AMS was used to carbon-date the age of the Shroud of Turin.
Tracing Chemicals
October / November 06
Volume 4 Number 5
It's an inconvenient truth for biomedical and pharmaceutical researchers: lab rats aren't people. Experiments on laboratory animals can go only so far in revealing the effects of carcinogens, pollutants and new drugs on humans.
Fortunately, scientists now have access to a technology—€”originally developed to determine the age of archeological specimens—€”that can accurately determine how toxins, nutrients and other substances interact with human physiology, without risking the health of human subjects.
Accelerator mass spectrometry, or AMS, is the same technology used to carbon-date the age of bones and historical artifacts such as the Shroud of Turin. In the last 15 years, researchers at Lawrence Livermore National Laboratory and their collaborators have been applying the technique to biological research in an increasingly wide variety of studies.
AMS works by tracking the passage of chemicals through humans without disturbing normal metabolic processes. A small quantity, or "microdose," of a substance—€”such as a new drug or vitamin—€”is tagged with a radioactive isotope such as carbon-14 and then ingested by human subjects. Researchers use AMS to measure the number of carbon-14 atoms in samples of urine, feces, saliva or blood over the course of hours, days and weeks. This lets them trace how much of the substance is absorbed, how it travels through the body, what organs it affects and how much is lost through excretion.
"There is a fundamental saying in toxicology that the dose makes the poison," said John Knezovich, director of the lab's Center for Accelerator Mass Spectrometry and the University of California's Toxic Substances Research Program. "Accordingly, we need to know the levels at which a chemical becomes toxic, and AMS gives us a sensitive tool for making this determination."
The technique is remarkably selective and precise: It can, for example, find one carbon-14 isotope among a quadrillion other carbon atoms. The first experiment to trace the vitamin folic acid in a human was able to track a single dose of just 35 micrograms, less than one-tenth of the recommended daily intake, for 200 days.
"The potential applications for biomedical AMS are virtually endless," said Livermore biologist Ken Turteltaub, one of the developers of AMS for biological work. "Because the samples are so small, we can look at isolated clusters of cells, such as breast cancer cells, and validate the results of animal studies.
AMS's sensitivity lets us assess the effects of hormones at low concentrations, and we can label bacteria and viruses and study the early effects of infections. Drug companies can use AMS to help set the right dosage for new medications. With accelerator mass spectrometry we can address problems that can't be solved otherwise."
One such problem is determining how well people, especially the elderly, are able to absorb vitamin B12. An estimated one million Americans over the age of 65 have a condition known as pernicious anemia, which interferes with their ability to properly absorb the vitamin and puts them at risk for developing debilitating fatigue and neurological problems.
The current definitive test for pernicious anemia, called the Schilling test, is cumbersome and unreliable; it requires patients to ingest a low but significant amount of radiation and to collect all urine produced in a 24-hour period.
A new test using AMS, developed by lab researchers and the University of California at Davis, uses a single drop of blood and involves negligible radiation exposure—€”an amount equivalent to that received on an intercontinental flight. The safe, simple test would allow physicians to easily diagnose patients with vitamin B12 absorption problems before anemia, dementia and other permanent damage develop.
"Accelerator mass spectrometry is the most exciting tool to enter clinical development in the past several decades," said Stephen R. Dueker, a former UC Davis nutritional biochemist who is currently the president of Vitalea Science of Woodland, Calif., a new company specializing in commercial applications of biomedical AMS. "The sky is the limit when you can get quantitative information from a single drop of blood," Dueker said.
"We've traced toxins, pesticides and nutrients in humans for academic studies for more than a decade at Lawrence Livermore," said CAMS nuclear physicist Bruce Buchholz, who designed dosimetry and conducted the accelerator mass spectrometry studies. "But this is the first clinical use of AMS."
To measure vitamin B12 absorption with the new test, the researchers had a healthy male subject drink water containing 1.5 micrograms of purified carbon-14-labeled vitamin B12 (14C-B12), an amount of vitamin B12 equivalent to that found in a two-ounce steak. Blood samples were taken every hour for the first 12 hours and then less frequently thereafter to assess absorption of the labeled vitamin. The blood samples were then analyzed at CAMS.
"Our study showed that labeled 14C-B12 is absorbed and metabolized normally and that we have a powerful new tool to assess vitamin B12 absorption," said Ralph Green, professor and chair in the Department of Pathology and Laboratory Medicine at UC Davis School of Medicine. "While Lawrence Livermore National Laboratory operates one of a handful of accelerator mass spectrometers in the United States applied to bioscience, I can envision a day when 14C-B12 is formulated into tablet form and small, 14C-labeled samples of blood from a finger-prick are mailed to a central laboratory for analysis. The new test promises to become the —€˜gold standard' for determining if a person suffers from malabsorption of vitamin B12."
With their initial studies behind them, the researchers have patented the technology and are now planning to conduct additional human studies and license the technology to companies that will make the pernicious anemia test available for practicing physicians.
CAMS was established in 1989 to monitor the spread of nuclear weapons by detecting telltale radioisotopes in air, water and soil samples; to study climate and geological records; and to develop AMS for use in biomedicine. The center is home to one of the world's most powerful and versatile AMS machines, a $2-million, 15-meter-long accelerator used for a wide variety of isotope studies in collaboration with university researchers from around the globe.
Because of its strong record of collaborative research, the center was awarded a National Institutes of Health research resource grant in 1999, which made CAMS the national center for biomedical applications of AMS. In 2001, CAMS scientists started using a smaller AMS machine specifically designed to analyze carbon-14 samples for biomedical research.
Recently CAMS researchers have used the spike in atmospheric carbon-14 resulting from above-ground nuclear testing from 1950 to 1963 to help determine the age of disaster victims and to trace the development of individual human cells. The amount of carbon-14 in tooth enamel, for example, helped forensic scientists narrow the search for victims of the 2004 Southeast Asia tsunami by calculating the age of the victims to an accuracy of within about 1.6 years.
"After a few days in the water, Buchholz said, "it's very hard to identify someone, You can't use (enamel dating) to identify a person that requires a DNA analysis, but you can narrow down the number of people you need to look at from a list of missing people.
"Unlike most other tissue, dental enamel doesn't turn over, Whatever carbon gets laid down in enamel during tooth formation stays there, so tooth enamel is a very good chronometer of the time of formation."
The same technique has allowed scientists to determine the age of DNA in brain cells in cadavers, and to confirm that brain cells live longer than most other cells. The research also shows that unlike other cells such as red blood cells, which often regenerate during a person's lifetime, all the neurons in our brains are generated before birth.
"When it comes to neurons," Buchholz said, "what you're born with is what you've got for life."
Charles Osolin is an LLNL public information officer.
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