title: Kinetics of 25-hydroperoxycholesterol formation during photo-oxidation of crystalline cholesterol.
abstract: BACKGROUND
  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 photo-oxidation of cholesterol for 28 days under fluorescent light. 25-OOH was estimated by an indirect method, using thin-layer chromatography coupled with gas chromatography-mass spectrometry.
  
  
  RESULTS
  Peroxide value (POV) and cholesterol oxidation products (COPs) 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; in particular, after photo-oxidation, 25-OOH (0.55 mg g(-1) ) and 25-OH (0.08 mg g(-1) ) were found in cholesterol, as well as seven other oxysterols, including 7-hydroxy and 5,6-epoxy derivatives. The application of kinetic models to the data showed good correlation with theoretical values, allowing derivation of the kinetic parameters for each oxidation route.
  
  
  CONCLUSIONS
  The results of this work confirm that cholesterol in the crystalline state involves different oxidation patterns as compared to cholesterol in solution. Moreover, the numerical fit proved that hydroperoxidation is the rate-limiting step in 25-OH formation.
authors.affiliation: Department of Biological Systems Engineering, Washington State University, Pullman, WA, 99164-6120, USA.
authors.forename: Ilce Gabriela
authors.initials: IG
authors.lastname: Medina-Meza
authors.forename: Maria Teresa
authors.initials: MT
authors.lastname: Rodriguez-Estrada
authors.forename: Giovanni
authors.initials: G
authors.lastname: Lercker
authors.forename: Carlo
authors.initials: C
authors.lastname: Barnaba
authors.forename: Hugo Sergio
authors.initials: HS
authors.lastname: García
mesh_terms: D002784:Cholesterol / Q000737:chemistry*; D002855:Chromatography, Thin Layer; D003460:Crystallization; D008401:Gas Chromatography-Mass Spectrometry; D006861:Hydrogen Peroxide / Q000737:chemistry; D006888:Hydroxycholesterols / Q000138:chemical synthesis* / Q000737:chemistry; D007700:Kinetics; D008027:Light; D010084:Oxidation-Reduction; D010545:Peroxides / Q000032:analysis; D010777:Photochemistry
chemical_list: D006888:Hydroxycholesterols; D010545:Peroxides; C007997:25-hydroxycholesterol; D002784:Cholesterol; D006861:Hydrogen Peroxide
keywords: GC-MS; kinetic modeling; oxysterols; photo-oxidation; thin-layer chromatography