Relationship between gluconeogenesis and phosphoenergetics in rat liver assessed by in vivo 13C and 31P NMR spectroscopy

Applications of NMR spectroscopy to systems biochemistry

The past decades of advancements in NMR have made it a very powerful tool for metabolic research. Despite its limitations in sensitivity relative to mass spectrometric techniques, NMR has a number of unparalleled advantages for metabolic studies, most notably the rigor and versatility in structure elucidation, isotope-filtered selection of molecules, and analysis of positional isotopomer distributions in complex mixtures afforded by multinuclear and multidimensional experiments. In addition, NMR has the capacity for spatially selective in vivo imaging and dynamical analysis of metabolism in tissues of living organisms. In conjunction with the use of stable isotope tracers, NMR is a method of choice for exploring the dynamics and compartmentation of metabolic pathways and networks, for which our current understanding is grossly insufficient. In this review, we describe how various direct and isotope-edited 1D and 2D NMR methods can be employed to profile metabolites and their isotopomer distributions by stable isotope-resolved metabolomic (SIRM) analysis. We also highlight the importance of sample preparation methods including rapid cryoquenching, efficient extraction, and chemoselective derivatization to facilitate robust and reproducible NMR-based metabolomic analysis. We further illustrate how NMR has been applied in vitro, ex vivo, or in vivo in various stable isotope tracer-based metabolic studies, to gain systematic and novel metabolic insights in different biological systems, including human subjects. The pathway and network knowledge generated from NMR- and MS-based tracing of isotopically enriched substrates will be invaluable for directing functional analysis of other 'omics data to achieve understanding of regulation of biochemical systems, as demonstrated in a case study. Future developments in NMR technologies and reagents to enhance both detection sensitivity and resolution should further empower NMR in systems biochemical research.

In vivo (1)H MRS and (31)P MRSI of the response to cyclocreatine in transgenic mouse liver expressing creatine kinase

Hepatocyte transplantation has been explored as a therapeutic alternative to liver transplantation, but a means to monitor the success of the procedure is lacking. Published findings support the use of in vivo (31)P MRSI of creatine kinase (CK)-expressing hepatocytes to monitor proliferation of implanted hepatocytes. Phosphocreatine tissue level depends upon creatine (Cr) input to the CK enzyme reaction, but Cr measurement by (1)H MRS suffers from low signal-to-noise ratio (SNR). We examine the possibility of using the Cr analog cyclocreatine (CCr, a substrate for CK), which is quickly phosphorylated to phosphocyclocreatine (PCCr), as a higher SNR alternative to Cr. (1)H MRS and (31)P MRSI were employed to measure the effect of incremental supplementation of CCr upon PCCr, γ-ATP, pH and Pi /ATP in the liver of transgenic mice expressing the BB isoform of CK (CKBB) in hepatocytes. Water supplementation with 0.1% CCr led to a peak total PCCr level of 17.15 ± 1.07 mmol/kg wet weight by 6 weeks, while adding 1.0% CCr led to a stable PCCr liver level of 18.12 ± 3.91 mmol/kg by the fourth day of feeding. PCCr was positively correlated with CCr, and ATP concentration and pH declined with increasing PCCr. Feeding with 1% CCr in water induced an apparent saturated level of PCCr, suggesting that CCr quantization may not be necessary for quantifying expression of CK in mice. These findings support the possibility of using (31)P MRS to noninvasively monitor hepatocyte transplant success with CK-expressing hepatocytes.

Gluconeogenesis and phosphoenergetics in rat liver during endotoxemia

BACKGROUND

During endotoxemia, glucose and energy metabolism varies depending on the stage, severity, and other conditions. In this study, gluconeogenesis from 13C-labeled alanine and phosphoenergetic state in rat liver during the acute phase of endotoxemia were concurrently observed by in vivo 13C and 31P NMR spectroscopy in a noninvasive manner.

MATERIALS AND METHODS

Lipopolysaccharide from Escherichia coli (10 mg/kg) was injected intravenously followed by infusion of [3-13C]alanine. In vivo 13C and 31P NMR spectra were alternately collected for 90 min with a 2.0 Tesla CSI Omega System.

RESULTS

In our experimental model, endotoxin increased the pulse rate without decreasing the blood pressure and elevated the blood sugar level, which suggests the so-called hyperdynamic state. Even under such conditions, a slight, but significant, impairment of the phosphoenergetic state in the liver (a decrease in ATP and an increase in Pi) was detected with 31P NMR spectroscopy. The 13C peaks of glucose C6 and Glu/Gln C2 of the liver in endotoxemia were significantly lower than those of the control, despite hyperglycemia in endotoxemia.

CONCLUSIONS

NMR spectroscopic studies suggest that the endotoxin caused the inhibition of gluconeogenic activity from the infused [3-13C]alanine and the TCA cycle accompanied by a deterioration in the phosphoenergetic state even in the hyperglycemic phase. Since the blood sugar level might be influenced by the systemic utilization of glucose, such direct measurements should prove important in the in vivo evaluation of glucose and energy metabolism in the liver.