Chapter 23 - Metals in the Environment

Abstract

Following a brief introduction to the toxicity of heavy metals introduced into our environment in the course of industrialization, the consequences of a number of nonbiological metal ions on the environment are discussed in biochemical detail. Cd2+ toxicity is directly associated with its perturbation of pathways modulating the homeostasis of essential transition metals, notably zinc, iron and copper. Hg2+, particularly in the form of methylmercury, the cause of Minamata disease in Japan, is a highly specific, irreversible inhibitor of Se-dependent enzymes (selenoenzymes). Since the affinity of Hg for Se is up to a 106 times higher than for sulphur, MeHg sequesters Se directly, impairing selenoenzyme activities and their synthesis. Chronic Pb2+ poisoning has been known since antiquity (even blamed for the fall of the Roman Empire). Pb2+ targets proteins that bind Ca2+ and Zn2+, including δ-aminolaevulinate synthase (haem biosynthesis) and synaptogmin (a Ca2+ sensor in neurotransmission). Al3+, the most abundant metal in the earth’s crust, is released by acid rain into ground water, and its toxicity involves disruption of the homeostasis of essential metal ions, notably Fe, Ca2+ and Mg2+. The sources of environmental metal toxicity are then briefly reviewed and we conclude with a light-hearted (though not for the victims) account of the use of metals like As, Hg, Sb, Pb and Tl, either inadvertently or, as many poisoners knew only too well, deliberately, as poisons.