This article is available online at http://www.jlr.orgIn order to metabolize long-chain fatty acids in pathways of  -oxidation or the synthesis of complex lipids, they must fi rst be activated to acyl-CoAs by long-chain acyl-CoA synthetases (ACSLs). The fi ve mammalian ACSL isoforms each have a specifi c substrate preference, subcellular location, and tissue distribution ( 1 ). In the heart, the ACSL1 isoform predominates, such that with defi ciency, total ACSL specifi c activity and fatty acid oxidation decrease by more than 90% ( 2 ). Because ACSL activity is required for the incorporation of fatty acids into phospholipids, we asked whether the ACSL1 isoform is also required for the synthesis and remodeling of cardiac phospholipids, particularly cardiolipin (CL).The mitochondrial phospholipid CL contributes to many aspects of mitochondrial function, including energy production through oxidative phosphorylation ( 3, 4 ), mitochondrial fi ssion and fusion ( 5, 6 ), and cellular apoptosis ( 7 ). Tetralinoleoyl-CL is the predominant CL species in the mammalian heart ( 8 ), but the mechanism by which this species is formed is unclear. Because the enzymes of CL synthesis lack acyl-chain specifi city, nascent CL contains a mixture of acyl chain lengths and degrees of unsaturation ( 9 ). To obtain mature CL, most remodeling occurs within the mitochondria by sequentially removing each acyl chain to form monolyso-CL (MLCL) and then replacing the missing fatty acid with linoleate (18:2). Linoleate is added by transacylation from a donor phospholipid ( 10 ) or by direct esterifi cation of a linoleoyl-CoA ( 11, 12 ).The transacylase tafazzin is believed to be responsible for cardiolipin remodeling. Mutations in tafazzin cause Barth syndrome, an X-linked cardiomyopathy characterized by skeletal muscle weakness and heart failure in Abstract Long-chain acyl-CoA synthetase 1 (ACSL1) contributes more than 90% of total cardiac ACSL activity, but its role in phospholipid synthesis has not been determined. Mice with an inducible knockout of ACSL1 ( Acsl1 T ؊ / ؊ ) have impaired cardiac fatty acid oxidation and rely on glucose for ATP production. Because ACSL1 exhibited a strong substrate preference for linoleate, we investigated the composition of heart phospholipids.