Late stage assay provider results from the probe development effort to identify inhibitors of PAFAH2: LC-MS/MS assay to assess binding of compounds to active site
Name: Late stage assay provider results from the probe development effort to identify inhibitors of PAFAH2: LC-MS/MS assay to assess binding of compounds to active site. ..more
BioActive Compound: 1
Source (MLPCN Center Name): The Scripps Research Institute Molecular Screening Center (SRIMSC)
Center Affiliation: The Scripps Research Institute (TSRI)
Assay Provider: Benjamin Cravatt, TSRI
Network: Molecular Libraries Probe Production Centers Network (MLPCN)
Grant Proposal Number: 1R01HL084366
Grant Proposal PI: Brian Bahnson, Univ. of Delaware, Benjamin Cravatt, TSRI
External Assay ID: PAFAH2_INH_LCMS
Name: Late stage assay provider results from the probe development effort to identify inhibitors of PAFAH2: LC-MS/MS assay to assess binding of compounds to active site.
Oxidative stress has been implicated as an underlying inflammatory factor in several disease pathologies, including cancer, atherosclerosis, aging, and various neurodegenerative disorders (1-5). Phospholipids in particular are susceptible to oxidative damage, and (per)oxidized phospholipids can have deleterious effects, including disruption of membrane bilayers and production of toxic byproducts (4, 6-8). One hypothesized pathway for removal of oxidatively damaged species involves hydrolysis by phospholipase A2-type enzymes. Candidate hydrolytic enzymes include the platelet-activating factor acetylhydrolases (PAFAHs) (4,9). The initial role assigned to the PAFAHs was the hydrolysis of platelet activating factor (PAF) (10,11), a potent pro-inflammatory phospholipid signaling molecule (12), which plays a role in myriad physiological processes including inflammation, anaphylaxis, fetal development, and reproduction (4,13). The PAFAHs are subdivided into three classes: plasma (p)PAFAH, and intracellular types 1 and 2. In terms of sequence homology, pPAFAH and PAFAH2 are close homologs and show little similarity to type 1 enzymes. This project aims to develop specific inhibitors for pPAFAH and three counterscreen enzymes: PAFAH2, PAFAH1b2, and PAFAH1b3.
pPAFAH is associated with inflammatory pathways involved in atherosclerosis, asthma, anaphylactic shock, and other allergic reactions (14,15). Numerous studies have also linked increases in pPAFAH concentration and/or activity to an increased risk of various cardiovascular diseases (16,17); however, the biological function of pPAFAH in the development of coronary heart diseases is controversial, with both anti- and proinflammatory roles attributed to it (18,19). Dr. Bahnson and colleagues recently reported the first high-resolution crystal structure of pPAFAH (20), and would like to expand their studies to co-crystallize pPAFAH with substrate-mimetic inhibitors to further define the active site and substrate specificity of pPAFAH. While one selective pPAFAH inhibitor has been reported (21), its properties are not suitable for the proposed studies.
PAFAH2 has also been shown to play a role in inflammatory processes via hydrolysis of oxidized phospholipids. Over-expression of PAFAH2 has been shown to reduce oxidative stress-induced cell death (22,23) and to mediate repair of oxidative-stress induced tissue injury (4). The enzyme also undergoes N-terminal myristoylation and subsequent translocation to the membrane under conditions of oxidative stress (22,23). This evidence suggests that PAFAH2 functions as an important, and perhaps primary, antioxidant enzyme in certain tissues (4); however, its substrate specificity and pathway involvement are far from being fully elucidated. Currently no suitable inhibitors exist for co-crystallization or biochemical studies of PAFAH2.
Given the complex biology of the PAFAH enzymes, chemical tools capable of interrogating enzyme architecture and providing precise temporal control over PAFAH activity are necessary for complete characterization of patho/physiological roles of the PAFAHs in phospholipid metabolism and inflammatory disease processes. Towards that goal, we developed a HTS assay for inhibitor discovery for four PAFAH enzymes: pPAFAH, PAFAH2, PAFAH1b2, and PAFAH1b3, based on their reactivity with the serine-hydrolase-specific fluorophosphonate (FP) (24) activity-based protein profiling (ABPP) probe. This reactivity can be exploited for inhibitor discovery using a competitive-ABPP platform, whereby small molecule enzyme inhibition is assessed by the ability to out-compete ABPP probe labeling (25). Competitive ABPP has also been configured to operate in a high-throughput manner via fluorescence polarization readout, FluoPol-ABPP (26). Following the HTS campaign, top inhibitors for each enzyme will be characterized and medchem optimized with the goal of delivering key reagents for elucidating the biology of the PAFAH enzymes.
