Determination of free and total cyst(e)ine in plasma of dogs and cats

Cristina L. Tôrres, Joshua W. Miller, Quinton Rogers

Research output: Contribution to journalArticlepeer-review

17 Scopus citations


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°C). Results: Initial free cyst(e)ine concentrations (mean ± SEM) were higher in canine plasma (77 ± 4 μmol/L) than in feline plasma (37 ± 3 μmol/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 μmol/L) and cats (296 μmol/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. Conclusion: These results emphasize the importance of removing plasma proteins within 1 hour after blood collection for reliable assay of free plasma cyst(e)ine.

Original languageEnglish (US)
Pages (from-to)228-233
Number of pages6
JournalVeterinary Clinical Pathology
Issue number4
StatePublished - 2004


  • Cat
  • Cysteine oxidation
  • Dog
  • Glutathione
  • Homocysteine
  • Plasma

ASJC Scopus subject areas

  • Biochemistry, Genetics and Molecular Biology(all)
  • veterinary(all)


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