In vivo enzymatic activity of acetylCoA synthetase in skeletal muscle revealed by 13C turnover from hyperpolarized [1-13C]acetate to [1-13C]acetylcarnitine

Abstract

a b s t r a c t a r t i c l e i n f oC]acetylcarnitine were recorded in vivo for 1 min. The kinetic rate constants related to the transformation of acetate into acetylcarnitine were deduced from the 3 s time resolution measurements using two approaches, either mathematical modeling or relative metabolite ratios. Conclusions: The conversion rates from hyperpolarized acetate into acetylcarnitine were quantified in vivo and, although separated by two enzymatic reactions, these rates uniquely defined the activity of ACS. The conversion rates associated with ACS were obtained using two analytical approaches, both methods yielding similar results. General significance: This study demonstrates the feasibility of directly measuring ACS activity in vivo and, since the activity of ACS can be affected by various pathological states such as cancer or diabetes, the proposed method could be used to non-invasively probe metabolic signatures of ACS in diseased tissue.

Direct noninvasive estimation of myocardial tricarboxylic acid cycle flux in vivo using hyperpolarized 13C magnetic resonance

“…13 ACE, 13 COA, 13 CAR, and 13 CIT represent the size of the corresponding 13 [35] and the assumption that the 13 C fractional enrichment of acetylcarnitine is proportional to the one of acetylCoA, i.e. 13 COA COA ¼ α 13 CAR CAR , which is supported by the rapid 13 C exchange between acetylCoA and acetylcarnitine observed by Schroeder et al [47], the system simplifies to (Fig.…”

“…The signal decay of precursor and metabolites results from a combination of the effects of longitudinal relaxation (characterized by the time constant T 1 ), RF excitation, and biochemical conversion and all time courses were corrected for the effect of repeated RF excitations. The T 1 of [1-13 C]acetylcarnitine was set to 14.9 s, as previously determined in vivo [35], and that of [5][6][7][8][9][10][11][12][13] C]citrate to 20 s [46]. The acetate signal decay was treated as free parameter.…”

“…All acquisitions were cardiac triggered and respiratory gated, resulting in a nominal T R between 3 and 3.5 s. The adiabatic pulse offset and power were calibrated to ensure that the RF excitation angle θ = 30°for all observed metabolites in the entire tissue of interest. Following in vivo data acquisition, a 200 μL liquid sample was extracted from the separator/infusion pump to retrospectively determine the precise injected [1-13 C]acetate concentration [35]. The blood volume of the animals was obtained using the relation between total blood volume and body weight published by Lee and Blaufox [44] allowing calculation of the blood 13 C acetate concentration.…”

“…The value of α is not required for determining V TCA , but based on the previous estimation that 60% of pyruvate-derived acetylCoA cycled through the acetylcarnitine pool before citrate formation [47], the factor α is expected to be at most 1/0.6. This mathematical model is an extension of the model we have previously used to analyze hyperpolarized acetate metabolism in skeletal muscle [35] and in the heart [48]. As for most 13 C MRS experiments, the signals originating from different cellular compartments cannot be separated and the model assumes that all processes take place within the same cellular compartment.…”

“…Unlike pyruvate, acetate uptake is independent of PDH activity. A study of hyperpolarized [1-13 C]acetate metabolism in resting rat skeletal muscle showed that the rate of acetylcarnitine 13 C labeling can be directly translated to the metabolic flux through ACS [35]. However, despite the fact that [1-13 C]acetylCoA was detected in the mouse heart and liver following the injection of hyperpolarized [1-13 C]acetate [36], 13 C label incorporation from acetate into myocardial TCA cycle intermediates has to date not been reported in vivo.…”