Cystinylglycine in plasma: diagnostic relevance for pyroglutamic acidemia, homocystinuria, and phenylketonuria.
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.
D001322:Autoanalysis; D004151:Dipeptides / Q000097:blood*; D006712:Homocystinuria / Q000097:blood* / Q000175:diagnosis; D006801:Humans; D010661:Phenylketonurias / Q000097:blood* / Q000175:diagnosis; D011760:Pyrrolidinones / Q000097:blood*; D011761:Pyrrolidonecarboxylic Acid / Q000097:blood*
D004151:Dipeptides; D011760:Pyrrolidinones; C022291:cystinylglycine; D011761:Pyrrolidonecarboxylic Acid