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Spin hyperpolarization enables real-time metabolic imaging of carbon-13-labeled substrates. While hyperpolarized L-(1-13 C)alaninamide is a probe of the cell-surface tumor marker aminopeptidase-N (APN, CD13), its activity in vivo has not been described. Scanning the kidneys of rats infused with hyperpolarized alaninamide shows both conversion to [1-13 C]alanine and several additional spectral peaks with distinct temporal dynamics. The (1-13 C)alaninamide chemical shift is pH-sensitive, with a pK a of 7.9 at 37 °C, and the peaks correspond to at least three different compartments of pH 7.46 ± 0.02 (1), 7.21 ± 0.02 (2), and 6.58 ± 0.05 (3). An additional peak was assigned to the carboxyamino adduct formed by reaction with dissolved CO 2 . Spectroscopic imaging showed nonuniform distribution, with the low-pH signal more concentrated in the inner medulla. Treatment with the diuretic acetazolamide resulted in significant pH shifts in compartment 1 to 7.38 ± 0.03 (p = 0.0057) and compartment 3 to 6.80 ± 0.05 (p = 0.0019). While the pH of compartment 1 correlates with blood pH, the pH of compartment 3 did not correspond to the pH of urine. In vitro experiments show that alaninamide readily enters blood cells and can detect intracellular pH. While carbamate formation depends on pH and pCO 2 , the carbamate-toalaninamide ratio did not correlate with either arterial blood pH or pCO 2 , suggesting that it may reflect variations in tissue pH and pCO 2 . This study demonstrates the feasibility of using hyperpolarized sensors to simultaneously image enzyme activity, pCO 2 , and pH in vivo.

D-amino acid oxidase (DAO) is a peroxisomal enzyme that catalyzes the oxidative deamination of several neutral and basic D-amino acids to their corresponding -keto acids. In most mammalian species studied, high DAO activity is found in the kidney, liver, brain and polymorphonuclear leukocytes, and its main function is to maintain low circulating D-amino acid levels. DAO expression and activity have been associated with acute and chronic kidney diseases and with several pathologies related to N-methyl-D-aspartate (NMDA) receptor hypo/hyper-function;however, its precise role is not completely understood. In the present study we show that DAO activity can be detected in vivo in the rat kidney using hyperpolarized D-[1-13 C]alanine.Following a bolus of hyperpolarized D-alanine, accumulation of pyruvate, lactate and bicarbonate was observed only when DAO activity was not inhibited. The measured lactate-to-D-alanine ratio was comparable to the values measured when the L-enantiomer was injected. Metabolites downstream of DAO were not observed when scanning the liver and brain. The conversion of hyperpolarized D-[1-13 C]alanine to lactate and pyruvate was detected in blood ex vivo, and lactate and bicarbonate were detected on scanning the blood pool in the heart in vivo; however, the bicarbonate-to-D-alanine ratio was significantly lower compared with the kidney. These results demonstrate that the specific metabolism of the two enantiomers of hyperpolarized [1-13 C]alanine in the kidney and in the blood can be distinguished, underscoring the potential of D-[1-13 C]alanine as a probe of D-amino acid metabolism. INTRODUCTIONWhile being less abundant than L-amino acids, D-amino acids are widespread in nature and take part in normal mammalian metabolism. Aside from D-serine and D-aspartate, which humans can synthesize endogenously, most D-amino acids are taken up through the diet, and their levels are regulated by D-amino acid oxidase (DAO) and D-aspartate oxidase. 1 DAO, a peroxisomal flavoenzyme, catalyzes the conversion of basic and neutral D-amino acids into the corresponding -keto acids in two steps. First, the D-amino acid is oxidized to an imino acid, and the flavin adenine dinucleotide coenzyme is concomitantly reduced. The imino acid then undergoes non-enzymatic hydrolysis to yield the -keto acid and ammonia. In mammals, DAO is abundantly present in the kidneys, where it is expressed in the epithelial cells of the S2 and S3 segments of the proximal tubules. Although it is also expressed in hepatocytes, in the central nervous system (CNS) and in polymorphonuclear (PMN) leukocytes, the physiological and pathological role of D-amino acids and DAO in these tissues is not completely understood. DAO has been shown to play a detoxifying role in the kidneys, as it helps maintain low D-amino acid levels. 2 Since it controls D-serine levels, DAO activity in the CNS has been associated with conditions affecting N-methyl-D-aspartate (NMDA) receptor signaling/dysfunction, including schizophrenia, 3 amyotrophic lateral sclerosis ...

