One of many nitroaniline fatty acid amides used to measure FAAH activity. 1 This is a relatively unselective enzyme that accepts a variety of amide head groups other than the ethanolamine of its nominal endogenous substrate AEA. Has also been shown to hydrolyze fatty acid amides with fewer carbons and fewer double bonds than arachidonate. Has the potential for fast and convenient measurement of FAAH activity using a 96-well plate spectrophotometer: exposure of ApNA to FAAH activity results in the release of p-nitroaniline.