Accurate quantification of dimethylamine (DMA) in human plasma and serum by GC–MS and GC–tandem MS as pentafluorobenzamide derivative in the positive-ion chemical ionization mode

Abstract

Dimethylamine (DMA) circulates in human blood and is excreted in the urine. Major precursor for endogenous DMA is asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide (NO) synthesis. ADMA is hydrolyzed to DMA and l-citrulline by dimethylarginine dimethylaminohydrolase (DDAH). In previous work, we reported a GC–MS method for the quantification of DMA in human urine. This method involves simultaneous derivatization of endogenous DMA and the internal standard (CD 3) 2NH by pentafluorobenzoyl chloride (PFBoylCl) and extraction of the pentafluorobenzamide derivatives by toluene. In the present work, we optimized this derivatization/extraction procedure for the quantitative determination of DMA in human plasma. Optimized experimental parameters included vortex time and concentration of PFBoylCl, carbonate and internal standard. The GC–MS method was thoroughly validated and applied to measure DMA concentrations in human plasma and serum samples. GC–MS quantification was performed by selected-ion monitoring of the protonated molecules at m/ z 240 for DMA and m/ z 246 for (CD 3) 2NH in the positive-ion chemical ionization mode. Circulating DMA concentration in healthy young women ( n = 18) was determined to be 1.43 ± 0.23 μM in serum, 1.73 ± 0.17 μM in lithium heparin plasma, and 9.84 ± 1.43 μM in EDTA plasma. DMA was identified as an abundant contaminant in EDTA vacutainer tubes (9.3 ± 1.9 nmol/monovette, n = 6). Serum and lithium heparin vacutainer tubes contained considerably smaller amounts of DMA (0.42 ± 0.01 and 0.95 ± 0.01 nmol/monovette, respectively, each n = 6). Serum is recommended as the most appropriate matrix for measuring DMA in human blood. The present GC–MS method should be useful for the determination of systemic and whole body DDAH activity by measuring circulating and excretory DMA in experimental and clinical studies.