Robert A. Goyer

Metallothioneins and their role in the metabolism and toxicity of metals

Abstract Recent investigations have provided considerable new information regarding the biological role of metallothioneins. The synthesis of this protein is induced in cells by certain metals. It can tightly bind with zinc, copper, cadmium, mercury or silver reducing the availability of diffusible forms of these metals within cells and therefore decreasing their toxic potential. The metallothioneins may also have an important role in regulating the normal absorption and homeostasis of zinc and copper. It is paradoxical, however, in that a protein synthesized within the cell to reduce toxicity, may, in itself, be toxic when excreted or leaked out from the cell to the extracellular space. Further studies are required to elucidate the mechanisms involved in these effects.

Cadmium-metallothionein-induced nephropathy.

Abstract Cadmium-metallothionein (Cd-Mt) was isolated from livers of rats repeatedly injected with small doses of cadmium (0.6 mg Cd/kg). Intraperitoneal injection of isolated Cd-Mt to control rats (1.2 mg Cd/kg) resulted in accumulation of a large percentage of the injected cadmium in the kidney (about 60 to 70%), whereas most Cd from the ip injection of a similar dose of CdCl 2 was recovered from the liver. The intracellular partitioning of Cd, whether given as Cd-Mt or CdCl 2 , was similar. Cd given as CdCl 2 was bound to large molecular weight proteins isolated from cytoplasm 4 hr after Cd injections; Cd given as Cd-Mt was isolated as Cd-Mt. Changes in the morphology of renal tubular cells are observed both by light and electron microscopy in these rats within 4 hr after Cd-Mt injection. Necrosis of proximal renal tubular lining cells was observed within 24 hr. No ultrastructural changes were found in renal tubular cells after CdCl 2 injection. These studies demonstrate that the circulating Cd-Mt complex was nephrotoxic. It may also have a major role in the pathogenesis of Cd-induced renal disease following long-term exposure to cadmium.

Mineral Arsenicals in Traditional Medicines: Orpiment, Realgar, and Arsenolite

Mineral arsenicals have long been used in traditional medicines for various diseases, yet arsenic can be highly toxic and carcinogenic. Arsenic in traditional medicines typically comes from deliberate addition for therapeutic purposes, mainly in the form of mineral arsenicals, including orpiment (As2S3), realgar (As4S4), and arsenolite (contains arsenic trioxide, As2O3). Inorganic arsenic is now accepted in Western medicine as a first line chemotherapeutic agent against certain hematopoietic cancers. This perspective analyzes the pharmacology and toxicology of these arsenicals used in traditional medicines. Orpiment and realgar are less soluble and poorly absorbed from the gastrointestinal tract, whereas the bioavailability of arsenic trioxide is similar to inorganic arsenic salts such as sodium arsenite. Pharmacological studies show that arsenic trioxide and realgar are effective against certain malignancies. Orpiment and realgar are used externally for various skin diseases. Realgar is frequently included as an ingredient in oral traditional remedies for its antipyretic, anti-inflammatory, antiulcer, anti-convulsive, and anti-schistosomiasis actions, but the pharmacological basis for this inclusion still remains to be fully justified. Toxicological studies show that cardiovascular toxicity is the major concern for arsenic trioxide and that the gastrointestinal and dermal adverse effects may occur after prolonged use of mineral arsenicals. Little is known regarding the possible secondary cancers resulting from the long-term use of any of these arsenicals. Similar to the safety evaluation of seafood arsenicals, total arsenic content alone appears to be insufficient for mineral arsenical safety evaluation. Arsenic speciation, bioavailability, and toxicity/benefit should be considered in evaluation of mineral arsenical-containing traditional medicines.

Mechanisms of lead and cadmium nephrotoxicity

Exposure to lead results in accumulation in proximal renal tubular lining cells in the form of morphologically discernible inclusion bodies which are lead-protein complexes. Acute nephrotoxicity consists of proximal tubular dysfunction and can be reversed by treatment with chelating agents. Chronic lead nephrotoxicity consists of interstitial fibrosis and progressive nephron loss, azotaemia and renal failure. Potential complications of lead nephropathy include gout and hypertension. Cadmium accumulates in renal tubular lining cells bound to metallothionein, a small protein containing 30% cystine. Metallothionein protects against nephrotoxicity by binding cadmium in a nontoxic form. Renal tubular dysfunction and chronic interstitial fibrosis occur when cadmium levels in the renal cortex exceed the critical concentration of about 200 micrograms/g. Recommendations are made for specific research needs.

Mercury in Traditional Medicines: Is Cinnabar Toxicologically Similar to Common Mercurials?

Mercury is a major toxic metal ranked top in the Toxic Substances List. Cinnabar, which contains mercury sulfide, has been used in Chinese traditional medicines for thousands of years as an ingredient in various remedies, and 40 cinnabar-containing traditional medicines are still used today. Little is known about toxicology profiles or toxicokinetics of cinnabar and cinnabar-containing traditional medicines, and the high mercury content in these Chinese medicines raises justifiably escalations of public concern. This minireview, by searching the available database of cinnabar and by comparing cinnabar with common mercurials, discusses differences in their bioavailability, disposition, and toxicity. The analysis showed that cinnabar is insoluble and poorly absorbed from the gastrointestinal tract. Absorbed mercury from cinnabar is mainly accumulated in the kidneys, resembling the disposition pattern of inorganic mercury. Heating cinnabar results in release of mercury vapor, which in turn can produce toxicity similar to inhalation of these vapors. The doses of cinnabar required to produce neurotoxicity are 1000 times higher than methyl mercury. Following long-term use of cinnabar, renal dysfunction may occur. Dimercaprol and succimer are effective chelation therapies for general mercury intoxication including cinnabar. Pharmacological studies of cinnabar suggest sedative and hypnotic effects, but the therapeutic basis of cinnabar is still not clear. In summary, cinnabar is chemically inert with a relatively low toxic potential when taken orally. In risk assessment, cinnabar is less toxic than many other forms of mercury, but the rationale for its inclusion in traditional Chinese medicines remains to be fully justified.