We present a 1H NMR investigation of spin dynamics in two finite integer spin molecular nanomagnetic rings, namely V7Zn and V7Ni. This study could be put in correlation with the problem of Haldane gap in infinite integer spin chains. While V7Zn is an approximation of a homometallic broken chain due to the presence of s = 0 Zn2+ ion uncoupled from nearest neighbor V2+s = 1 ions, the V7Ni compound constitutes an example of a closed periodical s = 1 heterometallic chain. From preliminary susceptibility measurements on single crystals and data analysis, the exchange coupling constant J/kB results in the order of few kelvin. At room temperature, the frequency behavior of the 1H NMR spin–lattice relaxation rate 1/T1 allowed to conclude that the spin–spin correlation function is similar to the one observed in semi-integer spin molecules, but with a smaller cutoff frequency. Thus, the high-T data can be interpreted in terms of, e.g., a Heisenberg model including spin diffusion. On the other hand, the behavior of 1/T1 vs temperature at different constant fields reveals a clear peak at temperature of the order of J/kB, qualitatively in agreement with the well-known Bloembergen–Purcell–Pound model and with previous results on semi-integer molecular spin systems. Consequently, one can suggest that for a small number N of interacting s = 1 ions (N = 8), the Haldane conjecture does not play a key role on spin dynamics, and the investigated rings still keep the quantum nature imposed mainly by the low number of magnetic centers, with no clear topological effect due to integer spins.

It was recently demonstrated that nonpersistent radicals can be generated in frozen solutions of metabolites such as pyruvate by irradiation with UV light, enabling radical-free dissolution dynamic nuclear polarization. Although pyruvate is endogenous, the presence of pyruvate may interfere with metabolic processes or the detection of pyruvate as a metabolic product, making it potentially unsuitable as a polarizing agent. Therefore, the aim of the current study was to characterize solutions containing endogenously occurring alternatives to pyruvate as UV-induced nonpersistent radical precursors for in vivo hyperpolarized MRI. The metabolites alphaketovalerate (αkV) and alpha-ketobutyrate (αkB) are analogues of pyruvate and were chosen as potential radical precursors. Sample formulations containing αkV and αkB were studied with UV-visible spectroscopy, irradiated with UV light, and their nonpersistent radical yields were quantified with electron spin resonance and compared with pyruvate. The addition of 13 C-labeled substrates to the sample matrix altered the radical yield of the precursors. Using αkB increased the 13 C-labeled glucose liquid-state polarization to 16.3% ± 1.3% compared with 13.3% ± 1.5% obtained with pyruvate, and 8.9% ± 2.1% with αkV. For [1-13 C]butyric acid, polarization levels of 12.1% ± 1.1% for αkV, 12.9% ± 1.7% for αkB, 1.5% ± 0.2% for OX063 and 18.7% ± 0.7% for Finland trityl, were achieved. Hyperpolarized [1-13 C]butyrate metabolism in the heart revealed label incorporation into [1-13 C]acetylcarnitine, [1- 13 C] acetoacetate, [1-13 C]butyrylcarnitine, [5-13 C]glutamate and [5-13 C]citrate. This study demonstrates the potential of αkV and αkB as endogenous polarizing agents for in vivo radical-free hyperpolarized MRI. UV-induced, nonpersistent radicals generated in endogenous metabolites enable high polarization without requiring radical filtration, thus simplifying the quality-control tests in clinical applications.