Ceruloplasmin copper homeostasis

Wilson s disease is a pathological accumulation of copper in tissue which is later released into the bloodstream, leading to anaemia, and final accumulation of copper in liver and brain. It is the result of a mutation in the Wilson s disease gene in chromosome 13 which ordinarily codes for a cation transporting ATPase so that copper can be incorporated into ceruloplasmin prior to excretion. Also known as ferroxi-dase, in acknowledgement of its primary function as an oxidoreductase responsible for electron transfer, this enzyme contains iron and, more importantly, six copper atoms. It accounts for the transport of 90% of copper in the plasma so any impairment in its production or efficacy has a major impact on copper homeostasis. The greatly reduced concentration of ceruloplasmin in the blood of Wilson s disease sufferers correlates with their inability to metabolize copper effectively. It leads to chronic liver disease, for which the only real cure is a liver transplant,... 

Recent reviews on copper homeostasis in E. col and yeast are available. Copper and iron metabolism are often intertwined. For instance, mammalian iron metabohsm depends on the copper protein, ceruloplasmin, a ferroxidase that facilitates iron efflux from cells see Copper Proteins Oxidases) Several important human diseases, including Menkes disease and Wilson s disease, result from mutations in copper transport see Metal-related Diseases of Genetic Origin) ... 

Ceruloplasmin Binds copper appears to be more important as a copper storage pool than as a transport protein integrates iron and copper homeostasis... 

The blue multicopper oxidases constitute a heterogeneous family of enzymes from different sources (7). In addition to the well characterized members of this family, ascorbate oxidase (45,46), laccase (47,48), and ceruloplasmin (49,50), all from higher organisms, two other proteins have attracted much recent interest FetSp, which is involved in iron uptake in yeast (51), and CueO, which is required for copper homeostasis in Escherichia coli (52). The characteristic reactivity of these enzymes is the one-electron oxidation of four substrate equivalents coupled to the four-electron reduction of dioxygen to water (1). These processes occur at a catalytic unit constituted by four copper atoms classified according to their spectroscopic properties in... 

Two inherited human diseases that represent abnormal copper metabolism are Menkes syndrome and Wilson s disease. Menkes syndrome, with symptoms similar to those of copper deficiency, is characterized by a progressive brain disease, abnormally low copper concentrations in Uver and other tissues, and diminished ability to transfer copper across the absorptive cells of the intestinal mucosa. Wilson s disease (hepatolenticular degeneration) is the only significant example of copper toxicity in humans. Wilson s disease is an autosomal recessive disorder that affects normal copper homeostasis and is characterized by excessive retention of hepatic copper, decreased concentration of serum ceruloplasmin, impaired biliary copper excretion, and hypercupremia. Systemic manifestations of Wilson s disease are hepatic and renal lesions and hemol5dic anemia. Certain strains of mutant rats with reduced excretion of... 

The rat ceruloplasmin cDNA clone described above was further used to identify extrahepatic sites of ceruloplasmin gene expression in the rat. Although the liver was originally postulated to be the sole tissue source of ceruloplasmin (189), the above study resulted in the detection of ceruloplasmin mRNA in the choroid plexus, yolk sac, placenta and testis of the rat. Since these tissues are situated at the interface between adjacent extracellular compartments, it has been suggested that ceruloplasmin may be involved in maintaining copper homeostasis in these compartments (189). 

Adults require 1-2 mg of copper per day, and eliminate excess copper in bile and feces. Most plasma copper is present in ceruloplasmin. In Wilson s disease, the diminished availability of ceruloplasmin interferes with the function of enzymes that rely on ceruloplasmin as a copper donor (e.g. cytochrome oxidase, tyrosinase and superoxide dismutase). In addition, loss of copper-binding capacity in the serum leads to copper deposition in liver, brain and other organs, resulting in tissue damage. The mechanisms of toxicity are not fully understood, but may involve the formation of hydroxyl radicals via the Fenton reaction, which, in turn initiates a cascade of cellular cytotoxic events, including mitochondrial dysfunction, lipid peroxidation, disruption of calcium ion homeostasis, and cell death. 

Contrary to popular belief, ceruloplasmin5, the principal copper-containing protein in plasma, ceruloplasmin, is not involved in copper transport. This is clearly underlined by the clinical observation that patients with aceruloplasminaemia (i.e. lacking ceruloplasmin in their blood) have perfectly normal copper metabolism and homeostasis. Copper is transported in plasma mostly by serum albumin with smaller amounts bound to low-molecular weight ligands like histidine. Likewise zinc is mostly transported in plasma bound to proteins (albumin and ot2-macroglobulin). 

More or less constant and in certain instances characteristic alterations of copper metabolism have been reported in a variety of human pathological states. Little is known about the mechanisms whereby the homeostasis of copper is disturbed in these conditions, although logical reasons can be given in a number of instances. These observations are restricted in most cases to a description of the alterations in the blood levels of copper and ceruloplasmin in the patients. Efforts to find an explanation for these changes and attempts to sort out the reasons for their development in the course of the disease are made infrequently. The reason is perhaps the tacit recognition that in most instances the disturbance in copper metabolism does not play a central role in either the causation or mechanism of the disease. 

Figure 4-4 The role of ceruloplasmin in iron homeostasis in humans. Ceruloplasmin con verts ferrous iron to ferric iron without the generation of radicals. Copper atoms in ceruloplasmin are represented by dark circles. The ferrox-idase activity of ceruloplasmin accelerates transferrin-iron binding facilitating iron distribution and may also play a role in iron export from tissues.

Copper is an essential nutrient needed for such diverse processes as mitochondrial respiration (cytochrome C), melanin biosynthesis (tyrosinase), dopamine metabolism (DOPA-P-monooxygenase), iron homeostasis (ceruloplasmin), antioxidant defense (snperoxyde dismutase), connective tissue formation (lysyl oxydase), and peptide amidation. 

Ceruloplasmin is a blue a-2 glycoprotein of 132 kD. This plasma protein is responsible for the binding of 90 to 95% of the blood plasma copper in vertebrates. In addition to its primary role in copper transport and homeostasis, it possesses a number of additional enzymatic activities (see ref. 179 for a recent review). The complete amino acid sequence of human ceruloplasmin has been determined (42), establishing that this large multicopper oxidase is synthesized as a single chain polypeptide, containing 1046 amino acid residues. 

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