Non-metallothionein-bound cadmium in the pathogenesis of cadmium nephrotoxicity in the rat

Male rats were injected SC with 0.6 mg Cd/kg/day for 5 days per week for 2, 4, 6, and 8 weeks. Liver and kidney were examined morphologically and analyzed for metallothionein, cadmium, zinc, and copper. Morphologic changes were found in kidney but not in liver. The earliest ultrastructural change consisted of myelin figures in vacuoles in cytoplasm of proximal tubular lining cells reflecting degeneration of membranes. This change occurred after 4 weeks with 801 +/- 25 nmol/g (89.9 micrograms/g) total kidney cadmium or 390 nmol/g (43.7 micrograms/g) of cadmium not bound to metallothionein. Similar changes were observed after 6 weeks but after 8 weeks pathological changes consisted of focal cellular necrosis and interstitial fibrosis. Other ultrastructural changes included altered mitochondria and increased numbers of microbodies. Renal cadmium after 8 weeks exposure was 1827 +/- 48 nmol/g (215.3 +/- 5.8 micrograms/g) or 628 nmol/g (70.2 micrograms/g) of cadmium not bound to metallothionein. Total cadmium was higher in liver than in kidney but partitioning between bound and nonbound cadmium differed in the two organs. The fraction not bound to metallothionein increased with time of exposure in both liver and kidney. However, total cadmium in the liver did not exceed potentially available binding sites of metallothionein, whereas total cadmium did exceed potentially available binding sites of metallothionein in the kidney where pathologic changes occurred. The results indicated that degeneration of cellular membranes is an early cellular effect of cadmium exposure followed later by toxicity to organelles, cellular necrosis, and interstitial fibrosis. Cadmium-induced cellular toxicity is more directly related to the fraction of cadmium in the kidney that is not bound to metallothionein than is total cadmium per se.

Renal ultrastructure, renal function, and parameters of lead toxicity in workers with different periods of lead exposure

Cramér, K., Goyer, R. A., Jagenburg, R., and Wilson, Marion H. (1974).British Journal of Industrial Medicine,31, 113-127. Renal ultrastructure, renal function, and parameters of lead toxicity in workers with different periods of lead exposure. Renal biopsies were obtained from five men with heavy occupational exposure to lead and compared with studies of their renal function and parameters of lead toxicity. Two men had lead exposure of less than one year while three men had been exposed for from four to more than 30 years. In addition, renal function studies were performed in two men from whom renal biopsies could not be obtained. Their lead exposures were five and 12 years, respectively. Significantly lower plasma levels, when compared with non-exposed controls, were found for proline, valine, tyrosine, and phenylalanine although no excessive aminoaciduria was found. Renal function tests were normal in all except for a reduced glomerular filtration rate (GFR) in one worker. Plasma ALA was measured by a new and highly specific method and ALA clearance was found to follow GFR closely. Those workers with prolonged lead exposure showed a lower urinary lead excreation. Typical lead-induced intranuclear inclusion bodies were found only in those with short exposure. The ultrastructural changes were localized to the proximal tubules, while the glomeruli were only nonspecifically affected. Mitochondrial changes were found in all men. Reasons for the decrease in inclusion body formation in chronic lead nephropathy are uncertain but may be due to an increased rate of renal cell turnover or a consequence of chelation therapy.

Ascorbic acid and EDTA treatment of lead toxicity in rats.

Ascorbic acid given orally to lead exposed rats has similar chelating properties as equimolar amounts of parenterally administered EDTA. Ascorbic acid in combination with EDTA is more than twice as effective as either chelating agent given alone and the combination is particularly effective in removing lead from the central nervous system.

Gastrointestinal absorption and organ distribution of oral cadmium chloride and cadmium‐metallothionein in mice

Body retention and tissue distribution of Cd from oral 109CdCl2 and 109Cd-metallothionein were studied in C57BL/6J mice given identical doses (60 microgram Cd and 2 muCi 109Cd). Measurement of 109Cd radioactivity in mice showed identical absorption but differences in tissue distribution of Cd from the two forms. A significantly greater deposition of Cd was observed in the kidney of mice fed Cd-metallothionein than in the CdCl2 group where a major portion of Cd was deposited in the liver. Most of the radioactive Cd in the renal and hepatic tissue and in the duodenum was bound to metallothionein 24 h after oral Cd-metallothionein, whereas only 30 and 38% of the cadmium in liver and kidney supernatants, respectively, were bound to metallothionein after oral CdCl2. These results suggest that the dietary form of Cd may influence the renal deposition of Cd.

Immunomodulation by Metals

A symposium entitled Immunomodulation by Metals was held at the 32nd Annual Meeting of the Society of Toxicology (SOT) in New Orleans, Louisiana. The symposium was co-sponsored by the Immunotoxicology and Metals Specialty Sections of SOT and was designed to describe the types of adverse immunological reactions which occur in response to environmental and/or occupational exposure to metals. Epidemiological evidence and underlying mechanisms responsible for the observed alterations were also discussed. The following is a summary of each of the individual presentations.