Metabolism of hyperpolarized ¹³C‐acetoacetate to β‐hydroxybutyrate detects real‐time mitochondrial redox state and dysfunction in heart tissue

Abstract

Mitochondrial dysfunction is considered to be an important component of many metabolic diseases yet there is no reliable imaging biomarker for monitoring mitochondrial damage in vivo. A large prior literature on inter‐conversion of β‐hydroxybutyrate and acetoacetate indicates that the process is mitochondrial and that the ratio reflects a specifically mitochondrial redox state. Therefore, the conversion of [1,3‐13C]acetoacetate to [1,3‐13C]β‐hydroxybutyrate is expected to be sensitive to the abnormal redox state present in dysfunctional mitochondria. In this study, we examined the conversion of hyperpolarized (HP) 13C‐acetoacetate (AcAc) to 13C‐β‐hydroxybutyrate (β‐HB) as a potential imaging biomarker for mitochondrial redox and dysfunction in perfused rat hearts. Conversion of HP‐AcAc to β‐HB was investigated using 13C magnetic resonance spectroscopy in Langendorff‐perfused rat hearts in four groups: control, global ischemic reperfusion, low‐flow ischemic, and rotenone (mitochondrial complex‐I inhibitor)‐treated hearts. We observed that more β‐HB was produced from AcAc in ischemic hearts and the hearts exposed to complex I inhibitor rotenone compared with controls, consistent with the accumulation of excess mitochondrial NADH. The increase in β‐HB, as detected by 13C MRS, was validated by a direct measure of tissue β‐HB by 1H nuclear magnetic resonance in tissue extracts. The redox ratio, NAD+/NADH, measured by enzyme assays of homogenized tissue, also paralleled production of β‐HB from AcAc. Transmission electron microscopy of tissues provided direct evidence for abnormal mitochondrial structure in each ischemic tissue model. The results suggest that conversion of HP‐AcAc to HP‐β‐HB detected by 13C‐MRS may serve as a useful diagnostic marker of mitochondrial redox and dysfunction in heart tissue in vivo.

Beta-Hydroxybutyrate, Friend or Foe for Stressed Hearts

“…Although most studies focus exclusively on β-OHB, the ketone body metabolism in the heart includes utilization of both β-OHB and acetoacetate, with the ratio of β-OHB to acetoacetate representing the mitochondrial NADH to NAD ratio (Chen et al, 2019). Acetoacetate's utilization is also increased in diabetic rat hearts (Abdurrachim et al, 2019b), which can enhance contractile function via an anti-oxidation mechanism (Squires et al, 2003).…”

Hyperpolarized ketone body metabolism in the rat heart

“…Such probes may therefore be considered to be inefficient for the study of ketone body metabolism, motivating the desire to look at the utilization of ketone bodies directly. We have therefore built on previous work and reported conference abstracts exploring the hyperpolarization of ketone bodies, and report a method to generate hyperpolarized [1‐ 13 C]acetoacetate and [1‐ 13 C]β‐hydroxybutyrate. We further investigate the metabolism of [1‐ 13 C]β‐hydroxybutyrate in the isolated, perfused rat heart, and [1‐ 13 C]acetoacetate in both the perfused heart and the rat heart in vivo .…”

Detection of myocardial medium‐chain fatty acid oxidation and tricarboxylic acid cycle activity with hyperpolarized [1–13C]octanoate

“…We report here for the first time the myocardial metabolism of hyperpolarized [1– 13 C]octanoate, with [1– 13 C]acetylcarnitine the most prominent metabolite. [1– 13 C]acetylcarnitine is similarly prominent with other hyperpolarized precursors of [1– 13 C]acetyl‐CoA, including [2‐ 13 C]pyruvate, [1– 13 C]acetate, [1– 13 C]butyrate, 3‐hydroxy[1– 13 C]butyrate, and [1– 13 C]‐ and [3‐ 13 C]acetoacetate . However, each follows a different route to acetyl‐CoA.…”