Cystinylglycine in plasma: diagnostic relevance for pyroglutamic acidemia, homocystinuria, and phenylketonuria

Clin Chim Acta. 1981 Nov 25;117(1):7-12. doi: 10.1016/0009-8981(81)90004-8.


Cystinylglycine, recently identified as a normal small peptide in human plasma, has diagnostic importance for several genetically determined disorders. We found cystinylglycine absent from the plasma of a patient with pyroglutamic acidemia, and the peptide was either absent or greatly reduced in plasma from patients with homosyctinuria. In the latter disorder, a different small peptide replaced cystinylglycine. It was identified as the mixed disulfide of homocysteine and cysteinylglycine. The mean plasma concentration of cystinylglycine was 13.6 +/- 3.6 mumol/l in adult control subjects, and concentrations of the mixed disulfide of homocysteine and cysteinylglycine varied between 2 and 10 mumol/l in the plasma of homocystinuric patients. Failure to separate cystinylglycine from phenylalanine with many rapid amino acid analyzer systems can lead to a misclassification of persons as heterozygotes for the phenylketonuria gene when heterozygosity testing is based on the phenylalanine/tyrosine molar ratio in fasting plasma.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Autoanalysis
  • Dipeptides / blood*
  • Homocystinuria / blood*
  • Homocystinuria / diagnosis
  • Humans
  • Phenylketonurias / blood*
  • Phenylketonurias / diagnosis
  • Pyrrolidinones / blood*
  • Pyrrolidonecarboxylic Acid / blood*


  • Dipeptides
  • Pyrrolidinones
  • cystinylglycine
  • Pyrrolidonecarboxylic Acid