ACETYL-L-CARNITINE FLUX TO LIPIDS IN CELLS ESTIMATED USING ISOTOPOMERSPECTRAL-ANALYSIS

Acetyl-L-carnitine is known as a reservoir of activated acetyl units a nd as a modulator of metabolic function. The objective of this study w as to quantify the fate of the acetyl moiety of acetyl-L-carnitine in lipogenic pathways. Lipogenesis was studied in an adipocyte model, dif ferentiated 3T3-L1 cells, and a hepatoma cell, HepG2 cells. Lipogenesi s and ketogenesis were examined in rat hepatocytes. Both de novo synth esis and elongation of fatty acids were investigated using gas chromat ography/mass spectrometry and [1,2-C-13]acetyl-L-carnitine. Comparison s were performed with [C-13]glucose and [C-13]acetate. Isotopomer Spec tral Analysis, a stable isotope method for differentiating between the enrichment of the precursor and the amount of synthesis was used to a nalyze the data. Acetyl-L-carnitine was generally less effective than acetate as a precursor for de novo lipogenesis. The effects of acetyl- L-carnitine were not identical to those of acetate plus carnitine as e xpected if acetyl-L-carnitine flux to acetyl CoA is controlled by carn itine acetyl transferase. Acetyl-L-carnitine (2-mM) contributed approx imately 10% of the lipogenic acetyl-CoA used for synthesis and elongat ion as well as 6% of the ketogenic acetyl-CoA. No differences were fou nd between the precursor enrichment for de novo lipogenesis and for el ongation of saturated fatty acids. Flux of acetyl-L-carnitine to lipid was increased, not decreased, by the ATP citrate lyase inhibitor, -hy droxycitrate. In contrast, flux of glucose to lipid was dramatically d ecreased by this inhibitor. These results indicate that flux of acetyl -L-carnitine to lipid can bypass citrate and utilize cytosolic acetyl- CoA synthesis.