Picolinyl ester fragmentation mechanism studies with application to the identification of acylcarnitine acyl groups following transesterification

Catabolism of Branched Chain Amino Acids Contributes Significantly to Synthesis of Odd-Chain and Even-Chain Fatty Acids in 3T3-L1 Adipocytes

The branched chain amino acids (BCAA) valine, leucine and isoleucine have been implicated in a number of diseases including obesity, insulin resistance, and type 2 diabetes mellitus, although the mechanisms are still poorly understood. Adipose tissue plays an important role in BCAA homeostasis by actively metabolizing circulating BCAA. In this work, we have investigated the link between BCAA catabolism and fatty acid synthesis in 3T3-L1 adipocytes using parallel ¹³C-labeling experiments, mass spectrometry and model-based isotopomer data analysis. Specifically, we performed parallel labeling experiments with four fully ¹³C-labeled tracers, [U-¹³C]valine, [U-¹³C]leucine, [U-¹³C]isoleucine and [U-¹³C]glutamine. We measured mass isotopomer distributions of fatty acids and intracellular metabolites by GC-MS and analyzed the data using the isotopomer spectral analysis (ISA) framework. We demonstrate that 3T3-L1 adipocytes accumulate significant amounts of even chain length (C14:0, C16:0 and C18:0) and odd chain length (C15:0 and C17:0) fatty acids under standard cell culture conditions. Using a novel GC-MS method, we demonstrate that propionyl-CoA acts as the primer on fatty acid synthase for the production of odd chain fatty acids. BCAA contributed significantly to the production of all fatty acids. Leucine and isoleucine contributed at least 25% to lipogenic acetyl-CoA pool, and valine and isoleucine contributed 100% to lipogenic propionyl-CoA pool. Our results further suggest that low activity of methylmalonyl-CoA mutase and mass action kinetics of propionyl-CoA on fatty acid synthase result in high rates of odd chain fatty acid synthesis in 3T3-L1 cells. Overall, this work provides important new insights into the connection between BCAA catabolism and fatty acid synthesis in adipocytes and underscores the high capacity of adipocytes for metabolizing BCAA.

cis-3,4-Methylene-heptanoylcarnitine: characterization and verification of the C8:1 acylcarnitine in human urine

Acylcarnitine profiles have been used to diagnose specific inherited metabolic diseases. For some acylcarnitines, however, the detailed structure of their acyl group remains a question. One such incompletely characterized acylcarnitine is cis-3,4-methylene-heptanoylcarnitine. To investigate this problem, we isolated the "C8:1" acylcarnitine from human urine, transesterified it to form its acyl picolinyl ester, and characterized it by GC/EI-MS. These results were compared to GC/EI-MS results from authentic standards we synthesized (cis-3,4-methylene-heptanoylcarnitine, trans-2-octenoylcarnitine, 3-octenoylcarnitine, cis-4-octenoylcarnitine, and trans-4-octenoylcarnitine). Only cis-3,4-methylene-heptanoylcarnitine matched the urinary "C8:1" acylcarnitine. The standards were then spiked in human urine, converted to pentafluorophenacyl esters, and detected by HPLC/MS. cis-3,4-Methylene-heptanoylcarnitine exactly matched the "C8:1" acylcarnitine in urine, whereas none of the other C8:1 acylcarnitine standards matched. Based on the data from GC/EI-MS and HPLC/MS, the "C8:1" acylcarnitine in human urine is shown to be cis-3,4-methylene-heptanoylcarnitine.