Activity assay of Purine Nucleoside Phosphorylases by LC-MS/MS

In the search for new therapeutics, fast and automated screening tools of chemical libraries are required for hit selection. Nucleoside phosphorylases (NPs, E.C. 2.4.2) are among the key enzymes in nucleotide salvage/recycling pathway. NPs catalyze the reversible cleavage of the glycosidic bond of (deoxy)ribonucleosides in the presence of inorganic orthophosphate (Pi) to generate the nucleobase and α-D-(deoxy)ribose-1-phosphate (see Scheme 1).[1] NPs are also essential for the metabolism of nucleotides in bacteria and other organisms. Nucleotide metabolic pathways in lower organisms represent reasonable targets for chemotherapy as they usually differ from the human counterparts.1b Inhibition of pathogen purine nucleoside phosphorylases (PNPs, E.C. 2.4.2.1) might result in the impairment of replicative processes, thus providing a new potential route to infection control.[2] Here we describe a novel LC-MS/MS method for the assessment of the activity of PNPs as an alternative to routinely used assays.1b Enzymatic activity was assessed by phosphorolysis of inosine to hypoxanthine (Scheme 1). Kinetic parameters (Km, Vmax, Kcat) were determined with respect to inosine and Pi. The assay was performed in a 96 well plate format with an overall reaction time of about 15 minutes per plate, followed by the application of HILIC-LC-MS/MS method for the rapid quantification of the produced hypoxanthine (less than 2 minutes for sample). For method development and validation, a PNP from Aeromonas hydrophila was used due to accumulated data on this enzyme by our team over the years.[3] The newly developed LC-MS/MS assay will be applied to the screening of potential inhibitors against pathogenic PNPs. [1] a. Pugmire, M. J. et al. Biochem. J. 2002, 361, 1; b. Bzowska, A. et al. Pharmacol. Ther. 2000, 88, 349. [2] a. de Moraes, M. C. et al. Anal. Bioanal. Chem. 2013, 405, 4871; b. Ducati, R. G. et al. Curr. Med. Chem. 2011, 18, 1258; c. Madrid, D. C. et al J. Biol. Chem. 2008, 283, 35899. [3] a. Ubiali, D. et al. Adv. Synth. Catal. 2012, 354, 96; b. Serra, I. et al. ChemPlusChem 2013, 78, 157; c. Calleri, E. et al. J. Chromatogr. B 2014, 968, 79.