Featuring ultimate sensitivity of high-resolution LC-MS analysis of phenolics in rat plasma.

Sensitive analysis of very low-molecular weight metabolites using liquid chromatography with quadrupole-time-of-flight mass spectrometry is challenging due to the high losses of ions in a time-of-flight analyzer. Improvement in sensitivity for these analytes via the optimization of advanced parameters, including quadrupole profile, ion guide parameters, and duty cycle, has been achieved. The optimization of the method was carried out using a large spectrum of structurally different compounds including (iso)flavonoids and their known metabolites. These compounds can be categorized into two major groups, that is, compounds with (iso)flavonoid core and low-molecular weight phenolics. The optimization of the duty cycle enabled up to a 15-fold increase in analyte responses while the contribution of tuning ion optics and quadrupole profile was negligible. The limits of quantifications of our new method were assessed using both standard solutions and rat plasma. They were decreased at least 10 times for several low-molecular weight phenolics enabling measurement of their concentrations in a range of 1-50 ng/mL in rat plasma after protein precipitation. Concurrently, the limits of quantifications for compounds with (iso)flavonoid core did not increase distinctly allowing their detection in a range of 0.5-10 ng/mL. The new method was used for the targeting of phenolics in biological samples from pharmacokinetics experiments.

Structure and function of nucleoside hydrolases from Physcomitrella patens and maize catalyzing the hydrolysis of purine, pyrimidine, and cytokinin ribosides.

We present a comprehensive characterization of the nucleoside N-ribohydrolase (NRH) family in two model plants, Physcomitrella patens (PpNRH) and maize (Zea mays; ZmNRH), using in vitro and in planta approaches. We identified two NRH subclasses in the plant kingdom; one preferentially targets the purine ribosides inosine and xanthosine, while the other is more active toward uridine and xanthosine. Both subclasses can hydrolyze plant hormones such as cytokinin ribosides. We also solved the crystal structures of two purine NRHs, PpNRH1 and ZmNRH3. Structural analyses, site-directed mutagenesis experiments, and phylogenetic studies were conducted to identify the residues responsible for the observed differences in substrate specificity between the NRH isoforms. The presence of a tyrosine at position 249 (PpNRH1 numbering) confers high hydrolase activity for purine ribosides, while an aspartate residue in this position confers high activity for uridine. Bud formation is delayed by knocking out single NRH genes in P. patens, and under conditions of nitrogen shortage, PpNRH1-deficient plants cannot salvage adenosine-bound nitrogen. All PpNRH knockout plants display elevated levels of certain purine and pyrimidine ribosides and cytokinins that reflect the substrate preferences of the knocked out enzymes. NRH enzymes thus have functions in cytokinin conversion and activation as well as in purine and pyrimidine metabolism.