol oxidation, has emerged as one of the important issues in food chemistry and biochemistry, because of its involvement in several human pathologies. This oxysterol is derived from both enzymatic and non-enzymatic pathways. However, the latter mechanism has been scarcely studied in either food or model systems. In this work, a kinetic model was developed to evaluate the formation of 25-OH and its precursor 25-hydroperoxycholesterol (25-OOH) during cholesterol photo-oxidation. About 1 mg of cholesterol was photo-oxidized by a fluorescent light for 28 days. Peroxide value (POV) and cholesterol oxidation products were determined. POV showed a hyperbolic behavior, typical of a crystalline system in which the availability of cholesterol is the limiting factor. Further reactions of hydroperoxides were followed; 25-OOH was estimated by an indirect method, using thin-layer chromatography (TLC) coupled with gas-chromatography-mass spectrometry (GC-MS). After photo-oxidation, 25-OOH (0.55 mg/g) and 25-OH (0.08 mg/g) were found in cholesterol, as well as seven other oxysterols, including 7-hydroxy and 5,6-epoxy derivatives. Moreover, the results of this work confirm that cholesterol in crystalline state involves different oxidation patterns as compared to cholesterol in solution.