PHYSICAL METHODS TO LOCATE METAL ATOMS IN BIOLOGICAL-SYSTEMS

Metal elements are involved in a number of cellular processes such as enzyme activation, transmembrane transport, water exchange, signal transduction, and intercellular communication. For instance, the alkaline and alkaline earth cations Na+, K+, Mg2+, and Ca2+ play a key role in the functioning of living systems; Cu, Fe, Zn, or Mn may be associated with proteins; heavy metals and radioactive Sr2+ or Cs+ can be dangerous pollutants; and lithium or cobalt may be used as specific inhibitors of biological processes. Quantitatively mapping, or depth-profiling, metal atoms in biological systems can thus help toward an understanding of physiological mechanisms. A variety of physical methods are progressively being adapted to the study of biological specimens. This chapter discusses X-ray fluorescence, analytical electron microscopy (X-ray microanalysis, electron energy loss spectrometry, and Auger spectrometry), analysis of tracks originating from nuclear events, nuclear probes, and secondary ion imaging methods. Among these different methods, some can detect most chemical elements, whereas others apply to only a few elements. Some are useful mainly on a histological scale, whereas, in other cases, the spatial resolution may be good enough for considering subcellular localizations. In vivo measurements may be possible in a few cases, but most often, the methods are meant for fixed preparations. It is then clearly requisite that the techniques used for sample preparation—fixation, dehydration, and staining—do not disturb the natural distribution of the metal ions under observation. © 1993, Elsevier Inc. All rights reserved.