Cadmium: From Toxicity to Essentiality

Cadmium is known for its toxicity in animals and man as it is not used in these species. Its only role in biology is as a zinc replacement at the catalytic site of a particular class of carbonic anhydrases in some marine diatoms. The toxicity of cadmium continues to be a significant public health concern as cadmium enters the food chain and it is taken up by tobacco smokers. The biochemical basis for its toxicity has been the objective of research for over 50 years. Cadmium damages the kidneys, the lungs upon inhalation, and interferes with bone metabolism. Evidence is accumulating that it affects the cardiovascular system. Cadmium is classified as a human carcinogen. It generates oxidative stress. This chapter discusses the chemistry and biochemistry of cadmium(II) ions, the only important state of cadmium in biology. This background is needed to interpret the countless effects of cadmium in laboratory experiments with cultured cells or with animals with regard to their significance for human health. Evaluation of the risks of cadmium exposure and the risk factors that affect cadmium’s biological effects in tissues is an on-going process. It appears that the more we learn about the biochemistry of cadmium and the more sensitive assays we develop for determining exposure, the lower we need to set the upper limits for exposure to protect those at risk. But proper control of cadmium’s presence and interactions with living species and the environment still needs to be based on improved knowledge about the mechanisms of cadmium toxicity; the gaps in our knowledge in this area are discussed herein.

Cadmium is at the end of the 4d-transition series, it is relatively mobile and acutely toxic to almost all forms of life. In this review we present a summary of information describing cadmium’s physical and chemical properties, its distribtion in crustal materials, and the processes, both natural and anthropogenic, that contribute to the metal’s mobilization in the biosphere. The relatively high volatility of Cd metal, its large ionic radius, and its chemical speciation in aquatic systems makes Cd particularly susceptible to mobilization by anthropogenic and natural processes. The biogeochemical cycle of Cd is observed to be significantly altered by anthropogenic inputs, especially since the beginning of the industrial revolution drove increases in fossil fuel burning and non-ferrous metal extraction. Estimates of the flux of Cd to the atmosphere, its deposition and processing in soils and freshwater systems are presented. Finally, the basin scale distribution of dissolved Cd in the ocean, the ultimate receptacle of Cd, is interpreted in light of the chemical speciation and biogeochemical cycling of Cd in seawater. Paradoxically, Cd behaves as a nutrient in the ocean and its cycling and fate is intimately tied to uptake by photosynthetic microbes, their death, sinking and remineralization in the ocean interior. Proximate controls on the incorporation of Cd into biomass are discussed to explain the regional specificity of the relationship between dissolved Cd and the algal nutrient phosphate (PO\(^{3-}_{4} \)) in oceanic surface waters and nutriclines. Understanding variability in the Cd/PO\( ^{3-}_{4} \) is of primary interest to paleoceanographers developing a proxy to probe the links between nutrient utilization in oceanic surface waters and atmospheric CO₂ levels. An ongoing international survey of trace elements and their isotopes in seawater will undoubtedly increase our understanding of the deposition, biogeochemical cycling and fate of this enigmatic, sometimes toxic, sometimes beneficial heavy metal.

This chapter reports an analysis of literature dedicated to the speciation of cadmium in various environmental compartments, i.e., atmosphere, natural waters, soils and sediments. The difficulty of the cadmium speciation studies, due to the variability of composition of different natural systems and to the low cadmium concentration in the environment, is highlighted. As an alternative approach, cadmium behavior is assessed by modelling its reactivity towards the main classes of ligands usually present in natural systems. The stability of cadmium complexes with various ligand classes is analyzed and modelled. Simple equations are proposed for the estimation of the stability of cadmium complexes with carboxylates, amines, amino acids, complexones, phosphates, phosphonates, and thiolates. The modelling ability of these equations is carefully analyzed. In addition, the sequestering ability of some ligands toward cadmium has been evaluated by the calculation of pL_0.5 (the total ligand concentration, as –log c _L, able to bind 50% of a metal cation), an empirical parameter recently proposed for an objective “quantification” of this ability in defined conditions (pH, ionic strength, temperature, composition of solution).

