Cd – Cadmium is found in igneous rocks 0.2 ppm; shale 0.3 ppm; sandstone 0.05ppm; limestone 0.035 ppm; fresh water 0.08 ppm; sea water 0.000 11 ppm; soils 0.06 ppm; marine plants 0.4 ppm; land plants 0.6 ppm; marine animals 0. 15 to 3.0 ppm; land animals 0.5 ppm (accumulates in kidney). Functions by stimulating the hatching of nematode cysts (worms). Cadmium bound proteins have been isolated from mollusks and the horse kidney.
Cadmium is a relatively toxic metal whose essentiality has only recently been investigated. The toxicity of the metal appears to be related to its ability to replace endogenous metal ions from cellular constituents, particularly proteins. Exposure to cadmium elicits, in target tissues, the synthesis of a group of specific proteins (the metallothioneins), which bind the metal with high affinity. The expression of the metallothionein gene is being actively studied at the molecular level and has proven to be a model system par excellence for investigating general aspects of gene regulation as well as the possible mechanisms whereby a metal may exert its specific biological effects.
The metabolism of cadmium is characterized by a lack of homeostatic regulation in mammals, with high body retention in certain tissues (such as liver and kidney) being characteristic of exposure to the metal. The nature of the distribution of cadmium in body tissues depends on a number of factors, but most significantly on the form of the metal given. Cadmium administered as cadmium salts or chelates accumulates preferentially in the liver after acute exposure. On the other hand, when administered as Cd-thionein, the metal accumulates primarily in the kidney.
The biochemical effects of cadmium are wide and varied. The metal does have inhibitory effects on several basic cellular processes, but these effects are variable and depend on whether the experiments are carried out in vivo or in vitro. Interactions of cadmium with the metabolism of various other metals (Zn, Cu, Fe, Se) have been extensively studied, and it is apparent that cadmium can interfere at almost any stage of a given metal’s metabolism. It is not clear, in many cases, whether these interferences are direct in the competitive sense of the word or indirect, via an alteration of biological activities that influence the metal in question but do not directly use it.
Cadmium was shown early in this century to be stimulatory to the growth of algae. More recently, the metal was shown to stimulate the growth of rats kept on a highly purified diet in metal-free environs. At least one report has appeared that suggests that cadmium deficiency may lead to a constellation of biological impairments. Although these reports are not sufficient to establish definitively a specific function for cadmium, the metal is a good candidate for essentiality.
Lead, Cadmium, and Arsenic
In the mid-1970s, Schwarz and others directed their attention to the study of the nutritional roles of lead, cadmium, and arsenic, three elements that until then had always been regarded only as poisonous. Schwarz could still report initial evidence for the biological essentiality of all three of them (Schwarz, 1977), but his death prevented completion of these studies. Although not all criteria of essentiality of Cd and Pb have been met, his observations were independently confirmed and extended by Anke et al. (1977) and Kirchgessner and Reichlmayr-Lais (1981). With arsenic, positive growth responses in young rats were observed (Schwarz, 1977), but at that time convincing evidence for the essentiality of arsenic had already been produced by two other groups (Anke et al., 1976; Nielsen et al., 1975). Nielsen’s arsenic-deficient rats exhibited rough fur, increased osmotic fragility of the erythrocytes, and abnormally enlarged spleens containing excessive amounts of iron. Anke’s As-deficient goats and pigs showed decreased fertility, low birth rates, and retarded growth. Lactating arsenic-deficient goats were also observed to die suddenly with myocardial damage. The discovery of the essentiality of arsenic and of the possible essentialities of lead and cadmium should of course not detract from the established fact that higher concentrations of these elements pose definite health hazards.
Distribution and Sources
Cadmium is a heavy metal that is widely distributed in the environment. Most Cd comes from Zn smelters and from the sludge obtained from the electrolyte refining of Zn. Cadmium-containing products, such as plastics, pigments, batteries, and alloys terminate in junk piles, creating a potential for biological injury through contamination of water supplies, air, and ingested feed. Relatively large quantities of Cd are found in commercial fertilizers containing phosphates and in water in galvanized or black polyethylene pipes (Flick et al., 1971).
Cadmium poses a potential environmental hazard of major importance. Most forages and plant materials fed to animals contain levels of Cd well below 0.5 ppm (dry basis) (NRC, 1980). Vegetables, nuts, and fruits are poor sources of Cd, with concentrations mostly ranging between 0.04 and 0.08 ppm. Cadmium levels in forages have been shown to be inversely related to distance from the highway. Concentrations of Cd in plants in Pb-mining regions averaged 1.51 ppm along the ore-truck routes, 0.58 at 90 meters, and less than 0.5 ppm for a distance of 180 meters or greater (Hemphill, 1974). Use of high-Cd sludges for fertilizing feedcrop lands has been shown to increase substantially the Cd content of crops. Plant Cd grown on sludge-fertilized soil was shown to be available for absorption by guinea pigs, mice, and sheep (Kostial, 1986).
Feed phosphate deposits from Florida (U.S.A.) typically contain 6 to 7 ppm Cd (D. J. Thompson, International Minerals and Chemical Corporation, 1979, personal communication). Present at levels of approximately 1% in finished feeds, these phosphates contribute 0.06 to 0.07 ppm Cd to the diet (NRC, 1980).
Cadmium as an Essential Element
Very limited data suggest that Cd may be an essential element. In rats fed a highly purified diet containing et al. (1977) fed rats a diet containing 0.0137 ppm Cd and reported higher weight gained for controls receiving adequate Cd. Cadmium deficiency in goats caused myasthenia (muscular debility), which was cured by repletion (Anke et al., 1983).