title: Fat transforms ascorbic acid from inhibiting to promoting acid-catalysed N-nitrosation.
abstract: BACKGROUND
  The major potential site of acid nitrosation is the proximal stomach, an anatomical site prone to a rising incidence of metaplasia and adenocarcinoma. Nitrite, a pre-carcinogen present in saliva, can be converted to nitrosating species and N-nitroso compounds by acidification at low gastric pH in the presence of thiocyanate.
  
  
  OBJECTIVE
  To assess the effect of lipid and ascorbic acid on the nitrosative chemistry under conditions simulating the human proximal stomach.
  
  
  METHODS
  The nitrosative chemistry was modelled in vitro by measuring the nitrosation of four secondary amines under conditions simulating the proximal stomach. The N-nitrosamines formed were measured by gas chromatography-ion-trap tandem mass spectrometry, while nitric oxide and oxygen levels were measured amperometrically.
  
  
  RESULTS
  In absence of lipid, nitrosative stress was inhibited by ascorbic acid through conversion of nitrosating species to nitric oxide. Addition of ascorbic acid reduced the amount of N-nitrosodimethylamine formed by fivefold, N-nitrosomorpholine by >1000-fold, and totally prevented the formation of N-nitrosodiethylamine and N-nitrosopiperidine. In contrast, when 10% lipid was present, ascorbic acid increased the amount of N-nitrosodimethylamine, N-nitrosodiethylamine and N-nitrosopiperidine formed by approximately 8-, 60- and 140-fold, respectively, compared with absence of ascorbic acid.
  
  
  CONCLUSIONS
  The presence of lipid converts ascorbic acid from inhibiting to promoting acid nitrosation. This may be explained by nitric oxide, formed by ascorbic acid in the aqueous phase, being able to regenerate nitrosating species by reacting with oxygen in the lipid phase.
authors.affiliation: Division of Cardiovascular and Medical Sciences, Western Infirmary, University of Glasgow, Glasgow, Scotland.
authors.forename: E
authors.initials: E
authors.lastname: Combet
authors.forename: S
authors.initials: S
authors.lastname: Paterson
authors.forename: K
authors.initials: K
authors.lastname: Iijima
authors.forename: J
authors.initials: J
authors.lastname: Winter
authors.forename: W
authors.initials: W
authors.lastname: Mullen
authors.forename: A
authors.initials: A
authors.lastname: Crozier
authors.forename: T
authors.initials: T
authors.lastname: Preston
authors.forename: K E L
authors.initials: KE
authors.lastname: McColl
mesh_terms: D001205:Ascorbic Acid / Q000494:pharmacology*; D002384:Catalysis / Q000187:drug effects; D004943:Esophagogastric Junction / Q000378:metabolism*; D006801:Humans; D006851:Hydrochloric Acid / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008055:Lipids / Q000494:pharmacology*; D008954:Models, Biological; D009569:Nitric Oxide / Q000378:metabolism; D009602:Nitrosamines / Q000378:metabolism*; D015538:Nitrosation / Q000187:drug effects; D010100:Oxygen / Q000378:metabolism