Etomoxir: A Promiscuous Fatty Acid Mimetic Chemoproteomic Probe

Etomoxir has been used for decades as a popular small molecule inhibitor of Carnitine Palmitoyltransferase I, Cpt1, to block mitochondrial fatty acid b-oxidation. To test the specificity of etomoxir, we generated click chemistry enabled reagents to label etomoxir binding proteins in situ. Etomoxir bound to Cpt1, but also bound to a large array of diverse proteins that metabolize and transport fatty acids in the cytoplasm, peroxisome and mitochondria. Many of the most abundant proteins identified in primary hepatocytes were peroxisomal. The loss of Pex5, required for the import of peroxisomal matrix proteins, eliminated many of these proteins. By utilizing the promiscuous, covalent and fatty acid mimetic properties of etomoxir, novel etomoxir targets of fatty acid w-oxidation were revealed following the loss of Pex5. These data demonstrate that etomoxir is not specific for Cpt1 and is not appropriate as a tool to distinguish the biological effects of fatty acid oxidation.