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-HC1), 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-HC1 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.