Introduction

YING QU, YU-FENG LI, RU BAI, CHUNYING CHEN, ZHIFANG CHAI

1.1 Background

The terms "-ome" and "-omics" have been widely adopted by scientists. The "omics" informally refers to the studies in biology, while the related "-omes" addresses the objects of study in such fields. The suffix "-ome" is thought to derive from the Latin prefix "omni-", meaning total or complete. Thus, "-omes" are intended to be a comprehensive description of all of the relevant components, both known and unknown, in a particular biomolecular subset. "Omes" can provide an easy short-hand to encapsulate a field; for example, a proteome refers to the protein complement of an entire organism, tissue type, or cell, and its associated field "proteomics" is clearly recognizable as relating to the study field of proteins on a large scale. The term "-omics" represents the rigorous study of various collections of molecules, biological processes, or physiological functions and structures as systems.

Compared to the well known genomics and proteomics, metallomics and metalloproteomics are relatively new fields. They are receiving great attention in the investigation of trace elements in biology and expected to develop as an interdisciplinary science complementary to genomics and proteomics. In the first chapter of this book, the history and definition of metallomics and metalloproteomics will be introduced. In addition, the current application of nuclear or nuclear-related analytical techniques for metallomics and metalloproteomics will be overviewed.

1.2 Metallomics and Metalloproteomics

1.2.1 Trace Elements, Chemical Species and Speciation Analysis

The term "trace elements" dates back to the early 20th century, in recognition of the fact that many elements occurred at such low concentrations that their presence could only just be detected. In analytical chemistry, a trace element is an element in a sample that has an average concentration of less than 100 parts per million atoms, or less than 100 µg g-1. In biochemistry, a trace element is a chemical element that is needed in minute quantities for the proper growth, development, and physiology of the organism and it is also referred to as a micronutrient. So far, 117 elements in total have been observed, of which 92 occur naturally on Earth. However, living organisms are composed of about 26 elements, and only six of those 26 make up practically all of the weight of most living things. The other 20 elements essential for life are present in very small amounts, some of them are in such tiny amounts that they are correspondingly called "trace elements". Besides their phenotypic and phylogenetic characteristics, only 11 elements appear to be approximately constant and predominant in all biological systems, which are called major elements. In the human body, these constitute 99.9% of the total number of atoms present, but just four of them (C, O, H, and N) correspond to 99% of the total and the other seven elements (Na, K, Ca, Mg, P, S, and Cl) represent only about 0.9%.

Trace elements play an important role in the functioning of life. Essential trace elements, acting as catalytic or structural components of larger molecules, have specific functions and are indispensable for life. In addition to the long-known deficiencies of iron and iodine, signs of deficiency for chromium, copper, zinc, and selenium have been identified in free-living populations. It is considered that marginal or severe trace element imbalances can be risk factors for several diseases of public health importance. However, the cause and effect relationships will depend on a more complete understanding of basic mechanisms of action, and more importantly, on better analytical procedures and functional tests to determine marginal trace element status in human.

The biological effect of an element is not only dependent on the total concentration, but also highly related to its chemical forms present in biological systems, e.g. the oxidation state, the nature of the ligands or even the molecular structure. Dramatic examples are chromium, tin and mercury, to name just a few. Cr(VI) ions are considered far more toxic than Cr(III). Although the inorganic forms of tin and mercury are less toxic or even do not show toxic properties, the alkylated forms are highly toxic. Dialkylmercury derivatives are considered extremely toxic, while mercuric selenide has a relatively low toxicity and accumulates as an apparently benign detoxification product in marine animals, and methylmercury cysteine proves to be much less toxic than methylmercury chloride in a zebra fish larvae model system. Therefore, to produce qualitative and quantitative information on chemical compounds that affect the quality of life, chemical forms of specific element should be considered carefully.

Nowadays, there is increasing awareness of the importance of the chemical form in which an element is present in biological systems. More and more chemical speciation information on a given element is demanded in most fields of research. In fact, many environmental, toxicological, pharmacological, nutritional, and biological issues today require reliable information on the actual chemical species present, rather than total element concentrations as usually provided by atomic techniques in routine laboratories. In recent years, trace element speciation has become a worldwide trend in current analytical chemistry. Often these different chemical forms of a particular element or its compounds are referred to as "species".

Species and speciation, words borrowed from the biological sciences originally, have been adopted by those in analytical chemistry and accepted in such diverse fields as toxicology, clinical chemistry, geochemistry, and environmental chemistry, expressing the idea that the specific chemical forms of an element should be considered individually. The concept of "speciation" dates back to 1954 when Goldberg introduced it to improve the understanding of the biogeochemical cycling of trace elements in seawater. Since...