title: Redox-based inactivation of cysteine cathepsins by compounds containing the 4-aminophenol moiety.
abstract: BACKGROUND
  Redox cycling compounds have been reported to cause false positive inhibition of proteases in drug discovery studies. This kind of false positives can lead to unusually high hit rates in high-throughput screening campaigns and require further analysis to distinguish true from false positive hits. Such follow-up studies are both time and resource consuming.
  
  
  RESULTS
  In this study we show that 5-aminoquinoline-8-ol is a time-dependent inactivator of cathepsin B with a k(inact)/K(I) of 36.7 ± 13.6 M(-1) s(-1) using enzyme kinetics. 5-Aminoquinoline-8-ol inhibited cathepsins H, L and B in the same concentration range, implying a non-specific mechanism of inhibition. Further analogues, 4-aminonaphthalene-1-ol and 4-aminophenol, also displayed time-dependent inhibition of cathepsin B with k(inact)/K(I) values of 406.4 ± 10.8 and 36.5 ± 1.3 M(-1) s(-1). No inactivation occurred in the absence of either the amino or the hydroxyl group, suggesting that the 4-aminophenol moiety is a prerequisite for enzyme inactivation. Induction of redox oxygen species (ROS) by 4-aminophenols in various redox environments was determined by the fluorescent probe 2',7'-dichlorodihydrofluorescein diacetate. Addition of catalase to the assay buffer significantly abrogated the ROS signal, indicating that H(2)O(2) is a component of the ROS induced by 4-aminophenols. Furthermore, using mass spectrometry, active site probe DCG-04 and isoelectric focusing we show that redox inactivation of cysteine cathepsins by 5-aminoquinoline-8-ol is active site directed and leads to the formation of sulfinic acid.
  
  
  CONCLUSIONS
  In this study we report that compounds containing the 4-aminophenol moiety inactivate cysteine cathepsins through a redox-based mechanism and are thus likely to cause false positive hits in the screening assays for cysteine proteases.
authors.affiliation: Department of Pharmaceutical Biology, Faculty of Pharmacy, University of Ljubljana, Ljubljana, Slovenia. bojana.mirkovic@ffa.uni-lj.si
authors.forename: Bojana
authors.initials: B
authors.lastname: Mirković
authors.forename: Izidor
authors.initials: I
authors.lastname: Sosič
authors.forename: Stanislav
authors.initials: S
authors.lastname: Gobec
authors.forename: Janko
authors.initials: J
authors.lastname: Kos
mesh_terms: D000627:Aminophenols / Q000494:pharmacology*; D002401:Cathepsin B / Q000037:antagonists & inhibitors* / Q000737:chemistry; D003545:Cysteine / Q000737:chemistry*; D005456:Fluorescent Dyes; D006801:Humans; D007525:Isoelectric Focusing; D007700:Kinetics; D013058:Mass Spectrometry; D010084:Oxidation-Reduction; D011480:Protease Inhibitors / Q000494:pharmacology*; D017382:Reactive Oxygen Species / Q000378:metabolism; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry
chemical_list: D000627:Aminophenols; D005456:Fluorescent Dyes; D011480:Protease Inhibitors; D017382:Reactive Oxygen Species; D011994:Recombinant Proteins; D002401:Cathepsin B; D003545:Cysteine; C026729:4-aminophenol
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