Determination of free and total cyst(e)ine in plasma of dogs and cats.
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.
Tôrres
Cristina L
CL
Department of Molecular Biosciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616, USA.
Miller
Joshua W
JW
Rogers
Quinton R
QR
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
D003545:Cysteine; D003553:Cystine; D004151:Dipeptides; D005978:Glutathione; D006710:Homocysteine; D010720:Phosphines; D019163:Reducing Agents; C080938:tris(2-carboxyethyl)phosphine; C028505:cysteinylglycine