Pyrimidines are metabolized in humans in four steps catalyzed by dihydropyrimidine dehydrogenase (DHPDH), dihydropyrimidinase (DHP), -ureidopropionase, and three aminotransferases. Typically, people with pyrimidine metabolism deficiencies lack sufficient amounts of DHPDH, characterized by excessive amounts of thymine and uracil in urine, and/or DHP, manifested by large amounts of dihydrothymine and dihydrouracil in urine.

Methods to determine the aforementioned defects include HPLC, two-dimensional TLC, and amino acid analysis. However, these methods are difficult to quantitate, lack appropriate sensitivity, or require further analysis to determine the specific pyrimidine deficiency. Now, researchers from the Kanazawa Medical University (Ishikawa, Japan) have developed a GC-MS procedure for diagnosing pyrimidine metabolism defects that is highly sensitive and requires no further analysis to make a differential diagnosis. (J. Chromatogr. B 2001, 1, 61–74).

The researchers took liquid urine or urine-soaked filter paper and treated it with urease to remove excess urea. They then spiked the treated urine with isotopically labeled uracil, orotate, and creatinine, simulating elevated levels in typical and moderate cases of DHPDH and DHP deficiencies. After deproteinization, centrifugation, and evaporation, the residue was trimethylsilated and injected into a benchtop GC-MS instrument. The resulting mass chromatograms show separate peaks for thymine, uracil, dihydrothymine, and dihydrouracil. The researchers concluded that this method allows the simultaneous and quantitative determination of the above compounds in urine to determine whether a patient has pyrimidine metabolism deficiencies.