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L1618Ch10Frame Page 211 Tuesday, August 13, 2002 5:47 PM CHAPTER 10 Toxic Elements 10.1 INTRODUCTION It is somewhat difficult to define what is meant by a toxic element. Some elements, such as white phosphorus, chlorine, and mercury, are quite toxic in the elemental state. Others, such as carbon, nitrogen, and oxygen, are harmless as usually encountered in their normal elemental forms. But, with the exception of those noble gases that do not combine chemically, all elements can form toxic compounds. A prime example is hydrogen cyanide. This extremely toxic compound is formed from three elements that are nontoxic in the uncombined form, and produce compounds that are essential constituents of living matter, but when bonded together in the simple HCN molecule constitute a deadly substance. The following three categories of elements are considered here: • Those that are notable for the toxicities of most of their compounds • Those that form very toxic ions • Those that are very toxic in their elemental forms Elements in these three classes are discussed in this chapter as toxic elements, with the qualification that this category is somewhat arbitrary. With a few exceptions, elements known to be essential to life processes in humans have not been included as toxic elements. 10.2 TOXIC ELEMENTS AND THE PERIODIC TABLE It is most convenient to consider elements from the perspective of the periodic table, which is shown in Figure 1.3 and discussed in Section 1.2. Recall that the three main types of elements, based on their chemical and physical properties as determined by the electron configurations of their atoms, are metals, nonmetals, and metalloids. Metalloids (B, Si, Ge, As, Sb, Te, At) show some characteristics of both metals and nonmetals. The nonmetals consist of those few elements in groups 4A to 7A above and to the right of the metalloids. The noble gases, only some of which form a limited number of very unstable chemical compounds of no toxicological significance, are in group 8A. All the remaining elements, including the lanthanide and actinide series, are metals. Elements in the periodic table are broadly distinguished between representative elements in the A groups of the periodic table and transition metals constituting the B groups, the lanthanide series, and the actinide series. Copyright © 2003 by CRC Press LLC L1618Ch10Frame Page 212 Tuesday, August 13, 2002 5:47 PM 10.3 ESSENTIAL ELEMENTS Some elements are essential to the composition or function of the body. Since the body is mostly water, hydrogen and oxygen are obviously essential elements. Carbon (C) is a component of all life molecules, including proteins, lipids, and carbohydrates. Nitrogen (N) is in all proteins. The other essential nonmetals are phosphorus (P), sulfur (S), chlorine (Cl), selenium (Se), fluorine (F), and iodine (I). The latter two are among the essential trace elements that are required in only small quantities, particularly as constituents of enzymes or as cofactors (nonprotein species essential for enzyme function). The metals present in macro amounts in the body are sodium (Na), potassium (K), and calcium (Ca). Essential trace elements are chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), copper (Cu), zinc (Zn), magnesium (Mg), molybdenum (Mo), nickel (Ni), and perhaps more elements that have not yet been established as essential. 10.4 METALS IN AN ORGANISM Metals are mobilized and distributed through environmental chemical processes that are strongly influenced by human activities. A striking example of this phenomenon is illustrated by the lead content of the Greenland ice pack. Starting at very low levels before significant industrialization had occurred, the lead content of the ice increased in parallel with the industrial revolution, showing a strongly accelerated upward trend beginning in the 1920s, with the introduction of lead into gasoline. With the curtailment of the use of leaded gasoline, some countries are now showing decreased lead levels, a trend that hopefully will extend globally within the next several decades. Metals in the body are almost always in an oxidized or chemically combined form; mercury is a notable exception in that elemental mercury vapor readily enters the body through the pulmonary route. The simplest form of a chemically bound metal in the body is the hydrated cation, of which + Na(H2O)6 is the most abundant example. At pH values ranging upward from somewhat less than seven (neutrality), many metal ions tend to be bound to one or more hydroxide groups; an example + + is iron(II) in Fe(OH)(H2O)5 . Some metal ions have such a strong tendency to lose H that, except at very low pH values, they exist as the insoluble hydroxides. A common example of this phenom- enon is iron(III), which is very stable as the insoluble hydrated iron(III) oxide, Fe2O3·xH2O, or hydroxide, Fe(OH)3. Metals can bond to some anions in body fluids. For example, in the strong hydrochloric acid medium of the stomach, some iron(III) may be present as HFeCl4, where the acid in the stomach prevents formation of insoluble Fe(OH)3 and a high concentration of chloride ion is available to bond to iron(III). Ion pairs may exist that consist of positively charged metal cations and negatively charged anions endogenous to body fluids. These do not involve covalent bonding between cations and anions, but rather an electrostatic attraction, such as in the ion pairs 2+ ¯ 2+ ¯ Ca HCO 3 or Ca Cl . 