Ceruloplasmin copper complexes

There is some evidence that localized copper imbalance is related to rheumatoid arthritis (RA) conditions. Low molecular weight copper complex concentrations in plasma and synovial fluids are increased as part of the body s natural response to RA. Secondly, when such increases are induced by copper administration, they have a powerful antiinflammatory effect. More data are required to show the medical significance of these changes. Three parameters which should be reported are (i) total copper levels (by atomic absorption spectrophotometry) (ii) total ceruloplasmin levels (by autoimmune assay) and (iii) the relative molecular weight distribution (by Sephadex gel filtration and ultrafiltration). 

Much of the copper in the plasma (60-95%) is bound to ceruloplasmin. The complex of copper and cenjloplasmir is assembled and secreted by the liver. A small fraction of plasma copper, under 7%, is weakly bound to albumin and to free amino acids, especially histidine, threonine, and glutamine. The copper bound to serum albumin is associated with a histidine residue near the amino terminus of the protein. The copper in red blood cells is bound to superoxide dismutase. 

As is also shown in Figure 6.1, an interleukin-1-mediated [29] acute-phase response to many diseases involves a release of copper-thioneine-stored copper from the liver as ceruloplasmin, copper amino-acid complexes, and a copper albumin complex to meet increased metabolic needs for copper, which exceed normal needs, and plasma copper concentrations increase 200-300% above normal, as illustrated in Figure 6.3. The appropriate increase coupled... 

A remarkable increase in SOD-mimetic activity was found in a comparison of synovial fluid from rheumatoid arthritis and osteoarthritic patients with normal control values [613]. The increase in SOD-mimetic activity correlated with increased rheumatoid disease activity and increasing progression of disease severity. There was also a good correlation between SOD-mimetic activity and C-reactive protein in synovial fluid from patients with rheumatoid arthritis. This SOD-mimetic activity may be attributed to either an elaboration of ceruloplasmin by synovial cells [614] or the liver along with copper albumin and amino-acid copper complexes which, in part, accounts for the established increase in synovial fluid copper and ceruloplasmin in rheumatoid arthritis [30] and it is well known that ceruloplasmin [102, 489, 615] as well as amino-acid and other small-molecular-weight copper complexes have SOD-mimetic activity [287-295, 327]. 

Golgi apparatus, endoplasmic reticulum, and plasma membrane, and are responsible for copper transport. A mutation of this gene is responsible for Wilson s disease. Copper is poorly incorporated into the ceruloplasmin when translocase is defective. Metal ions are also sequestrated into lyso-somes, especially under conditions of copper overload (Mohan etal. 1995). The liver, which is the only true storage site that may be mobilized in the case of a negative copper balance, retains 20% of body copper. Muscles and brain account for 40% and 20%, respectively, but this copper is not available to assess in copper balance maintenance. A carrier-mediated facilitated diffusion system for uptake of copper complexes, amino acids and small peptides was identified in the rat hypothalamus (Harttler and Barnea 1988). Copper transport into the bile takes place in association with the biliary excretion of glutathione (Freedman etal. 1989). 

The importance of coordination in the biochemistry of essential metallic elements may be illustrated by numerous examples of metal complexes of which the following are representative the iron complex hemoglobin and numerous enzymes containing the heme and related structures such as catalases, peroxidases and cytochromes and the iron-containing proteins ferritin, transferrin, and hemosiderin the zinc complexes zinc-insulin, carbonic anhydrase and the carboxypeptidases the cobalt complex vitamin B12 the copper complex, ceruloplasmin the molybdenum-containing enzymes, xanthine oxidase, and nitrate reductase DNA-metal ion complexes. 

The somewhat paradoxical situation of having enhanced copper levels in advanced arthritis, and the moderate advantages of using copper complexes in treatment, is mirrored by the apparent contradiction of D-penicillamine efficacy, despite its clinical use for treatment and excretion of excess copper in Wilson s disease [46]. There is little evidence that penicillamine copper complexes have any relevance in vivo. Ceruloplasmin copper is unavailable for chelation and, indeed, D-penicillamine is not particularly effective at scavenging Cu(II) from BSA [84]. 

As with all antiarthritic drugs, the situation is not clear. Biochemical effects of copper are general, and no one target, such as a particular protein, is recognizable. The copper complexes are presumably a means of further increasing the copper content, because the species are expected to be rather labile. The introduction of exogenous copper will also affect thiol content and redox state of the cell, and some biochemical responses listed above may be a consequence of this altered state. Besides ceruloplasmin and albumin, major binding sites of Cu(II) are histidine and cysteine [94, 95] and some possibilities for the mechanism of action have been summarized [64]. 

