High-Throughput Screening for the Discovery of Inhibitors of Fatty Acid Amide Hydrolase Using a Microsome-Based Fluorescent Assay

Fluorescence-Based Enzyme Activity Assay: Ascertaining the Activity and Inhibition of Endocannabinoid Hydrolytic Enzymes

The endocannabinoid system, known for its regulatory role in various physiological processes, relies on the activities of several hydrolytic enzymes, such as fatty acid amide hydrolase (FAAH), N-acylethanolamine-hydrolyzing acid amidase (NAAA), monoacylglycerol lipase (MAGL), and α/β-hydrolase domains 6 (ABHD6) and 12 (ABHD12), to maintain homeostasis. Accurate measurement of these enzymes' activities is crucial for understanding their function and for the development of potential therapeutic agents. Fluorometric assays, which offer high sensitivity, specificity, and real-time monitoring capabilities, have become essential tools in enzymatic studies. This review provides a comprehensive overview of the principles behind these assays, the various substrates and fluorophores used, and advances in assay techniques used not only for the determination of the kinetic mechanisms of enzyme reactions but also for setting up kinetic assays for the high-throughput screening of each critical enzyme involved in endocannabinoid degradation. Through this comprehensive review, we aim to highlight the strengths and limitations of current fluorometric assays and suggest future directions for improving the measurement of enzyme activity in the endocannabinoid system.

A fluorogenic substrate for the detection of lipid amidases in intact cells

Lipid amidases of therapeutic relevance include acid ceramidase (AC), N-acylethanolamine-hydrolyzing acid amidase, and fatty acid amide hydrolase (FAAH). Although fluorogenic substrates have been developed for the three enzymes and high-throughput methods for screening have been reported, a platform for the specific detection of these enzyme activities in intact cells is lacking. In this article, we report on the coumarinic 1-deoxydihydroceramide RBM1-151, a 1-deoxy derivative and vinilog of RBM14-C12, as a novel substrate of amidases. This compound is hydrolyzed by AC (appKm = 7.0 μM; appVmax = 99.3 nM/min), N-acylethanolamine-hydrolyzing acid amidase (appKm = 0.73 μM; appVmax = 0.24 nM/min), and FAAH (appKm = 3.6 μM; appVmax = 7.6 nM/min) but not by other ceramidases. We provide proof of concept that the use of RBM1-151 in combination with reported irreversible inhibitors of AC and FAAH allows the determination in parallel of the three amidase activities in single experiments in intact cells.

Fluorimetric Assay of FAAH Activity

Fatty acid amide hydrolase (FAAH) is the enzyme responsible for the degradation of anandamide (N-arachidonoylethanolamine, AEA) to arachidonic acid (AA) and ethanolamine. The method described here measures FAAH activity through the fluorometric arachidonoyl-7-amino-4-methyl-coumarin amide (AAMCA) substrate, which allows a simple and sensitive assay suitable for high-throughput screening tests. FAAH catalyzes the hydrolysis of AAMCA producing AA and the highly fluorescent compound 7-amino-4-methylcoumarin (AMC).

A NIR fluorescent probe for fatty acid amide hydrolase bioimaging and its application in development of inhibitors

Fatty acid amide hydrolase (FAAH) is primarily responsible for the inactivation of fatty acid ethanolamide (FAE) and is involved in a variety of biological functions related to diseases of the nervous system. Herein, we developed a highly selective and sensitive FAAH-activated near-infrared fluorescent probe named DAND and achieved the real-time detection and imaging of FAAH activity in complex biosystems. Moreover, a visual high-throughput screening method was established using DAND, piperine was identified as a novel inhibitor of FAAH. Based on the interaction of piperine with FAAH, a more potent FAAH inhibitor (11f) was designed and synthesized which possessed an IC₅₀ value of 0.65 μM. Furthermore, 11f could attenuate the liposaccharide (LPS)-induced activation of BV2 cells, exhibiting an excellent anti-inflammatory activity. These results indicated that DAND could be used as a promising molecular tool for exploring FAAH activity and for rapidly screening potential FAAH inhibitors. In addition, piperine and its derivatives could serve as potential candidate drugs for the treatment of neurodegenerative diseases in the future.

