Response to Burgess

Assessment of Hepatic Mitochondrial Oxidation and Pyruvate Cycling in NAFLD by 13C Magnetic Resonance Spectroscopy

“…While it is well established that infusions of 13 C acetate over the course of several hours will lead to significant 13 C labeling in plasma glutamine (Landau et al, 1993; Schumann et al, 1991) our 13 C MRS studies were performed over a relatively short duration (<100 minutes), which resulted in negligible 13 C labeling of hepatic [5- 13 C]glutamine (Fig. 1, insert) (Befroy et al, 2015). Furthermore our in vivo MRS method for estimating rates of hepatic mitochondrial oxidation depends primarily on the incorporation of 13 C label from [1- 13 C]acetate into hepatic [5- 13 C]glutamate, which is independent of any 13 C labeling into hepatic CO 2 (Befroy et al, 2014, Befroy et al, 2015).…”

“…Rates of hepatic mitochondrial oxidation were assessed in both groups by monitoring the rate of 13 C label incorporation into hepatic [5- 13 C]glutamate and [1- 13 C]glutamate by in vivo 13 C MRS during an intravenous infusion of [1- 13 C]acetate as previously described (Befroy et al, 2014). Our measurements of rates of hepatic mitochondrial oxidation depend primarily on the kinetics of enrichment in hepatic [5- 13 C]glutamate and are independent of hepatic anaplerotic flux (V ANA ) and hepatic pyruvate kinase flux (V PK ) rates (Befroy et al, 2014; Befroy et al, 2015). Following the start of the [1- 13 C]acetate infusion plasma 13 C acetate enrichments (Supplemental Fig.…”

“…We also directly assessed rates of hepatic pyruvate cycling defined by: (V PK +V ME )/(V PC +V PDH ), where V PK is pyruvate kinase flux, V ME is malic enzyme flux, V PC is pyruvate carboxylase flux and V PDH is pyruvate dehydrogenase flux, by monitoring the relative 13 C labeling in hepatic [2- 13 C]alanine relative to hepatic [2- 13 C]glutamate during an intravenous infusion of [3- 13 C]lactate (Befroy et al, 2015; Perry et al, 2016). Following the start of the [3- 13 C]lactate infusion plasma 13 C lactate enrichments (Supplemental Fig.…”

“…1, insert) (Befroy et al, 2015). Furthermore our in vivo MRS method for estimating rates of hepatic mitochondrial oxidation depends primarily on the incorporation of 13 C label from [1- 13 C]acetate into hepatic [5- 13 C]glutamate, which is independent of any 13 C labeling into hepatic CO 2 (Befroy et al, 2014, Befroy et al, 2015). …”

“…In addition given the very high rates of hepatic pyruvate cycling previously reported by Sunny et al we also directly assessed rates of hepatic pyruvate cycling relative to anaplerosis in subgroups of Control and NAFLD subjects. This was accomplished by monitoring the 13 C labeling in hepatic [2- 13 C]alanine relative to hepatic [2- 13 C]glutamate during an intravenous infusion of [3- 13 C]lactate (Befroy et al, 2015, Perry et al, 2016). …”