Identification of Urinary Modified Nucleosides and Ribosylated Metabolites in Humans Via Combined ESI-FTICR MS and ESI-IT MS Analysis

The physiological response of the human body to several diseases can be reflected by the metabolite pattern in biological fluids. Cancer, like other diseases accompanied by metabolic disorders, causes characteristic effects on cell turnover rate, activity of modifying enzymes, and RNA/DNA modifications. This results in an altered excretion of modified nucleosides and biochemically related compounds. In the course of our metabolic profiling project, we screened 24-h urine of patients suffering from lung, rectal, or head and neck cancer for previously unknown ribosylated metabolites. Therefore, we developed a sample preparation procedure based on boronate affinity chromatography followed by additional prepurification with preparative TLC. The isolated metabolites were analyzed by ion trap mass spectrometry (IT MS) and Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). IT MS was applied for LC-auto MS3 screening runs and MSn(n = 4–6) syringe pump infusion experiments, yielding characteristic fragmentation patterns. FTICR MS measurements enabled the calculation of corresponding molecular formulae based on accurate mass determination (mass accuracy: 1–5 ppm for external and sub-ppm values for internal calibration). We were able to identify 22 metabolites deriving from cellular RNA metabolism and related metabolic pathways like histidine metabolism, purine biosynthesis, methionine/polyamine cycle, and nicotinate/nicotinamide metabolism. The compounds 1-ribosyl-3-hydroxypyridinium, 1-ribosyl-pyridinium, and 3-ribosyl-1-methyl-l-histidinium as well as a series of ribosylated histamines, conjugated to carboxylic acids at the Nω-position were found as novel urinary constituents. The occurrence of the modified nucleosides 2-methylthio-N6-(cis-hydroxyisopentenyl)-adenosine, 5-methoxycarbonylmethyl-2-thiouridine, N6-methyl-N6-threonylcarbamoyladenosine, and 2-methylthio-N6-threonylcarbamoyladenosine in human urine is verified for the first time