Albumin copper binding

Ceruloplasmin contains substantial amounts of copper, but albumin appears to be more important with regard to its transport. Both Wilson disease and Menkes disease, which reflect abnormahties of copper metabohsm, have been found to be due to mutations in genes encoding copper-binding P-type ATPases. 

Despite the positive effects of optimal levels of copper, deleterious effects may occur if a threshold level is exceeded. Wilson s disease (hepatolenticularic degeneration) is one of the diseases linked to the excess of copper in the body. It results from a dysfunction of the copper transmission process, which occurs due to a lack of suitable enzyme to catalyze the process of copper deletion from detached bonds with albumins and binding to ceruloplasma. The condition leads to neuron degradation, liver cirrhosis, and occurrence of colorful rings on the cornea (DiDonato and Sarkar, 1997). 

Albumin is a major transport facilitator of hydrophobic compounds which would otherwise disrupt cellular membranes. These compounds include free fatty acids and bilirubin as well as hormones such as cortisol, aldosterone, and thyroxine when these materials have exceeded the capacity of proteins normally associated with them. Albumin also binds ions, including toxic heavy metals and metals such as copper and zinc which are essential for normal physiological functioning but may be toxic in quantities in excess of their binding capacity for their carrier proteins. Binding of protons is the basis for the buffering capacity of albumin. 

Hypoproteinemia may result in low levels of serum calcium, ceruloplasmin, and transferrin. Because losses of iron are at most 0.5-1.0 mg/24 hr, even with the heaviest proteinuria, other factors must operate to produce iron deficiency and microcytic hypochromic anemia. Although the copper-binding protein ceruloplasmin is lost in the urine in nephrotic subjects and its plasma levels are low, plasma and red cell copper concentrations are usually normal. Zinc circulates mainly bound to albumin and also to transferrin, and thus the reported reduction zinc concentration in plasma, hair, and white cells in nephrotic patients is not surprising. 

Copper is absorbed from food in the upper small intestine. The absorption is primarily dependent on the quantity of the copper present in the diet. High intake of zinc diminishes copper absorption by inducing metallothionein formation in the mucosal cells. Metallothioneins, due to their high affinity for copper, bind it preferentially and the bound copper is lost during the sloughing of cells from the villi. Copper accumulation in patients with Wilson s disease can be reduced by giving oral zinc acetate, which decreases absorption (discussed later). Absorbed copper is transported to the portal blood where it is bound to albumin (and probably transcuprein), amino acids, and small peptides. Copper binds to albumin at the N-terminal tripeptide (Asp-Ala-His) site. The recently absorbed copper is taken up by the liver, which plays a central role in copper homeostasis. 

Many, perhaps all, proteins have some degree of copper-complexing property. Of the serum proteins, albumin and transferrin have known copper-binding properties. When, however, copper is added to serum in excess of a 1 1 molar ratio in respect to albumin, then, in addition to albumin and transferrin, several other globulins will also bind some copper. 

Physiological free copper concentrations are certainly decisive for determining a role for copper in PrP cellular function. It is important to recall the properties of Cu2+ in buffer solution. The solubility product (Ksp) of Cu(OH)2 at physiological pH is in the order of 10 20. i.e., rather small. It is therefore likely that it is bound to other compounds like, e.g., human serum albumin which binds copper in the picomolar range [92]. To evaluate whether PrP is a functional copper binding protein one has to determine the relative concentration of membrane attached PrPc and that of albumin in the cerebrospinal fluid which is about 3 pM [93]. However, the free concentration of copper has not been determined. Inside cells... 

The specificity of the iron mobilization response in the perfusate system was also studied. Only Cp among the compounds tested proved to have any activity in the perfused livers. No activity was shown by 30 fxM apo-transferrin, HCOa", 10 /xM CUSO4, 5 mM glucose, 0.6 mM fructose, 120 iiM citrate, or 36 fxM bovine serum albumin, 21 fxM CUSO4. Further experiments designed to test apo-Cp, other copper oxidases, and other iron or copper binding compounds are in progress. 

