AMS as a Tool in the Biological Sciences

Radioisotope labeling has been an important tool in the biological sciences and will continue to be used for many complex problems. Such studies have traditionally been carried out by labeling chemicals of interest with specific rare radioisotopes of elements typically found in organics (³H, ¹¹C, ¹⁴C, ³²P, and ³⁵S for example) because they can usually be incorporated into biomolecules without modifying their natural properties and they have low natural abundances. Isotope quantification is also advantageous since it is independent of the physical properties of the labeled chemical and is distinctive in a complex biological matrix.

Detection of radioisotopes is traditionally performed by decay-counting, which is often limited by high backgrounds, as well as low specificity and low decay counting efficiency. The possible advantages of mass spectrometry (MS) (where individual nuclei are counted independent of decay) for long-lived radioisotope detection relative to decay counting have been recognized for 30 years. However, until the advent of AMS, measurement of ¹⁴C by MS has been fraught with difficulty, mostly due to problems in resolution and isobaric interference.

Over the last ten years, AMS has evolved as a biomedical tool, offering the required sensitivity, selectivity, and precision to address questions that alternative methodologies have been unable to achieve in practice.

AMS was originally applied in the life sciences to overcome limitations in detection sensitivity for studying the molecular damage caused by exposure to low levels of environmental carcinogens and pollutants. For example, AMS can be used to conduct metabolite analysis at the picomole to the attomole level and is also being used to identify macromolecular targets for drugs and toxic compounds.

The high sensitivity of AMS is allowing our collaborators to address important issues in nutrition, pharmacology and comparative medicine. For example, it has been possible to trace a physiological dose of beta-carotene in humans for 200 days. In addition, the synergy between pyridostigmine, a nerve agent, and a pyrethrin pesticide has been quantified in rats treated at human equivalent dosages. AMS can also help to establish the human relevance of animal models because the metabolism of xenobiotics can be studied directly in humans. In fact, initial studies using ¹⁴C-labeled agents suggest that activities as low as a few nCi/person can be used to assess metabolism, and activities as low as 100 nCi/person can be used to address macromolecular binding in the study of candidate drugs or toxicants. This level of radioactivity is less than that from a single day’s exposure to background ionizing radiation, or a chest x-ray.

Similar applications can be envisioned in other disciplines. In general, the use of smaller samples should allow the study of exfoliated tissues, isolated cell subpopulations, and precious tissues of human or animal origin. The high sensitivity of AMS also allows for the quantitative study of ligand—macromolecule interactions at physiologically relevant concentrations, for studying effects such as hormones at low concentrations or where the receptor is present in low copy number, and for studying early events in the pathology of infection by labeling bacteria and viruses. The increased sensitivity should also facilitate the use of compounds that are difficult to synthesize at high specific activity or cannot be used in large amounts. This Resource seeks to both develop the methodology and instrumentation to make AMS into a general use tool for biomedical researchers and to make it available for investigators needing access to techniques for the ultra-trace analysis of radioisotopes in biological studies.

For information about this page contact:
Karolyn Burkhart-Schultz, burkhartschultz1@llnl.gov
UCRL-MI-140299