27.1: Copper (24b.0)

The adult human body contains 70–80mg of copper. Of this, approximately 25% is in the skeletal muscle, 15% in the skin, 15% in the bone marrow, 19% in the skeleton, 8.0 to 15% in the liver, and 8.0% in the brain. The liver and brain contain the highest concentrations of copper. Like iron, copper can be found in two valences, Cu⁺ and Cu²⁺, and it is this ability to gain and lose electrons that give the enzymes into which it is incorporated their function, catalyzing oxidation-reduction reactions. Examples of copper enzymes (cupro-enzymes) are given in (Table 24b.1).

Table 24b.1 Function of some copper enzymes with oxidation and reduction activity in humans. From Uauy et al., American Journal of Clinical Nutrition 67S: 952S–959S, 1998

Copper Enzyme	Function
Ceruloplasmin (Ferroxidase I)	Copper transport, iron mobilization, antioxidant
Cytochrome c oxidase	Electron transport, terminal oxidase
Superoxide dismutase	Free radical detoxication
Tyrosinase	Synthesis of melanin
Lysyl oxidase	Collagen and elastin cross-linking
Amine oxidases	Deamination of primary amines
Dopamine-β-monooxygenase	Dopamine→norepi-nephrine
Peptidylglycine monooxygenase	α-Amidation of neuro peptides

• Functions of copper

Many clinical features of copper deficiency can be explained by changes in the activities of cupro­enzymes. For instance, the ferr­oxidases, of which there are several examples, are involved in oxidising Fe²⁺ to Fe³⁺ to facilitate iron incorporation into trans­ferrin. Cerulo­plasmin, a serum protein, was the first to be identified. A reduction in cerulo­plasmin (ferr­oxidase I) (EC 1.16.3.1) impairs the transport of iron to the erythro­poietic sites, leading to hypo­chromic anemia. Hephaestin is located on the baso­lateral membrane of gut cells (Vulpe et al., 1999).The membrane bound form of cerulo­plasmin is pre­dominantly located in the brain (Jeong and David 2003),while zyklopen is central to trans-placental transfer of iron, though it is also located in retinal cells (Chen et al., 2010).

Deficiency of the cupro­enzyme lysyl oxidase (EC 1.4.3.13) may be responsible for the skeletal and vascular defects reported in copper deficiency. Lysyl oxidase is essential for the normal maturation of collagen, specifically in the steps involving the formation of lysine-derived cross-links (O'Dell, 1976). The latter are required for the formation of strong, flexible con­nective tissues (Rucker and Murray, 1978).

Depigment­ation is often associated with a deficiency of tyro­sinase (EC 1.14.18.1), an enzyme involved in the production of melanin. The latter is necessary for protection against excess ultra­violet exposure. The central nervous system disturb­ances in copper deficiency are the result of myelin­ization derange­ments or of abnormal catechol­amine levels and associated decreased activity of dopamine β mono­oxygen­ase (EC 1.14.17.1) or reduced activity of cyto­chrome c oxid­ase (EC 1.9.3.1) (Mason, 1979).

• Absorption and metabolism of copper

The major site of copper absorption is the small intestine, although some copper is also absorbed from the stomach. Absorption is inversely related to the amount of copper in the diet, decreasing the likelihood of deficiency and toxicity (Turnlund et al., 1989). For dietary intakes above 5mg/d, absorption is < 20%, whereas for intakes < 1mg/d, absorp­tion is as high as 50% (Figure 24b.1).

Figure 24b.1. Copper absorption as a percentage and in mg/d in relation to a dietary copper intake varying from 0.8–7.5 mg/d. Redrawn from Turnlund et al., American Journal of Clinical Nutrition 67: 960S–964S, 1998

As noted above, on the apical membrane, facing the gut, copper is taken up by a carrier mediated process, involving a transporter named Ctr1 (Kuo et al., 2006). Following uptake, the Cu is transferred across the cell and excreted into the portal circulation using an energy-dependent process and a Cu-ATPase, known as ATP7A (Harris, 2000). This enzyme is deficient in Menkes disease patients, and its absence is responsible for the subsequent copper deficiency. It is mostly located in the gut, but it also plays an important role in trans-placental transport and may be involved in uptake across the blood brain barrier (Hardman et al., 2004).

Once absorbed, copper is trans­ported via the portal blood, bound to albumin and trans­cuprein, mainly to the liver. In the liver, copper is incorporated into cerulo­plasmin, and then dis­tributed to other tissues. For further details, see the review by Linder et al. (1998) and Harris (2000).

At present, the mechanisms of copper regulation are only beginning to be under­stood. Tissue copper con­cen­trations are main­tained within a narrow range, through homeo­static mechanisms that operate at the level of intestinal absorption and biliary excretion. Of the two levels, biliary excretion is the more important. As more copper is absorbed, turnover is faster, and, hence, more copper is excreted into the gastro­intes­tinal tract. Very little copper is excreted in the urine, and urinary losses do not help regulate copper stores. Relatively little copper is stored in the body, compared with other trace elements such as iron (Turnlund, 1998; McArdle et al. 1999).

Copper is taken into the gut cell through Ctr1. In the cell, and in fact in all other cells apart from red blood cells, copper is bound to one of a series of chaperones. The first to be identified was Atox1, followed by several others, each of which takes copper from the membrane transporter to specific target proteins (Palumaa, 2013). The copper is trans­ported across the cell, excreted into the portal circulation through the enzyme which is defective in Menkes disease, ATP7A and trans­ferred to the liver bound to albumin, trans­cuprein or as a low-molecular weight complex with histidine or other amino acids (Gambling et al., 2011).

