Fatty Acid Chain Elongation in Palmitate-perfused Working Rat Heart

Background: In [16,16,16-d(3)]palmitic acid (M+3) perfused working hearts, M+3 stearoylcarnitine is formed. Results: Rat hearts chain-elongate palmitate to stearate and arachidate. Furthermore, rat heart mitochondria catalyze the malonyl/acetyl-CoA-dependent chain elongation of palmitoyl-CoA. Conclusion: The two-carbon units for fatty acid chain elongation are derived from mitochondrial fatty acid -oxidation. Significance: This pathway may represent the molecular basis for heart-specific fatty acid remodeling. Rat hearts were perfused with [1,2,3,4-C-13(4)]palmitic acid (M+4), and the isotopic patterns of myocardial acylcarnitines and acyl-CoAs were analyzed using ultra-HPLC-MS/MS. The 91.2% C-13 enrichment in palmitoylcarnitine shows that little endogenous (M+0) palmitate contributed to its formation. The presence of M+2 myristoylcarnitine (95.7%) and M+2 acetylcarnitine (19.4%) is evidence for -oxidation of perfused M+4 palmitic acid. Identical enrichment data were obtained in the respective acyl-CoAs. The relative C-13 enrichment in M+4 (84.7%, 69.9%) and M+6 (16.2%, 17.8%) stearoyl- and arachidylcarnitine, respectively, clearly shows that the perfused palmitate is chain-elongated. The observed enrichment of C-13 in acetylcarnitine (19%), M+6 stearoylcarnitine (16.2%), and M+6 arachidylcarnitine (17.8%) suggests that the majority of two-carbon units for chain elongation are derived from -oxidation of [1,2,3,4-C-13(4)]palmitic acid. These data are explained by conversion of the M+2 acetyl-CoA to M+2 malonyl-CoA, which serves as the acceptor for M+4 palmitoyl-CoA in chain elongation. Indeed, the C-13 enrichment in mitochondrial acetyl-CoA (18.9%) and malonyl-CoA (19.9%) are identical. No C-13 enrichment was found in acylcarnitine species with carbon chain lengths between 4 and 12, arguing against the simple reversal of fatty acid -oxidation. Furthermore, isolated, intact rat heart mitochondria 1) synthesize malonyl-CoA with simultaneous inhibition of carnitine palmitoyltransferase 1b and 2) catalyze the palmitoyl-CoA-dependent incorporation of C-14 from [2-C-14]malonyl-CoA into lipid-soluble products. In conclusion, rat heart has the capability to chain-elongate fatty acids using mitochondria-derived two-carbon chain extenders. The data suggest that the chain elongation process is localized on the outer surface of the mitochondrial outer membrane.