Characterization of fatty acid amide hydrolase activity by a fluorescence-based assay

Fatty acid amide hydrolase (FAAH) is involved in many human diseases, particularly cancer, pain and inflammation as well as neurological, metabolic and cardiovascular disorders. Therefore, FAAH is an attractive target for the development of low-molecular-weight inhibitors as therapeutics, which requires robust assays that can be used for high-throughput screening (HTS) of compound libraries. Here, we report the development of a fluorometric assay based on FAAH's ability to effectively hydrolyze medium-chain fatty acid amides, introducing N-decanoyl-substituted 5-amino-2-methoxypyridine (D-MAP) as new amide substrate. D-MAP is cleaved by FAAH with an 8-fold larger specificity constant than the previously reported octanoyl-analog Oc-MAP (V_max/K_m of 1.09 and 0.134 mL min^-1 mg^-1, respectively), with both MAP derivatives possessing superior substrate properties and much increased aqueous solubility compared to the respective p-nitroaniline compounds D-pNA and Oc-pNA. The new assay with D-MAP as substrate is highly sensitive using a lower enzyme concentration (1 μg mL^-1) than literature-reported fluorimetric FAAH assays. In addition, D-MAP was validated in comparison to the substrate Oc-MAP for the characterization of FAAH inhibitors by means of the reference compounds URB597 and TC-F2 and was shown to be highly suitable for HTS in both kinetic and endpoint assays (Z' factors of 0.81 and 0.78, respectively).

Mustard vesicants alter expression of the endocannabinoid system in mouse skin

Vesicants including sulfur mustard (SM) and nitrogen mustard (NM) are bifunctional alkylating agents that cause skin inflammation, edema and blistering. This is associated with alterations in keratinocyte growth and differentiation. Endogenous cannabinoids, including N-arachidonoylethanolamine (anandamide, AEA) and 2-arachidonoyl glycerol (2-AG), are important in regulating inflammation, keratinocyte proliferation and wound healing. Their activity is mediated by binding to cannabinoid receptors 1 and 2 (CB1 and CB2), as well as peroxisome proliferator-activated receptor alpha (PPARα). Levels of endocannabinoids are regulated by fatty acid amide hydrolase (FAAH). We found that CB1, CB2, PPARα and FAAH were all constitutively expressed in mouse epidermis and dermal appendages. Topical administration of NM or SM, at concentrations that induce tissue injury, resulted in upregulation of FAAH, CB1, CB2 and PPARα, a response that persisted throughout the wound healing process. Inhibitors of FAAH including a novel class of vanillyl alcohol carbamates were found to be highly effective in suppressing vesicant-induced inflammation in mouse skin. Taken together, these data indicate that the endocannabinoid system is important in regulating skin homeostasis and that inhibitors of FAAH may be useful as medical countermeasures against vesicants.

Stable isotope liquid chromatography-tandem mass spectrometry assay for fatty acid amide hydrolase activity

Fatty acid amide hydrolase (FAAH) is the main enzyme responsible for the hydrolysis of the endocannabinoid anandamide (arachidonoyl ethanolamide, AEA) to arachidonic acid (AA) and ethanolamine (EA). Published FAAH activity assays mostly employ radiolabeled anandamide or synthetic fluorogenic substrates. We report a stable isotope liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay for specific, sensitive, and high-throughput capable FAAH activity measurements. The assay uses AEA labeled with deuterium on the EA moiety (d₄-AEA) as substrate and measures the specific reaction product tetradeutero-EA (d₄-EA) and the internal standard ¹³C₂-EA. Selected reaction monitoring of m/z 66→m/z 48 (d₄-EA) and m/z 64→m/z 46 (¹³C₂-EA) in the positive electrospray ionization mode after liquid chromatographic separation on a HILIC (hydrophilic interaction liquid chromatography) column is performed. The assay was developed and thoroughly validated using recombinant human FAAH (rhFAAH) and then was applied to human blood and dog liver samples. rhFAAH-catalyzed d₄-AEA hydrolysis obeyed Michaelis-Menten kinetics (K(M)=12.3 μM, V(max)=27.6 nmol/min mg). Oleoyl oxazolopyridine (oloxa) was a potent, partial noncompetitive inhibitor of rhFAAH (IC₅₀=24.3 nM). Substrate specificity of other fatty acid ethanolamides decreased with decreasing length, number of double bonds, and lipophilicity of the fatty acid skeleton. In human whole blood, we detected FAAH activity that was inhibited by oloxa.