10.4.1 Complex Ions and Chelates With the exception of group lA metals and the somewhat lesser exception of group 2A metals, there is a tendency for metals to form complexes with electron donor functional groups on ligands consisting of anionic or neutral inorganic or organic species. In such cases, covalent bonds are formed between the central metal ion and the ligands. Usually the resulting complex has a net – charge and is called a complex ion; FeCl4 is such an ion. In many cases, an organic ligand has two or more electron donor functional groups that may simultaneously bond to a metal ion to form a complex with one or more rings in its structure. A ligand with this capability is called a chelating agent, and the complex is a metal chelate. Copper(II) ion forms such a chelate with the anion of the amino acid glycine, as shown in Figure 10.1. This chelate is very stable. Copyright © 2003 by CRC Press LLC L1618Ch10Frame Page 213 Tuesday, August 13, 2002 5:47 PM H O H O H H CC – CC NO* NO * H H H H + Cu2+ Cu H – H H H * O N * O N CC H CC H O H O H Glycinate anions Copper chelate Figure 10.1 Chelation of Cu2+ by glycinate anion ligands to form the glycinate chelate. Each electron donor group on the glycinate anion chelating agents is designated with an asterisk. In the chelate, the central copper(II) metal ion is bonded in four places and the chelate has two rings composed of the five-atom sequence Cu–O–C–C–N. Organometallic compounds constitute a large class of metal-containing species with properties quite different from those of the metal ions. These are compounds in which the metal is covalently bonded to carbon in an organic moiety, such as the methyl group, –CH3. Unlike metal complexes, which can reversibly dissociate to the metal ions and ligands, the organic portions of organometallic compounds are not normally stable by themselves. The chemical and toxicological properties of organometallic compounds are discussed in detail in Chapter 12, so space will not be devoted to them here. However, it should be mentioned that neutral organometallic compounds tend to be lipid soluble, a property that enables their facile movement across biologic membranes. They often remain intact during movement through biological systems and so become distributed in these systems as lipid-soluble compounds. A phenomenon not confined to metals, methylation is the attachment of a methyl group to an element and is a significant natural process responsible for much of the environmental mobility of some of the heavier elements. Among the elements for which methylated forms are found in the environment are cobalt, mercury, silicon, phosphorus, sulfur, the halogens, germanium, arsenic, selenium, tin, antimony, and lead. 10.4.2 Metal Toxicity Inorganic forms of most metals tend to be strongly bound by protein and other biologic tissue. Such binding increases bioaccumulation and inhibits excretion. There is a significant amount of tissue selectivity in the binding of metals. For example, toxic lead and radioactive radium are accumulated in osseous (bone) tissue, whereas the kidneys accumulate cadmium and mercury. Metal ions most commonly bond with amino acids, which may be contained in proteins (including enzymes) or polypeptides. The electron-donor groups most available for binding to metal ions are amino and carboxyl groups (see Figure 10.2). Binding is especially strong for many metals to thiol (sulfhydryl) groups; this is particularly significant because the –SH groups are common components of the active sites of many crucial enzymes, including those that are involved in cellular energy output and oxygen transport. The amino acid that usually provides –SH groups in enzyme active sites is cysteine, as shown in Figure 10.2. The imidazole group of the amino acid histidine is a common feature of enzyme active sites with strong metal-binding capabilities. The absorption of metals is to a large extent a function of their chemical form and properties. Pulmonary intake results in the most facile absorption and rapid distribution through the circulatory system. Absorption through this route is often very efficient when the metal is in the form of respirable particles less than 100 µm in size, as volatile organometallic compounds (see Chapter 12) or (in the case of mercury) as the elemental metal vapor.

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