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Web Date: December 16, 2011

Researchers Spy On The Tricarboxylic Acid Cycle

Medical Imaging: A new imaging probe could help reveal metabolic differences between normal and diseased tissue
Department: Science & Technology
News Channels: Analytical SCENE, Biological SCENE, JACS In C&EN
Keywords: magnetic resonance imaging, magnetic resonance spectroscopy, hyperpolarization, tricarboxylic acid cycle, TCA
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Lighting Up Metabolites
A probe, 1-13C-diethyl succinate, with hydrogen atoms in specific spin states (red) can enter and report on changes to the tricarboxylic acid cycle in mice.
Credit: J. Am. Chem. Soc
20111216lnj1-TCAprobe
 
Lighting Up Metabolites
A probe, 1-13C-diethyl succinate, with hydrogen atoms in specific spin states (red) can enter and report on changes to the tricarboxylic acid cycle in mice.
Credit: J. Am. Chem. Soc

Diseases such as cancer or Alzheimer’s can disrupt the energy-producing pathways inside cells. To help spot these shifts, researchers have developed a small molecule that allows them to use magnetic resonance techniques to monitor metabolism in animals (J. Am. Chem. Soc., DOI: 10.1021/ja2040865). They hope that the probe someday could help researchers and doctors study and diagnose disease.

As one of the major metabolic pathways in cells, the tricarboxylic acid (TCA) cycle steadily converts metabolites from fats, proteins, and sugars into energy-rich molecules. But in tumor cells, the pathway runs differently: Levels of the TCA enzymes and metabolites fluctuate, although researchers have not uncovered the molecular details.

To study changes in a person or animal’s metabolism, clinicians currently rely on imaging techniques like positron emission tomography (PET) and magnetic resonance spectroscopy. But the small molecule probes used in both techniques have limitations, says Pratip Bhattacharya of Huntington Medical Research Institutes, in Pasadena, Calif. PET probes can only provide information about glucose uptake and phosphorylation, two metabolic steps upstream of the TCA cycle. Researchers think that data on the TCA cycle itself would be more diagnostically useful. The small molecules used in magnetic resonance spectroscopy do report on the cycle, but they produce weak signals that require several hours in the magnetic resonance scanner to collect adequate data.

Bhattacharya and his team wanted to develop a magnetic resonance probe that produces a robust signal to relay information on metabolites in the TCA cycle. The molecule they came up with, 1-13C-diethyl succinate, is a labeled analog of succinate, a key intermediate in the TCA pathway. They envisioned that when the molecule slipped inside a cell, it would enter the cycle and the researchers could follow how it changed chemically based on the magnetic resonance signal produced by its 13C atom. To cut down on analysis times, the chemists boosted this signal 5,000-fold through a process called hyperpolarization, in which they placed hydrogen atoms with specific spin states adjacent to the 13C atom.

To test their probe, the scientists injected it into the bloodstream of 13 anesthetized, healthy mice placed inside a magnetic resonance imaging machine. Using two magnetic resonance methods, the researchers could spot the location in the body of the 13C-labeled molecule and determine if it had changed chemically. The data suggested that once the compound is inside the animal, enzymes hydrolyze it into succinate. Over time, other enzymes further metabolize the probe to aspartate, malate, and fumarate, key metabolites of the TCA cycle.

The team also injected an inhibitor of a TCA enzyme into the mice and could spot changes in the magnetic resonance signals minutes later. Bhattacharya says this very quick feedback suggests that someday doctors could use the probe to monitor patients’ response to drugs.

Pierre-Gilles Henry, a neuroscientist at the University of Minnesota, Twin Cities, calls the new probe exciting and points out that the study is the first to spot the metabolites malate and fumarate using magnetic resonance imaging in a live animal. To study neurodegenerative diseases, however, the researchers will need to test whether the probe can cross the blood-brain barrier, he cautions.

 
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