Analyses of cadmium concentrations in biological material are performed using inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption spectrometry (AAS), but also electrochemical methods, neutron activation analysis (NAA), and X-ray fluorescence spectrometry (XRF). The predominant sample matrices include blood, plasma, serum, and urine, as well as hair, saliva, and tissue of kidney cortex, lung, and liver. While cadmium in blood reveals rather the recent exposure situation, cadmium in urine reflects the body burden and is an indicator for the cumulative long term exposure.

After chronic exposure, cadmium accumulates in the human body and causes kidney diseases, especially lesions of proximal tubular cells. A tubular proteinuria causes an increase in urinary excretion of microproteins. Excretions of retinol binding protein (RBP), β2-microglobulin (β2-M), and α1-microglobulin are validated biomarkers for analyzing cadmium effects. For this purpose, immunological procedures such as ELISA, and radio- and latex-immunoassays are used.

However, proteinuria is not specific to cadmium, but can also occur after exposure to other nephrotoxic agents or due to various kidney diseases. In summary, cadmium in urine and blood are the most specific biomarkers of cadmium exposure. A combination of parameters of exposure (cadmium in blood, cadmium in urine) and parameters of effect (e.g., β2-M, RBP) is required to reveal cadmium-induced nephrological effects.

Cadmium is one of the highly toxic transition metals for human beings and is known as a human carcinogen. Once humans are exposed to Cd²⁺ on a chronic basis, Cd²⁺ primarily accumulates in the liver and kidney where it forms complexes with small peptides and proteins via sulfhydryl groups. Complexed Cd²⁺ or the ionic Cd²⁺ is then taken up by target cells and tissues and exerts the toxicity. However, the question of how non-essential Cd²⁺ crosses the cell membranes remains unanswered. Furthermore, the molecular mechanism of Cd²⁺-induced physiological signaling disruption in cells is still not fully elucidated. Investigations of Cd²⁺ uptake kinetics, distributions, and concentrations in cells require chemical tools for its detection. Because of the easy use and high spatiotemporal resolution, optical imaging using fluorescence microscopy is a well-suited method for monitoring Cd²⁺ in biological samples. This chapter summarizes design principles of small molecule fluorescent sensors for Cd²⁺ detection in aqueous solution and their photophysical and metal-binding properties. Also the applications of probes for fluorescence imaging of Cd²⁺ in a variety of cell types are demonstrated.

Our laboratories have actively published in this area for several years and the objective of this chapter is to present as comprehensive an overview as possible. Following a brief review of the basic principles associated with ¹¹³Cd NMR methods, we will present the results from a thorough literature search for ¹¹³Cd chemical shifts from metalloproteins. The updated ¹¹³Cd chemical shift figure in this chapter will further illustrate the excellent correlation of the ¹¹³Cd chemical shift with the nature of the coordinating ligands (N, O, S) and coordination number/geometry, reaffirming how this method can be used not only to identify the nature of the protein ligands in uncharacterized cases but also the dynamics at the metal binding site. Specific examples will be drawn from studies on alkaline phosphatase, Ca²⁺ binding proteins, and metallothioneins.

In the case of Escherichia coli alkaline phosphatase, a dimeric zinc metalloenzyme where a total of six metal ions (three per monomer) are involved directly or indirectly in providing the enzyme with maximal catalytic activity and structural stability, ¹¹³Cd NMR, in conjunction with ¹³C and ³¹P NMR methods, were instrumental in separating out the function of each class of metal binding sites. Perhaps most importantly, these studies revealed the chemical basis for negative cooperativity that had been reported for this enzyme under metal deficient conditions. Also noteworthy was the fact that these NMR studies preceeded the availability of the X-ray crystal structure.

In the case of the calcium binding proteins, we will focus on two proteins: calbindin D_9k and calmodulin. For calbindin D_9k and its mutants, ¹¹³Cd NMR has been useful both to follow actual changes in the metal binding sites and the cooperativity in the metal binding. Ligand binding to calmodulin has been studied extensively with ¹¹³Cd NMR showing that the metal binding sites are not directly involved in the ligand binding. The ¹¹³Cd chemical shifts are, however, exquisitely sensitive to minute changes in the metal ion environment.

In the case of metallothionein, we will reflect upon how ¹¹³Cd substitution and the establishment of specific Cd to Cys residue connectivity by proton-detected heteronuclear ¹H-¹¹³Cd multiple-quantum coherence methods (HMQC) was essential for the initial establishment of the 3D structure of metallothioneins, a protein family deficient in the regular secondary structural elements of α-helix and β-sheet and the first native protein identified with bound Cd. The ¹¹³Cd NMR studies also enabled the characterization of the affinity of the individual sites for ¹¹³Cd and, in competition experiments, for other divalent metal ions: Zn, Cu, and Hg.

