Metallothioneins copper storage

The primary organs for copper storage are the liver and spleen, where the metal is found in the cytosol in superoxide dismutase see Copper Proteins with Type Sites) or metallothionein see Metallothiondns) In response to a copper challenge, yeast adaptively synthesizes metallothionein to detoxify the metal. Copper is also bound, transported, and assimilated into tissues by ceruloplasmin. 

As stated previously, the total normal cytoplasmic free copper concentration is less than 10 18 M or less than one copper ion per cell. In thermodynamic terms, almost all hydrated copper ions are immediately and tightly coordinated by amino acids or biopolymers—peptides, proteins, and other species with free sulfur ligands. An excess of copper ions activates metallothionein synthesis for storage or removal of the excess. Copper chaperones mediate transfer of copper ions from extracellular or storage locations to their target proteins. Instability of copper ion concentrations in vivo results in various disease states. Three of these—FALS, Menkes, and Wilson s diseases—are described below. 

When animals are fed experimental diets lacking copper or zinc, their copper or zinc status rapidly declines, suggesting that there is not a storage pool of these metals. Thus, while the small, cysteine-rich protein metallothionein (see below) can avidly bind zinc and copper, this may reflect its role in detoxification rather than as a specific storage form. This is reflected by the fact that metallothionein genes are typically expressed at a basal level, but their transcription is strongly induced by heavy metal load. 

The addition of copper, zinc, cadmium or mercury to animals results in the synthesis of a cysteine-rich protein called metallothionein.1147-1149 These proteins have been isolated from a number of sources, and have molecular weights in the range 6000 to 12 000 with a cysteine content of about 30-35% of the total amino acid content. They have also been found in microorganisms and plants. These proteins are thought to play an important role in the storage of zinc and copper, and as a result of their storage capacity, are able to bind and detoxify cadmium and mercury. 

Other proteins play important roles in controlling free metal concentrations in the cytosol. Of particular importance are ferritin and metallothionein. Ferritin is the major iron storage protein in the cell. It is critical that cytosolic iron is kept at low levels, because iron can catalize the Fenton reaction, which generates the most toxic of the ROS—the hydroxyl radical. Copper and superoxide can also participate in the fenton reaction. Metallothioneins are another important family of proteins that helps control cytosolic concentrations of metals such as Cu and Cd. Glutathione is another peptide that controls free Hg and Cd levels in the cell (Figure 21.7). 

These results together with published data [63,64] render the suggestion that copper and its storage protein — metallothionein — play similar roles in PC tempting. 

With few exceptions, metallothioneins consist of relatively simple amino acids, aromatic amino acids and histidine only being found in a small number of species [329]. This amino acid composition suggests that metallothioneins evolved early in the evolution of life, probably even before the oxygenation of the atmosphere. A further clue is one of their functions. As metal-transport and storage proteins, thioneins are capable of binding metal ions but release them relatively easily as well. Metallothioneins can therefore be considered a transition from non-metal to metalloproteins. It is improbable, however, that the known copper proteins evolved from copper metallothioneins as there are no homologies between them and other copper proteins or enzymes. 

Apart from its catalytical function, Cu Zn superoxide dismutases was suggested to play a role as copper-transport and/or storage protein Copper depleted E Znj-superoxide dismutase (E = Empty) can be reconstituted using Cu(I)-thiolate proteins (metallothioneins)... 

Copper is stored by the liver in the storage protein metallothionein and excreted by the transport protein ceruloplasmin into the bile. When 10 rat hepatocytes were incubated with 50 piM Cu " for 2 h, the formation of reactive oxygen species as determined by oxidation of dichlorofluorescin diacetate to dichlorofluorescein increased from 90 5 intensity units in the controls to 412 9 intensity units in the metal-treated cells (Pourahmad and O Brien 2000). Malondialdehyde UV absorption increased from 0.048 0.006 to 0.662 0.012 units obtained at X ax = 532 nm (P <0.001). The ED50 concentrations found for Cu and Cd (i.e. 50% membrane lysis in 2 h) were 50 and 20 pM, respectively (Pourahmad and O Brien 2000). However, reactive oxygen species formation, GSH oxidation and hpid peroxidation were induced by Cu at these concentrations more rapidly than by Cd. The dechne of mitochondrial membrane potential though occurred at the same time and to the same extent for both metals. 

Physiologically important metal ions include iron, copper, cobalt and nickel. Normally, metal ions are not present in free solution to any significant extent, but are bound to transport proteins (in plasma) or storage proteins and enzymes (in cells). Thus, iron is bound to transferrin (in plasma) and haemosiderin and ferritin in tissues, copper is bound to caeruloplasmin in plasma, and metallothionein in plasma binds a wide variety of metal ions. The adverse effects of iron overload (section 4.5.1) are the result of free iron, not bound to storage proteins, acting as a source of oxygen radicals. 

---