Assay of extracellular metallothionein

The Bioinorganic Chemistry of Mammalian Metallothioneins

The functions, purposes, and roles of metallothioneins have been the subject of speculations since the discovery of the protein over 60 years ago. This article guides through the history of investigations and resolves multiple contentions by providing new interpretations of the structure-stability-function relationship. It challenges the dogma that the biologically relevant structure of the mammalian proteins is only the one determined by X-ray diffraction and NMR spectroscopy. The terms metallothionein and thionein are ambiguous and insufficient to understand biological function. The proteins need to be seen in their biological context, which limits and defines the chemistry possible. They exist in multiple forms with different degrees of metalation and types of metal ions. The homoleptic thiolate coordination of mammalian metallothioneins is important for their molecular mechanism. It endows the proteins with redox activity and a specific pH dependence of their metal affinities. The proteins, therefore, also exist in different redox states of the sulfur donor ligands. Their coordination dynamics allows a vast conformational landscape for interactions with other proteins and ligands. Many fundamental signal transduction pathways regulate the expression of the dozen of human metallothionein genes. Recent advances in understanding the control of cellular zinc and copper homeostasis are the foundation for suggesting that mammalian metallothioneins provide a highly dynamic, regulated, and uniquely biological metal buffer to control the availability, fluctuations, and signaling transients of the most competitive Zn(II) and Cu(I) ions in cellular space and time.

The bioinorganic chemistry of cadmium in the context of its toxicity

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.

Trace elements in three marine birds breeding on Reunion Island (Western Indian ocean): part 2-factors influencing their detoxification

Seabird tissues collected between 2002 and 2004 from Barau's Petrel (Pterodroma baraui), Audubon's Shearwater (Puffinus lherminieri bailloni), and White-Tailed Trop icbird (Phaethon lepturus) colonies on Reunion Island were analyzed for metallothioneins (MTs) and trace element content. The subcellular distribution between soluble and insoluble fractions of Cd, Cu, Fe, Mn, Se, and Zn was determined in liver and kidney. In both, the soluble fraction of the cell concentrated most of the Cd and Se, whereas Fe, Mn, and Zn were preferentially accumulated in the insoluble fraction. The distribution of these elements varied with the tissue, age of the bird, and species. Furthermore, the distributions of Fe and Mn were somewhat influenced by the bird's physical condition. MT levels were measured in the soluble fraction after heat denaturation. The levels of these proteins varied from 5.5 +/- 2.7 mg x g(-1) dry weight (dw) to 11.4 +/- 6.2 mg x g(-1) dw depending on the species and the tissue considered. MT levels were significantly different between liver and kidney only in the White-Tailed Tropicbird. In the three species, MT levels in kidney were significantly higher in adult than juvenile birds. The bird's weight also had an influence on hepatic and renal MT levels, but not the sex nor the reproductive status. The implication of MTs in Cu and Zn homeostasis and Cd and Hg detoxification are discussed. In addition, clues on Hg regulation by Se were found, especially in Barau's Petrel, where the levels of these two elements were significantly correlated.

Lipid raft-dependent endocytosis of metallothionein in HepG2 cells

Human hepatocellular carcinoma (HepG2) cells take up metallothionein (MT) by endocytosis. MT co-localizes with albumin but not with transferrin, indicating uptake via a non-classical pathway rather than via clathrin-mediated endocytosis. A lipid raft-dependent uptake is indicated by pravastatin inhibition of cholesterol synthesis and methyl-beta-cyclodextrin inhibition of cholesterol translocation to the plasma membrane, reducing MT uptake by 29% and 69%, respectively. Subcellular fractionation after MT uptake reveals significant amounts of MT in vesicular fractions including lysosomes but virtually no MT in the cytosol. Metals bound to MT are released into the cytosol, however. The findings define a pathway for cellular metal acquisition. Together with results from other studies demonstrating secretion of MT from different cells and the presence of MT in extracellular fluids, the results suggest a function of MT in intercellular communication.

Further observations on the etiology of pre-eclampsia: mobilization of toxic cadmium-metallothionein into the serum during pregnancy

Cadmium-metallothionein, mobilized from the liver, might be the toxic serum factor associated with pre-eclampsia. We base this on four documented concepts. First, during pregnancy, maternal physiology adjusts to assure the fetus of the proper amounts of nutrients necessary for growth. Our focus is on zinc and progesterone. Second, because zinc and cadmium are similar, they compete for binding sites. Our focus is on the storage protein metallothionein. Third, the manifestations of cadmium toxicity closely mimic the manifestations of toxemia (i.e. hypertension, proteinuria, edema). Our focus is on cadmium-induced endovasculitis. Fourth is the concept that metallothionein-bound cadmium can be mobilized from the liver into the serum during pregnancy as it follows the mobilization of metallothionein-bound zinc. Our focus is on the extreme toxicity of extracellular cadmium-metallothionein. We correlate these four concepts into a rational theory on the etiology of toxemia, and we suggest a method of proof.