This chapter provides a review of the literature on structural information from crystal structures determined by X-ray diffractometry of cadmium(II) complexes containing ligands of potential biological interest. These ligands fall into three broad classes, (i) those containing N-donors such as purine or pyrimidine bases and derivatives of adenine, guanine or cytosine, (ii) those containing carboxylate groups such as α-amino acids, in particular the twenty essential ones, the water soluble vitamins (B-complex) or the polycarboxylates of EDTA type ligands, and (iii) S-donors such as thiols/thiolates or dithiocarbamates. A crystal and molecular structural analysis has been carried out for some representative complexes of these ligands, specifically addressing the coordination mode of ligands, the coordination environment of cadmium and, in some significant cases, the intermolecular interactions.

Cadmium(II), commonly classified as a relatively soft metal ion, prefers indeed aromatic-nitrogen sites (e.g., N7 of purines) over oxygen sites (like sugar-hydroxyl groups). However, matters are not that simple, though it is true that the affinity of Cd²⁺ towards ribose-hydroxyl groups is very small; yet, a correct orientation brought about by a suitable primary binding site and a reduced solvent polarity, as it is expected to occur in a folded nucleic acid, may facilitate metal ion-hydroxyl group binding very effectively. Cd²⁺ prefers the guanine(N7) over the adenine(N7), mainly because of the steric hindrance of the (C6)NH₂ group in the adenine residue. This Cd²⁺-(N7) interaction in a guanine moiety leads to a significant acidification of the (N1)H meaning that the deprotonation reaction occurs now in the physiological pH range. N3 of the cytosine residue, together with the neighboring (C2)O, is also a remarkable Cd²⁺ binding site, though replacement of (C2)O by (C2)S enhances the affinity towards Cd²⁺ dramatically, giving in addition rise to the deprotonation of the (C4)NH₂ group. The phosphodiester bridge is only a weak binding site but the affinity increases further from the mono- to the di- and the triphosphate. The same also holds for the corresponding nucleotides. Complex stability of the pyrimidine-nucleotides is solely determined by the coordination tendency of the phosphate group(s), whereas in the case of purine-nucleotides macrochelate formation takes place by the interaction of the phosphate-coordinated Cd²⁺ with N7. The extents of the formation degrees of these chelates are summarized and the effect of a non-bridging sulfur atom in a thiophosphate group (versus a normal phosphate group) is considered. Mixed ligand complexes containing a nucleotide and a further mono- or bidentate ligand are covered and it is concluded that in these species N7 is released from the coordination sphere of Cd²⁺. In the case that the other ligand contains an aromatic residue (e.g., 2,2’-bipyridine or the indole ring of tryptophanate) intramolecular stack formation takes place. With buffers like Tris or Bistris mixed ligand complexes are formed. Cd²⁺ coordination to dinucleotides and to dinucleoside monophosphates provides some insights regarding the interaction between Cd²⁺ and nucleic acids. Cd²⁺ binding to oligonucleotides follows the principles of coordination to its units. The available crystal studies reveal that N7 of purines is the prominent binding site followed by phosphate oxygens and other heteroatoms in nucleic acids. Due to its high thiophilicity, Cd²⁺ is regularly used in so-called thiorescue experiments, which lead to the identification of a direct involvement of divalent metal ions in ribozyme catalysis.

Cadmium(II) ions form complexes with all natural amino acids and peptides. The thermodynamic stabilities of the cadmium(II) complexes of the most common amino acids and peptides are generally lower than those of the corresponding zinc(II) complexes, except the complexes of thiolate ligands. The coordination geometry of the cadmium(II) amino acid complexes is generally octahedral with the involvement of the amino and carboxylate groups in metal binding. In the case of simple peptides, both octahedral and tetrahedral complexes can be formed depending on the steric conditions. The terminal amino group and the subsequent carbonyl-O atom are the primary binding sites and there is no example for cadmium(II)-induced peptide amide deprotonation and coordination. The various hydrophobic and polar side chains do not have a significant impact on the structural and thermodynamic parameters of cadmium(II) complexes of amino acids and peptides. β-carboxylate function of aspartic acid and imidazole-N donors of histidyl residues slightly enhance the thermodynamic stability of cadmium(II)-peptide complexes. The most remarkable effects of side chains are, however, connected to the involvement of thiolate residues in cadmium(II) binding. Stability constants of the cadmium(II) complexes of both L-cysteine and its peptides and related ligands are significantly higher than those of the zinc(II) complexes. Thiolate donor functions can be bridging ligands too, resulting in the formation of polynuclear cadmium(II) complexes.

