Renal metabolism of hyperpolarized [1-13C]aspartate

Aspartic acid is involved in several central metabolic pathways, including gluconeogenesis, the urea cycle, de novo nucleotide synthesis, the malate-aspartate shuttle, and via transmination the TCA cycle. The conversion in vivo of hyperpolarized [1-13C]pyruvate to aspartate, via pyruvate carboxylation to oxaloacetate, has been reported in rat kidney [1] and in the mouse liver [2]. Additionally, the metabolism of hyperpolarized [1-13C]aspartate to oxaloacetate has been noted in cultured cells supplemented with 2-oxoglutarate [3]. Aspartate is also of interest with its conversion to succinate in ischemic tissue implicated in subsequent reperfusion injury [4]. Here we report initial experiments with hyperpolarized [1-13C]aspartate in the rat kidney. [1-13C]Aspartic acid was formulated with OX063 trityl radical as in [3] as the tris salt, with the addition of glycerol (14% v/v) to improve glassing. The formulation is highly viscous and dissolves poorly, limiting the usable dose. Experiments were performed in a 9.4T animal scanner and a 7T polarizer operating at 1K, with automated rapid transfer and infusion. Rats (n=2) were isoflurane anesthesized, a femoral vein catheterized, and a quadrature 13C surface coil placed over the kidney. Respiration-triggered (TR ~3s) pulse-acquire (BIR4-30o) scans were started at the time of infusion of [1-13C]aspartate (~16 μmol/kg in 1.2 ml). Three metabolites were detected, with chemical shifts of 183.51 ppm (1.5%), 182.31 ppm (0.5%) and 181.26 ppm (0.6%), corresponding to [1-13C]malate, [4-13C]malate and an unidentified species provisionally assigned as N-acetyl[1-13C]aspartate. [4- 13C]aspartate was at natural abundance, but [4-13C]malate results from conversion to fumarate or succinate and label scrambling, and the greater prominence of [1-13C]malate indicates oxaloacetate as an intermediate. These results show the potential of hyperpolarized [1- 13C]aspartate as an in vivo metabolic probe.