Metabolic flux analysis of acetylcarnitine turnover and mitochondrial oxidation of [2-13C]acetate in rat skeletal muscle in vivo measured by 13C MRS

INTRODUCTION: Acetylcarnitine is a necessary intermediate in the mitochondrial oxidation of acetate, since it transports the acetyl moiety across the mitochondrial membrane [1]. It has only been observed using hyperpolarized methods [2,3] or with localized polarization transfer sequences [5]. Glutamate has been used as an indicator of citric acid cycle (TCA) fluxes following [2-C]acetate infusion [6,7,8], however, the detection of [5-C]glutamate is hindered by an overlapping [1-C]acetate resonance in hyperpolarized MRS studies and the detection of the glutamate C3 and C4 resonances are challenging due to large lipid resonances. Here we used localized DEPT at high field to monitor C enrichment and isotope turnover in glutamate and acetylcarnitine in skeletal muscle in vivo following [2-C]acetate infusion. Two different modeling approaches were evaluated to obtain metabolic fluxes of mitochondrial acetate oxidation, either with or without the C labeling of acetylcarnitine and the enzymatic fluxes of acetylCoA synthesase (ACS) and acetylcarnitine transferase (CAT).