1. Ames, B.N., Dietary carcinogens and anticarcinogens. Oxygen radicals and degenerative diseases. Science, 1983. 221(4617): p. 1256-64.
2. Halliwell, B. and J.M. Gutteridge, Role of free radicals and catalytic metal ions in human disease: an overview. Methods Enzymol., 1990. 186: p. 1-85.
3. Harman, D., The aging process. Proc. Natl. Acad. Sci. U. S. A., 1981. 78(11): p. 7124-8.
4. Kono, N., et al., Protection against oxidative stress-induced hepatic injury by intracellular type II platelet-activating factor acetylhydrolase by metabolism of oxidized phospholipids in vivo. J. Biol. Chem., 2008. 283(3): p. 1628-36.
5. Southorn, P.A. and G. Powis, Free radicals in medicine. II. Involvement in human disease. Mayo. Clin. Proc., 1988. 63(4): p. 390-408.
6. Kinnunen, P.K., On the principles of functional ordering in biological membranes. Chem. Phys. Lipids, 1991. 57(2-3): p. 375-99.
7. Uchida, K., 4-Hydroxy-2-nonenal: a product and mediator of oxidative stress. Prog. Lipid Res., 2003. 42(4): p. 318-43.
8. Fruhwirth, G.O., A. Loidl, and A. Hermetter, Oxidized phospholipids: from molecular properties to disease. Biochim. Et Biophys. Acta, 2007. 1772(7): p. 718-36.
9. Nigam, S. and T. Schewe, Phospholipase A(2)s and lipid peroxidation. Biochim. Et Biophys. Acta, 2000. 1488(1-2): p. 167-81.
10. Blank, M.L., et al., A specific acetylhydrolase for 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (a hypotensive and platelet-activating lipid). J. Biol. Chem., 1981. 256(1): p. 175-8.
11. Farr, R.S., et al., Preliminary studies of an acid-labile factor (ALF) in human sera that inactivates platelet-activating factor (PAF). Clin. Immunol. Immunopathol., 1980. 15(3): p. 318-330.
12. Zimmerman, G.A., et al., The platelet-activating factor signaling system and its regulators in syndromes of inflammation and thrombosis. Crit. Care Med., 2002. 30(5 Suppl): p. S294-301.
13. Prescott, S.M., et al., Platelet-activating factor and related lipid mediators. Annu. Rev. Biochem., 2000. 69: p. 419-45.
14. Karasawa, K., et al., Plasma platelet activating factor-acetylhydrolase (PAF-AH). Prog. Lipid Res., 2003. 42(2): p. 93-114.
15. Leitinger, N., Oxidized phospholipids as triggers of inflammation in atherosclerosis. Mol. Nutr. Food Res., 2005. 49(11): p. 1063-71.
16. Anderson, J.L., Lipoprotein-associated phospholipase A2: an independent predictor of coronary artery disease events in primary and secondary prevention. Am. J. Cardiol., 2008. 101(12A): p. 23F-33F.
17. Sudhir, K., Clinical review: Lipoprotein-associated phospholipase A2, a novel inflammatory biomarker and independent risk predictor for cardiovascular disease. J. Clin. Endocrinol. Metab., 2005. 90(5): p. 3100-5.
18. Wilensky, R.L. and C.H. Macphee, Lipoprotein-associated phospholipase A(2) and atherosclerosis. Curr. Opin. Lipidol., 2009. 20(5): p. 415-20.
19. Karabina, S.A. and E. Ninio, Plasma PAF-acetylhydrolase: an unfulfilled promise? Biochim. Et Biophys. Acta, 2006. 1761(11): p. 1351-8.
20. Samanta, U. and B.J. Bahnson, Crystal structure of human plasma platelet-activating factor acetylhydrolase: structural implication to lipoprotein binding and catalysis. J. Biol. Chem., 2008. 283(46): p. 31617-24.
21. Blackie, J.A., et al., The identification of clinical candidate SB-480848: a potent inhibitor of lipoprotein-associated phospholipase A2. Bioorg. Med. Chem. Lett., 2003. 13(6): p. 1067-70.
