title: Determination of free and total cyst(e)ine in plasma of dogs and cats.
abstract: BACKGROUND
  In human blood, the amino acid cysteine forms disulfide bonds with itself and with other sulfhydryl compounds in their free form and with sulfhydryls in protein. Protein-bound cysteine is lost when plasma proteins are removed before amino acid analysis.
  
  
  OBJECTIVE
  The purpose of this study was to assess the time course and extent of cyst(e)ine (cysteine + half-cystine) loss in dog and cat plasma.
  
  
  METHODS
  An equal volume of 6% sulfosalicylic acid was added to plasma aliquots at 0, 2, 4, 10, 16, 24, 36, 48, 60, and 72 hours after separation of blood cells. Tris-2-carboxyethyl-phosphine hydrochloride (TCEP - HCl), a reducing agent, was used to regenerate total plasma cyst(e)ine after 3 months of sample storage (-20 degrees C).
  
  
  RESULTS
  Initial free cyst(e)ine concentrations (mean +/- SEM) were higher in canine plasma (77 +/- 4 micromol/L) than in feline plasma (37 +/- 3 micromol/L). Free plasma cyst(e)ine concentrations in dogs and cats decreased after first-order kinetics, with a half-life of 23 and 69 hours, respectively. Total plasma cysteine after TCEP - HCl treatment was similar for dogs (290 micromol/L) and cats (296 micromol/L), but the percentage of free cysteine was higher (P = .02) in dogs (27%) than in cats (13%). Over half of the cyst(e)ine, homocysteine, cysteinylglycine, and glutathione were bound in vivo to plasma proteins.
  
  
  CONCLUSIONS
  These results emphasize the importance of removing plasma proteins within 1 hour after blood collection for reliable assay of free plasma cyst(e)ine.
authors.affiliation: Department of Molecular Biosciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616, USA.
authors.forename: Cristina L
authors.initials: CL
authors.lastname: Tôrres
authors.forename: Joshua W
authors.initials: JW
authors.lastname: Miller
authors.forename: Quinton R
authors.initials: QR
authors.lastname: Rogers
mesh_terms: D000818:Animals; D002415:Cats / Q000097:blood*; D003545:Cysteine / Q000097:blood*; D003553:Cystine / Q000097:blood*; D004151:Dipeptides / Q000097:blood; D004285:Dogs / Q000097:blood*; D005978:Glutathione / Q000097:blood; D006710:Homocysteine / Q000097:blood; D008297:Male; D010720:Phosphines; D019163:Reducing Agents; D013997:Time Factors
chemical_list: D003545:Cysteine; D003553:Cystine; D004151:Dipeptides; D005978:Glutathione; D006710:Homocysteine; D010720:Phosphines; D019163:Reducing Agents; C080938:tris(2-carboxyethyl)phosphine; C028505:cysteinylglycine