Detection and characterization of modified nucleosides in serum and urine of uremic patients using capillary liquid chromatography-frit-fast atom bombardment mass spectrometry

Why Is the GFR So High?: Implications for the Treatment of Kidney Failure

The high GFR in vertebrates obligates large energy expenditure. Homer Smith's teleologic argument that this high GFR was needed to excrete water as vertebrates evolved in dilute seas is outdated. The GFR is proportional to the metabolic rate among vertebrate species and higher in warm-blooded mammals and birds than in cold-blooded fish, amphibians, and reptiles. The kidney clearance of some solutes is raised above the GFR by tubular secretion, and we presume secretion evolved to eliminate particularly toxic compounds. In this regard, high GFRs may provide a fluid stream into which toxic solutes can be readily secreted. Alternatively, the high GFR may be required to clear solutes that are too large or too varied to be secreted, especially bioactive small proteins and peptides. These considerations have potentially important implications for the understanding and treatment of kidney failure.

Urinary Nucleosides and Deoxynucleosides

Urinary nucleosides and deoxynucleosides are mainly known as metabolites of RNA turnover and oxidative damage of DNA. For several decades these metabolites have been examined for their potential use in disease states including cancer and oxidative stress. Subsequent improvements in analytical sensitivity and specificity have provided a reliable means to measure these unique molecules to better assess their relationship to physiologic and pathophysiologic conditions. In fact, some are currently used as antiviral and antitumor agents. In this review we provide insight into their molecular characteristics, highlight current separation techniques and detection methods, and explore potential clinical usefulness.

Using UHPLC Q-Trap/MS as a complementary technique to in-depth mine UPLC Q-TOF/MS data for identifying modified nucleosides in urine

Modified nucleosides, metabolites of RNA, are potential biomarkers of cancer before the appearance of morphological abnormalities. It is of great significance to comprehensively detect and identify nucleosides in human urine for discovery of cancer biomarkers. However, the lower abundance, the greater polarity and the matrix effects make it difficult to detect urinary nucleosides. In this paper, an integrated method consisted of sample preparation followed by ultraperformance liquid chromatography quadrupole time-of-flight mass spectrometry (UPLC Q-TOF/MS) detection and primary identification, then ultra-high performance liquid chromatography coupled with hybrid triple quadrupole linear ion trap mass spectrometer (UHPLC Q-Trap/MS) further identification and validation were introduced. Firstly, to enrich the nucleosides and eliminate the urine matrix effects, different sorbent materials of solid phase extraction (SPE) and the elution conditions were screened. Secondly, UPLC Q-TOF/MS was used to acquire mass data in MS^E mode. The structural formulas of nucleosides in urine sample were primarily identified according to retention time, accurate mass precursor ions and fragment ions from in-house database and online database. Thirdly, the preliminary identified nucleoside structures lacking of characteristic fragment ions were verified by UHPLC Q-Trap/MS in multiple reaction monitoring trigger enhanced product ion scan (MRM-EPI) and neutral loss scan (NL). At last, phenylboronic acid (PBA)-based SPE was utilized due to its higher MS signal and weaker matrix effects under optimized extraction conditions. Fifty-five nucleosides were primarily identified by UPLC Q-TOF/MS, among which 50 nucleosides were confirmed by UHPLC Q-Trap/MS. Five nucleosides, namely 4',5'-didehydro-5'-deoxyadenosine, 4',5'-didehydro-5'-deoxyinosine, isonicotinamide riboside, peroxywybutosine and hydroxywybutosine, were found from urine for the first time. The results will expand the Human Metabolome Database (HMDB).

Update of uremic toxin research by mass spectrometry

Mass spectrometry (MS) has been successfully applied for the identification and quantification of uremic toxins and uremia-associated modified proteins. This review focuses on the recent progress in the MS analysis of uremic toxins. Uremic toxins include low-molecular weight solutes, protein-bound low-molecular weight solutes, and middle molecules (peptides and proteins). Based on MS analysis of these uremic toxins, the pathogenesis of the uremic symptoms will be elucidated to prevent and manage the symptoms. Notably, protein-bound uremic toxins such as indoxyl sulfate, p-cresyl sulfate, and 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid have emerged as important targets of therapeutic removal. Hemodialysis even with a high-flux membrane cannot efficiently remove the protein-bound uremic toxins because of their high albumin-binding property. The accumulation of these protein-bound uremic toxins in the blood of dialysis patients might play an important role in the development of uremic complications such as cardiovascular disease. Indoxyl sulfate is the most promising protein-bound uremic toxin as a biomarker of progress in chronic kidney disease. Novel dialysis techniques or membranes should be developed to efficiently remove these protein-bound uremic toxins for the prevention and management of uremic complications.

Recent progress in the analysis of uremic toxins by mass spectrometry

Mass spectrometry (MS) has been successfully applied for the identification and quantification of uremic toxins and uremia-associated modified proteins. This review focuses on recent progress in the analysis of uremic toxins by using MS. Uremic toxins include low-molecular-weight compounds (e.g., indoxyl sulfate, p-cresol sulfate, 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid, asymmetric dimethylarginine), middle-molecular-weight peptides, and proteins modified with advanced glycation and oxidation. These uremic toxins are considered to be involved in a variety of symptoms which may appear in patients with stage 5 chronic kidney disease. Based on MS analysis of these uremic toxins, the pathogenesis of the uremic symptoms will be elucidated to prevent and manage the symptoms.

Identification and determination of nucleosides in rat brain microdialysates by liquid chromatography/electrospray tandem mass spectrometry

A liquid chromatography/tandem mass spectrometry (LC/MS/MS) method has been developed for the determination of brain basal nucleosides (inosine, guanosine and adenosine) in microdialysates from the striatum and cortex of freely moving rats. A microdialysis probe was surgically implanted into the striatum or cortex of individual rats and Ringer's solution was used as the perfusion medium at a flow rate of 0.3 or 0.5 microl/min. The samples were then analyzed off-line by LC/MS/MS experiments. The separation of inosine, guanosine and adenosine was carried out on a cyano column using a mobile phase of 10 mM ammonium acetate, 1% acetic acid and 8% methanol at a flow rate of 0.4 ml/min. Analytes were detected by electrospray ionization tandem mass spectrometry in the positive ion mode. The detection limit for inosine, guanosine and adenosine was 80, 80 and 40 pg on column, respectively. With this method, the intercellular basal inosine, guanosine and adenosine concentrations in striatum and cortex of rat were determined.