A fluorescence-based assay for fatty acid amide hydrolase compatible with high-throughput screening

A novel fluorescent assay to continuously monitor fatty acid amide hydrolase (FAAH) activity that is simple, sensitive, and amenable to high-throughput screening (HTS) of compound libraries is described in this article. Stable Chinese hamster ovary (CHO) cell lines expressing either human FAAH or an inactive mutant, FAAH-S241A, were established. Arachidonyl 7-amino, 4-methyl coumarin amide (AAMCA), a novel fluorogenic substrate for FAAH, was designed and synthesized. FAAH catalyzes the hydrolysis of AAMCA to generate arachidonic acid and a highly fluorescent 7-amino, 4-methyl coumarin (AMC). The assay was done at 25 °C by incubating whole cell or microsomal preparations from FAAH-expressing cells with AAMCA. Release of AMC was monitored continuously using a fluorometer. Microsomal FAAH catalyzed the hydrolysis of AAMCA with an apparent Km of 0.48 μM and Vmax of 58 pmol min−1 mg protein−1. The assay is specific for FAAH given that microsomes prepared from cells expressing FAAH-S241A or vector alone had no significant activity against AAMCA. Furthermore, the activity was inhibited by URB-597, an FAAH-specific inhibitor, in a concentration-dependent manner with an IC50 of 33.5 nM. The assay was optimized for HTS and had a Z′ value ranging from 0.7 to 0.9. The assay is also compatible with ex vivo analysis of FAAH activity.