Focusing on brain-tumor phosphates.

Focusing on brain-tumor phosphates

High-energy phosphates are the "energy currency' of cells, fueling everything from protein synthesis to cell movement. Getting an accurate reading of the levels of these molecules is crucial to understanding how cells create and use energy.

Using nuclear magnetic resonance (NMR) spectroscopy, Klaus Roth and his colleagues at the San Francisco Veterans Administration Medical Center have obtained the first noninvasive, quantitative measurements of energy-storing phosphates in human brains and brain tumors. Roth spoke in Chicago at a meeting of the Radiological Society of North America.

NMR spectroscopy, which is based on the behavior of nuclei in an applied magnetic field, is not the only way of quantifying levels of energy-storing molecules. In fact, there are other methods that are much more sensitive. But according to Gregory Karczmar, a physical chemist at the same center, NMR is the only practical way of studying metabolic reactions in vivo without taking biopsies. Moreover, while NMR spectroscopy has been used before to study phosphates in the brain, those studies could only provide ratios of compounds. In contrast, Roth's group can measure absolute levels by comparing the brain's NMR signals with those from a sample placed near the patient's head and containing a known concentration of phosphorus atoms.

"Our goal is to apply these quantitiative measurements to get a quick test of the effectiveness of chemotherapy,' says Roth, whose group is one of a number using NMR to look at energy-storing molecules in living tissue. At present a physician must wait two or three weeks to see whether a tumor has shrunk in response to treatment. "But if chemotherapy works, there are biochemical changes you can observe in the tissue within days,' says Roth. Animal studies show that the high-energy phosphate concentrations reflect most biochemical changes inside cells. If these levels drop, a physician will know that tumor tissue has died and the chemotherapy is working.

Roth also would like to use NMR spectra in humans to detect the early stages and extent of ischemia, in which the flow of oxygen-carrying blood to tissue is blocked. Normally, cells use oxygen to make adenosine triposphate, a high-energy phosphate, but when there isn't enough they use glucose to produce it. This process also produces lactic acid and a drop in pH, which can be measured with NMR by noting the distance between NMR peaks of inorganic phosphorus and phospho-creatine.

Karczmar thinks that basic researchers, such as those studying energy use and fatigue in muscles, will also be interested in Roth's quantitative measurements. Such measurements would enable them to get into the nitty-gritty of cell bioenergetics--which involves various reactions and their properties, such as speed and direction. While the technique itself isn't novel, he says, "Roth has gone much farther than past work by doing the very difficult calculations' necessary for interpreting NMR data.

Photo: From an NMR spectrum, Roth's group can measure the levels of these phosphate compounds: phosphocreatine (PCr), adenosine triphosphate ( , , ), inorganic phosphate (Pi), phosphomonoester (PME) and phosphodiester (PDE).