Accurate quantification of dimethylamine (DMA) in human urine by gas chromatography–mass spectrometry as pentafluorobenzamide derivative: Evaluation of the relationship between DMA and its precursor asymmetric dimethylarginine (ADMA) in health and disease

Dimethylamine [DMA, (CH3)2NH)] is abundantly present in human urine. Main sources of urinary DMA have been reported to include trimethylamine N-oxide, a common food component, and asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide (NO) synthesis. ADMA is excreted in the urine in part unmetabolized and in part after hydrolysis to DMA by dimethylarginine dimethylaminohydrolase (DDAH). Here we describe a GC–MS method for the accurate and rapid quantification of DMA in human urine. The method involves use of (CD3)2NH as internal standard, simultaneous derivatization with pentafluorobenzoyl chloride and extraction in toluene, and selected-ion monitoring of m/z 239 for DMA and m/z 245 for (CD3)2NH in the electron ionization mode. GC–MS analysis of urine samples from 10 healthy volunteers revealed a DMA concentration of 264 ± 173 μM equivalent to 10.1 ± 1.64 μmol/mmol creatinine. GC–tandem MS analysis of the same urine samples revealed an ADMA concentration of 27.3 ± 15.3 μM corresponding to 1.35 ± 1.2 μmol/mmol creatinine. In these volunteers, a positive correlation (R = 0.83919, P = 0.0024) was found between urinary DMA and ADMA, with the DMA/ADMA molar ratio being 10.8 ± 6.2. Elevated excretion rates of DMA (52.9 ± 18.5 μmol/mmol creatinine) and ADMA (3.85 ± 1.65 μmol/mmol creatinine) were found by the method in 49 patients suffering from coronary artery disease, with the DMA/ADMA molar ratio also being elevated (16.8 ± 12.8). In 12 patients suffering from end-stage liver disease, excretion rates of DMA (47.8 ± 19.7 μmol/mmol creatinine) and ADMA (5.6 ± 1.5 μmol/mmol creatinine) were found to be elevated, with the DMA/ADMA molar ratio (9.17 ± 4.2) being insignificantly lower (P = 0.46). Between urinary DMA and ADMA there was a positive correlation (R = 0.6655, P < 0.0001) in coronary artery disease, but no correlation (R = 0.27339) was found in end-stage liver disease.