| Abstract
| - Enzymes regulate biological processes through the conversion of specific substrates to products.Therefore, of fundamental interest for every enzyme is the elucidation of its natural substrates. Here, wedescribe a general strategy for identifying endogenous substrates of enzymes by untargeted liquidchromatography−mass spectrometry (LC−MS) analysis of tissue metabolomes from wild-type and enzyme-inactivated organisms. We use this method to discover several brain lipids regulated by the mammalianenzyme fatty acid amide hydrolase (FAAH) in vivo, including known signaling molecules (e.g., theendogenous cannabinoid anandamide) and a novel family of nervous system-enriched natural products,the taurine-conjugated fatty acids. Remarkably, the relative hydrolytic activity that FAAH exhibited forlipid metabolites in vitro was not predictive of the identity of specific FAAH substrates in vivo. Thus,global metabolite profiling establishes unanticipated connections between the proteome and metabolomethat enable assignment of an enzyme's unique biochemical functions in vivo.
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