This chapter describes an approach using designed proteins to understand the structure, spectroscopy, and dynamics of proteins that bind Cd(II). We will show that three-stranded coiled coils (3SCCs) based on the parent peptides TRI (Ac-G(LKALEEK)₄G-NH₂) or GRAND (Ac-G(LKALEEK)₅G-NH₂) have been essential for understanding how Cd(II) binds to thiolate-rich environments in proteins. Examples are given correlating physical properties such as the binding constants or deprotonation constants relating to structure. We present a scale that relates ¹¹³Cd NMR chemical shifts to structures extracted from ^111mCd PAC experiments. In addition, we describe motional processes that help transport from the helical interface of proteins into the hydrophobic interior of helical bundles. These studies help clarify the chemistry of Cd(II) in relation to metal-regulated gene expression and detoxification.

Metallothioneins (MTs) are low-molecular-mass cysteine-rich proteins with the ability to bind mono- and divalent metal ions with the electron configuration d ¹⁰ in form of metal-thiolate clusters. MTs are thought, among others, to play a role in the homeostasis of essential Zn(II) and Cu(I) ions. Besides these metal ions also Cd(II) can be bound to certain MTs in vivo, giving rise to the perception that another physiological role of MTs is in the detoxification of heavy metal ions. Substitution of the spectroscopically silent Zn(II) ions in metalloproteins by Cd(II) proved to be an indispensable tool to probe the Zn(II) sites in vitro. In this review, methods applied in the studies of structural and chemical properties of Cd-MTs are presented. The first section focuses on the physical basis of spectroscopic techniques such as electronic absorption, circular dichroism (CD), magnetic CD, X-ray absorption, and perturbed angular correlation of γ-rays spectroscopy, as well as mass spectrometry, and their applications to Cd-MTs from different organisms. The following is devoted to the discussion of metal binding affinities of Cd-MTs, cluster dynamics, the reactivity of bound Cd(II) ions with metal ion chelators and of thiolate ligands with alkylating and oxidizing agents. Finally, a brief summary of the known three-dimensional structures of Cd-MTs, determined almost exclusively by multinuclear NMR techniques, is presented. Besides Cd-MTs, the described methods can also be applied to the study of metal binding sites in other metalloproteins.

Plants are categorized in three groups concerning their uptake of heavy metals: indicator, excluder, and hyperaccumulator plants, which we explain in this chapter, the former two groups briefly and the hyperaccumulators in detail. The ecological role of hyperaccumulation, for example, the prevention of herbivore attacks and a possible substitution of Zn by Cd in an essential enzyme, is discussed. As the mechanisms of cadmium hyperaccumulation are a very interesting and challenging topic and many aspects are studied worldwide, we provide a broad overview over compartmentation strategies, expression and function of metal transporting proteins and the role of ligands for uptake, transport, and storage of cadmium. Hyperaccumulators are not without reason a topic of great interest, they can be used biotechnologically for two main purposes which we discuss here for Cd: phytoremediation, dealing with the cleaning of anthropogenically contaminated soils as well as phytomining, i.e., the use of plants for commercial metal extraction. Finally, the outlook deals with topics for future research in the fields of biochemistry/biophysics, molecular biology, and biotechnology. We discuss which knowledge is still missing to fully understand Cd hyperaccumulation by plants and to use that phenomenon even more successfully for both environmental and economical purposes.

Cadmium is an important pollutant in the environment, toxic to most organisms and a potential threat to human health: Crops and other plants take up Cd from the soil or water and may enrich it in their roots and shoots. In this review, we summarize natural and anthropogenic reasons for the occurrence of Cd toxicity, and evaluate the observed phytotoxic effects of plants growing in Cd-supplemented soil or nutrient solution. Cd-induced effects include oxidative stress, genotoxicity, inhibition of the photosynthetic apparatus, and inhibition of root metabolism. We explain proposed and possible interactions between these modes of toxicity. While discussing recent and older studies, we further emphasize the environmental relevance of the experiments and the physiological response of the plant.