We also need to know how metals are transported into cells and stored. Iron has been investigated most intensively. Mammals bind and transport iron by the serum protein transferrin and store it in ferritin. One protein can bind around 4500 Fe ions. Copper is taken up by the serum protein ceruloplasmin. Also albumin can bind and transport metal ions. The cystein-rich protein metallothioneine is formed in cells if toxic Cd and are present. This aprotein protects cells against the toxic effects of such metal ions. 

Plasma copper exists in two forms 5% is loosely bound to albumin, and the remainder exists in the form of copper complexes. Copper forms ionic bonds with either an imidazole or a carboxyl group of the amino acid of albumin. Loosely bound copper reacts readily with dithiocarbamate, and therefore has been called the directly reacting copper. It is generally assumed but not established that the albumin that binds this copper plays an important role in transporting copper in the blood. 

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]. 

Another important function of albumin is its ability to bind various ligands. These include free fatty acids (FFA), calcium, certain steroid hormones, bilirubin, and some of the plasma tryptophan. In addition, albumin appears to play an important role in transport of copper in the human body (see below). A vatiety of drugs, including sulfonamides, penicilhn G, dicumarol, and aspirin, are bound to albumin this finding has important pharmacologic implications. 

Copper is an essential trace element. It is required in the diet because it is the metal cofactor for a variety of enzymes (see Table 50—5). Copper accepts and donates electrons and is involved in reactions involving dismu-tation, hydroxylation, and oxygenation. However, excess copper can cause problems because it can oxidize proteins and hpids, bind to nucleic acids, and enhance the production of free radicals. It is thus important to have mechanisms that will maintain the amount of copper in the body within normal hmits. The body of the normal adult contains about 100 mg of copper, located mostly in bone, liver, kidney, and muscle. The daily intake of copper is about 2—A mg, with about 50% being absorbed in the stomach and upper small intestine and the remainder excreted in the feces. Copper is carried to the liver bound to albumin, taken up by liver cells, and part of it is excreted in the bile. Copper also leaves the liver attached to ceruloplasmin, which is synthesized in that organ. 

Many copper(II) complexes, including Cu(DIPS)2 (DIPS = diisopro-pylsalicylate), Cu(salicylate)2, and Cu(Gly-His-Lys), are also active in superoxide dismutation (437, 438), but their use in vivo is limited by dissociation of Cu(II) and binding to natural ligands such as albumin (439). In contrast, the activity of Fe-93 is not affected by albumin (439, 440). 

Recent publications signal the continued interest in the function of this protein. It has been called a stress enzyme, involved in influenza virus infection (Tomas and Toparceanu, 1986). An explanation for Wilson s disease in terms of a genetic defect resulting in failure to convert from a neonatal (i.e., low) level of ceruloplasmin and copper to a normal adult level has been reported (Srai et al., 1986). Tissue specificity for the binding of ceruloplasmin to membranes was demonstrated in a study investigating the possible role of ceruloplasmin-specific receptors in the transfer of copper from ceruloplasmin to other copper-containing proteins (Orena et al, 1986). Ceruloplasmin has been shown to be effective in transferring copper to Cu,Zn-SOD in culture (Dameron and Harris, 1987), as has copper albumin. In view of the variable content of copper in this protein, it is not clear which copper is transferred. 

The net charge on albumin appears to be more significant than the nature of the substrate when considering how much protein initially binds to the aqueous/solid interface. More protein adsorbed onto copper, nickel, and germanium substrates at pH 4.8, where albumin has no net surface charge, than at pH 4.0 or pH 7.4. Since no charge effects exist between the macromolecules adsorbed on the surface, high protein densities at the aqueous/solid interface would be expected. Copper and nickel appeared to accumulate the same quantities of albumin independent of the pH studied. 

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