The purpose of this study was to investigate the rapid metabolic conversion of hyperpolarized (HP) [1-13C]α-ketobutyrate, a molecular analog of pyruvate, in mouse liver in vivo as compared to [1-13C]pyruvate. Previously, it has been noted that in liver, there is relatively less conversion of [1-13C]α-ketobutyrate to its reduction product, [1-13C]hydroxybutyrate when compared to the conversion of [1-13C]pyruvate to [1-13C]lactate¹. This difference in conversion likely represents a different LDH activity in liver. In this study, we examine the decarboxylation of ketobyrate into bicarbonate, which we have found to be unexpectedly elevated when compared to pyruvate, presumably also via PDH and/or a related enzyme.Protocol was approved by the local institutional animal care and use committee. A sample containing 15 mM of trityl radical OX063 (Oxford Intruments, Miamisburg, OH) and either [1-13C]ketobutyric acid (αkB) (Cambridge Isotopes Laboratories, Andover, MA) or [1-13C]pyruvic acid (Sigma Aldrich, Andover, MA) was polarized in an Oxford HyperSense (Abingdon, UK) operating at 3.35T and 1.3K. During the dissolution process, the frozen sample was rapidly melted using 4.5 ml of a superheated buffer that neutralized the acid for a solution with neutral pH and a final akB or pyruvate concentration of 80mM. Three CD1 mice (7 month-old) were injected with 350 ul of either αkB or pyruvate solution via tail vein catheter over 12 s. The dynamic acquisition of fifteen 13C spectra started 14 s after the beginning of the HP solution injection using a 14T MRI scanner (Varian, Inc) and a volume 1H-13C birdcage coil. Respiratory gating was employed. Acquisition parameters include: 8 mm axial slice placed on the liver, receiver BW=20 kHz, 4 k points, TR=3 s, and flip angle=30º. The ratio of bicarbonate to ketobutyrate and pyruvate signals was computed, as well as ratios of reduction and transamination products to ketobutyrate and pyruvate.The mean ratio of bicarbonate-to-ketobutyrate was 2.8 times greater than the ratio of bicarbonate-to-pyruvate, indicating a much greater extent of decarboxylation of ketobutyrate than pyruvate, via PDH and/or a related enzyme such as branched chain α-keto acid dehydrogenase2. Ketobutyrate has a different route of entry into the TCA cycle than pyruvate. Decarboxylation of ketobutyrate generates propionyl-CoA (instead of acetyl-CoA), feeding the TCA cycle via succinyl-CoA. The mean ratio of reduction product hydroxybutyrate-to-ketobutyrate was 0.65 times smaller than the ratio of lactate to pyruvate, which is expected due to the prevalence of LDHA expressed M subunits of LDH in mouse liver, which exhibit substantial substrate preference for pyruvate. Very little transamination product (α-aminobutyrate) was observed in the spectra. The chronological order of the injections was switched to exclude possible residual effects from the prior injection. The large amount of conversion to bicarbonate indicates a potentially significant ability to monitor both these reduction and decarboxylation activities on a localized basis, which is not possible using HP pyruvate (outside of the heart), due to limited conversion of pyruvate to bicarbonate.