After absorption in the intestine, copper reaches the bloodstream where most of it (80% or more) becomes bound to ceruloplasmin, a protein (globulin-copper) complex. The rest is loosely bound to albumen and transported to various tissues. 

The copper transport function of ceruloplasmin has been documented in several reviews (e.g. see refs. 15, 42, 43) and a transport function established. The turnover of ceruloplasmin allows copper ions to move from the major sites of ceruloplasmin synthesis in liver cells [44,45] to peripheral tissues for incorporation into copper-dependent enzymes [46,47], but transport mechanisms may also be active which involve copper atoms in the intact protein. However, the complexity of the protein has made it difficult to determine which, if any, of the six integral copper atoms are involved in copper delivery or whether there exist additional... 

Copper is part of several essential enzymes including tyrosinase (melanin production), dopamine beta-hydroxylase (catecholamine production), copper-zinc superoxide dismutase (free radical detoxification), and cytochrome oxidase and ceruloplasmin (iron conversion) (Aaseth and Norseth 1986). All terrestrial animals contain copper as a constituent of cytochrome c oxidase, monophenol oxidase, plasma monoamine oxidase, and copper protein complexes (Schroeder et al. 1966). Excess copper causes a variety of toxic effects, including altered permeability of cellular membranes. The primary target for free cupric ions in the cellular membranes are thiol groups that reduce cupric (Cu+2) to cuprous (Cu+1) upon simultaneous oxidation to disulfides in the membrane. Cuprous ions are reoxidized to Cu+2 in the presence of molecular oxygen molecular oxygen is thereby converted to the toxic superoxide radical O2, which induces lipoperoxidation (Aaseth and Norseth 1986). 

Factor IXa causes a rapid activation of factor X only if Ca2+, phospholipid,553 554 and the accessory factor Villa555 are present. The IXa Villa complex acts on X about 2 x 105 times faster than does IXa alone. This complex cleaves the same bonds in X as does the VIIa Va complex formed in the tissue factor pathway.514 The 2332-residue factor VIII and factor V have similar structures that include three repeats of a domain homologous to the blue copper-containing plasma protein ceruloplasmin (Chapter 16).556-559 Tyrosine 1680 of VIII apparently must be converted to a sulfate ester for full activity.560... 

Transferrins bind Fe2+ weakly and it is likely that a transferrin- Fe2+-HC03- complex formed initially undergoes oxidation to the Fe3+-C032- complex within cells and within the bloodstream. A conformational change closes the protein around the iron ions.56 In yeast the previously mentioned copper oxidoreductase encoded by the FET3 gene appears to not only oxidize Fe2+ but also transfer the resulting Fe3+ to transferrin. Ceruloplasmin may play a similar role in mammals.33... 

Because Cu2+ is the most tightly bound metal ion in most chelating centers (Table 6-9), almost all of the copper present in living cells is complexed with proteins. Copper is transported in the blood by a 132-kDa, 1046-residue sky-blue glycoprotein called ceruloplasmin.471475-477 This one protein contains 3% of the total body copper. 

In in vitro studies penicillamine inhibited angiotensin-con-verting enzyme (ACE) and carboxypeptidase (930). Penicillamine interferes with the functions of the copper-containing enzyme ceruloplasmin, and some of the penicillamine- and copper-containing complexes formed in vivo have a superoxide dismutase effect (931). In patients with scleroderma, penicillamine normalized collagen metabolism, by inhibiting beta-galactosidase activity (932). 

Thrombin (MW 39,000) is a proteolytic enzyme of the serine protease group. It is derived from prothrombin, a circulating plasma protein, through the proteolytic action of a complex consisting of the proteolytic enzyme factor X (or factor Xa), another protein called factor V (accelerator protein), calcium, and phospholipid. Factor V has recently been identified as the plasma copper protein ceruloplasmin or a similar protein (see Chapter 6). 

The disturbance of copper excretion, primarily due to a defect in the billiary excretion, is consistent with the biochemical findings in patients with Wilson disease. Urinary copper excretion is increased owing to total body overload of copper. Renal dysfunction includes albuminuria and renal rickets. Incorporation of copper in ceruloplasmin is impaired. Thus, there is a greater proportion of copper bound to albumin and amino acid complexes in the serum. But the overall copper concentration in serum is low. Ceruloplasmin is a multicopper oxidase see Copper Proteins Oxidases) that... 

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