22. Matsuzawa, A., et al., Protection against oxidative stress-induced cell death by intracellular platelet-activating factor-acetylhydrolase II. J. Biol. Chem., 1997. 272(51): p. 32315-20.
23. Marques, M., et al., Identification of platelet-activating factor acetylhydrolase II in human skin. J. Invest. Dermatol., 2002. 119(4): p. 913-9.
24. Jessani, N., et al., Enzyme activity profiles of the secreted and membrane proteome that depict cancer cell invasiveness. Proc. Natl. Acad. Sci. U. S. A., 2002. 99(16): p. 10335-40.
25. Leung, D., et al., Discovering potent and selective reversible inhibitors of enzymes in complex proteomes. Nat. Biotechnol., 2003. 21(6): p. 687-91.
26. Bachovchin, D.A., et al., Identification of selective inhibitors of uncharacterized enzymes by high-throughput screening with fluorescent activity-based probes. Nat. Biotechnol., 2009. 27(4): p. 387-94.
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The purpose of this assay is to assess the covalent nature of an inhibitor compound belonging to the urea triazole scaffold and determine whether or not it labels the active site serine of PAFAH2. In this assay, purified enzyme is reacted with inhibitor compound, digested with trypsin, and the resulting peptides are analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The resulting data are analyzed to identify sites of covalent labeling.
Two aliquots (25 uL) of 50 uM ABHD11 were prepared. To one aliquot was added inhibitor (0.5 uL of 10 mM in DMSO), giving a final concentration of 200 uM. To the second (control) aliquot was added DMSO (0.5 uL). Reactions were gently vortexed and incubated at room temperature for 30 minutes. To each reaction was added solid urea (50 mg), followed by freshly prepared aqueous ammonium bicarbonate (75 uL of 25 mM). The reactions were vortexed until the urea was dissolved. Final urea concentration was approximately 8 M. To each reaction was added freshly prepared TCEP (5 uL of 100 mM in water), and the reactions were incubated at 30 C for 30 minutes. To each reaction was then added freshly prepared IAA (10 uL of 100 mM in water), and the reactions were incubated for 30 min at room temperature in the dark. Aqueous ammonium bicarbonate (375 uL of 25 mM) was added to reduce the urea concentration to 2 M. To each reaction was added sequencing grade modified trypsin (1 ug), and reactions were incubated at 37 C for 12 hours. Formic acid was added to 5% (v/v) final.
An Agilent 1200 series quaternary HPLC pump and Thermo Scientific LTQ-Orbitrap mass spectrometer were used for sample analysis. A fraction (10 uL) of the protein digest for each sample was pressure-loaded onto a 100 micron fused-silica column (with a 5 micron in-house pulled tip) packed with 10 cm of Aqua C18 reversed-phase packing material. Chromatography was carried out using an increasing gradient of aqueous acetonitrile containing 0.1% formic acid over 125 minutes. Mass spectra were acquired in a data-dependent mode with dynamic exclusion enabled.
The MS/MS spectra generated for each run were searched against a human protein database concatenated to a reversed decoy database using Sequest. A static modification of +57.021 was specified cysteine, and a variable modification of +97.053 was specified for serine to account for possible probe labeling by AA39-2. The resulting peptide identifications were assembled into protein identifications using DTASelect, and filters were adjusted to maintain a false discovery rate (as determined by number of hits against the reversed database) of < 1%. Any modified peptides identified in the DMSO-treated sample were discarded as spurious hits.
PubChem Activity Outcome and Score:
Compounds observed to covalently modify the active site serine of PAFAH2 were considered active. Compounds for which no covalent modification was observed were considered inactive.
The PubChem Activity Score is assigned a value of 100 for active compounds, and 0 for inactive compounds.
The PubChem Activity Score range for active compounds is 100-100. There are no inactive compounds.
List of Reagents:
PAFAH2 protein (human recombinant protein provided by Assay Provider)
DPBS (CellGro 21-030-CV)
Urea (Fisher U15-3)
Ammonium bicarbonate (Acros, AC37093-0010)
TCEP (Tris[2-carboxyethyl]phosphine hydrochloride; Sigma C4706)
IAA (iodoacetamide; Sigma I1149)
Trypsin (Promega V5111)
Fused-silica (Agilent 160-2635-10)
Aqua C18 (Phenomenex 04A-4299)
Acetonitrile (Fisher A955-4)
Formic acid (Fluka 06440)
This assay was performed in the laboratory of the Assay Provider with powder samples of compounds.
Data Table (Concise)