The detrimental health effects of cadmium (Cd) were first described in the mid 19th century. As part of industrial developments, increasing usage of Cd has led to widespread contamination of the environment that threatens human health, particularly today. Rather than acute, lethal exposures, the real challenge in the 21st century in a global setting seems to be chronic low Cd exposure (CLCE), mainly from dietary sources. Ubiquity of Cd makes it a serious environmental health problem that needs to be thoroughly assessed because it already affects or will affect large proportions of the world’s population. CLCE is a health problem that affects increasingly organ toxicity, especially nephrotoxicity, without a known threshold, implying that there is currently no safe limit for CLCE. In this chapter, we summarize current knowledge on the sources of Cd in the environment, describe the entry pathways for Cd into mammalian organisms, sum up the major organs targeted by acute or chronic Cd exposure and review the impact of Cd on organ function and human health. We also aim to put early pioneering studies on Cd poisoning into perspective in the context of recent ground-breaking prospective long-term population studies, which link CLCE to leading causes of diseases in modern societies – cancer, diabetes, and cardiovascular diseases, and of state-of-the-art studies detailing cellular and molecular mechanisms of acute and chronic Cd toxicity.

Cadmium is an established human and animal carcinogen. Most evidence is available for elevated risk for lung cancer after occupational exposure; however, associations between cadmium exposure and tumors at other locations including kidney, breast, and prostate may be relevant as well. Furthermore, enhanced cancer risk may not be restricted to comparatively high occupational exposure, but may also occur via environmental exposure, for example in areas in close proximity to zinc smelters. The underlying mechanisms are still a matter of manifold research activities. While direct interactions with DNA appear to be of minor importance, elevated levels of reactive oxygen species (ROS) have been detected in diverse experimental systems, presumably due to an inactivation of detoxifying enzymes. Also, the interference with proteins involved in the cellular response to DNA damage, the deregulation of cell growth as well as resistance to apoptosis appears to be involved in cadmium-induced carcinogenicity. Within this context, cadmium has been shown to disturb nucleotide excision repair, base excision repair, and mismatch repair. Particularly sensitive targets appear to be proteins with zinc-binding structures, present in DNA repair proteins such as XPA, PARP-1 as well as in the tumor suppressor protein p53. Whether or not these interactions are due to displacement of zinc or due to reactions with thiol groups involved in zinc complexation or in other critical positions under realistic exposure conditions remains to be elucidated. Further potential mechanisms relate to the interference with cellular redox regulation, either by enhanced generation of ROS or by reaction with thiol groups involved in the regulation of signaling pathways. Particularly the combination of these multiple mechanisms may give rise to a high degree of genomic instability evident in cadmium-adapted cells, relevant not only for tumor initiation, but also for later steps in tumor development.

The distribution of cadmium in the ocean is very similar to that of major nutrients suggesting that it may be taken up by marine phytoplankton at the surface and remineralized at depth. This interpretation is supported by recent data on Cd isotope distribution showing an increase in the ¹¹²Cd/¹¹⁰Cd ratio in Cd-depleted surface water. While at high concentrations, Cd is toxic to phytoplankton as it is to many organisms, at relatively low concentrations, Cd can enhance the growth of a number of phytoplankton species under zinc limitation. Kinetic studies suggest that Cd is taken up through either the Mn or the Zn transport system, depending on the ambient concentrations of these metals. In addition to inorganic Cd complexes (including the free Cd²⁺ ion), Cd complexes with relatively weak organic ligands may also be bioavailable. Cd is very effective to induce the production of phytochelatin and other thiols in phytoplankton, probably as a detoxification mechanism as well as a control of Cd homeostasis in cells. The only known biological function of Cd is to serve as a metal cofactor in Cd-carbonic anhydrase (CDCA) in diatoms. The expression of CDCA is regulated by Cd and Zn availabilities and by the pCO₂/pH of the ambient seawater in cultured diatoms and natural assemblages. The conformation of CDCA active site is similar to that of β-CA and both Zn and Cd can be used as its metal cofactor and exchanged for each other. Understanding of the biological role of Cd in marine phytoplankton provides insights into the biogeochemical cycling of this element in the ocean.