Within the liver, Cu is stored as metallo­thionein, re-excreted in the bile or incorp­orated into cerulo­plasmin. Excretion and Cp incorp­oration both involve a second Cu-ATPase, ATP7B. ATPB is a membrane protein, which changes location in the liver ER, depending on whether the copper being trans­ported is excreted or incorp­orated into cerulo­plasmin (McArdle et al.,1999).

• Copper deficiency in humans

Copper deficiency in humans is rare. It has been described in infants recovering from malnutrition (Graham and Cordano, 1969) and in premature and low-birth-weight infants fed cow's milk (al‑Rashid and Spangler, 1971; Griscom et al., 1971; Ashkenazi et al., 1973). Copper deficiency has also been reported in patients receiving prolonged total paren­teral nutrition (TPN) unsup­plemented with copper (Karpel and Peden, 1972; Dunlap et al., 1974; Vilter et al., 1974) and in a woman receiving a normal diet sup­plemented with antacids (Anonymous, 1984). Severely handicapped patients (n=60) receiving an enteral diet with only 15µg/1000kcal Cu for 12–66mo also developed copper deficiency (Higuchi et al., 1988).

The earliest clinical manifestation of copper deficiency in humans is persistent neutropenia, usually followed by hypochromic microcytic anemia, scurvy-like bone changes, and osteoporosis (in infants) (Cordano et al., 1964; al-Rashid and Spangler, 1971; Karpel and Peden, 1972; Higuchi et al., 1988). The anemia probably results from defective iron mobilization and hemoglobin formation related to lower cerulo­plasmin activity (Danks, 1988), or to increased susceptibility to oxidative damage (Johnson and Kramer, 1987).

Less frequent features of copper deficiency include hypo­pigmen­tation of the hair and hypo­tonia (low muscle tone), as well as impaired growth, an increased incidence of infections, and changes in the phago­cytic capacity of the neutro­phils (Uauy et al., 1998). In some experimental depletion-repletion studies, changes in blood clotting factors (Milne and Nielsen, 1996) and in markers of bone meta­bolism (Baker et al., 1999) have also been reported. Low copper intakes have been implicated as a risk factor in the develop­ment of cardio­vascular disease (Lukaski et al., 1988; Klevay, 2000).

A few cases of adults with sickle cell disease who were receiving prolonged zinc therapy have devel­oped hema­tological signs of copper deficiency(Prasad et al., 1978). Clinical features of copper deficiency, with the exception of anemia, occur in infants with Menkes kinky hair syndrome (KHS), an X-linked, recessive, hereditary disorder of copper metabolism with an intra­cellular defect of copper utilization (Danks et al., 1972).

Most of the reports of copper deficiency in humans have resulted from inadequate dietary intakes of copper, often associated with pro­longed diarrhea, which prevents reabsorp­tion of copper from the bile. Hence, it is not surprising that subjects with mal­absorp­tion syn­dromes — including tropical sprue, celiac disease, cystic fibrosis, and short-bowel syndrome resulting from intestinal resection — are at risk of copper deficiency (Rodriguez et al., 1985).

Diseases associated with chronic loss of proteins, such as nephritic syndrome and protein-losing enter­opathy, may also cause copper deficiency as a result of loss of cerulo­plasmin and of copper bound to albumin. High intakes of fructose and sucrose exacer­bate the effects of copper deficiency, but the importance of this effect in humans is unclear (O'Dell, 1990).

• Food sources and dietary intakes

Copper is widely distributed in foods; the richest food sources are oysters, other shellfish, organ meats (e.g., liver, kidney), nuts, and dried legumes. Milk and dairy products are low in copper. The copper level in plants is not influenced by the copper content of the soil in which they grow (Institute of Medicine, 2001). Drinking water may contribute a relatively large amount of copper to the daily intake, especially if the water is slightly acidic or very soft and is supplied from copper pipes (Lönnerdal, 1996).

Copper deficiency among the general population has not been described. Dietary intakes of copper for adult European populations range from 0.9 to 2.3mg/d (van Dokkum, 1995). Data are available on the copper intakes of the U.S. population based on the Continuing Survey of Food Intakes by Individuals (CSFII 1994–1996) and NHANES III (IOM, 2001) and for the U.K. population, based on the National Diet and Nutrition Surveys (Gregory et al., 1990; Gregory et al., 1995; Gregory et al., 2000; Finch et al., 1998; Whitton et al., 2011). Copper intakes of infants fed breast milk or cow's milk are often low. More recently, the European Food Safety Authority has published data on dietary intake values in the human population (EFSA Panel on Dietetic Products 2015).

The composition of mixed western diets has little effect on the bioavailability of copper in humans. Copper deficiency has been observed in some cases of excessively high intakes of iron or zinc supplements (Fischer et al., 1984; Barclay et al., 1991; Botash et al., 1992) induced by their adverse effect on copper absorption. There is some evidence that the adverse effect of high dietary iron intake on copper absorption may occur only when copper status is low or marginal (Cohen et al., 1985).

• Effect of high intakes of copper

Under normal circumstances, copper levels in the body are tightly regulated. However, several cases of accidental copper toxicity in humans have been described, arising from ingestion of copper sulfate (Chuttani et al., 1965), acidic drinks in prolonged contact with copper (Paine, 1968), or drinking water with an unusually high copper concentration (800µg/L) (Salmon and Wright, 1971). Adverse effects include nausea, gastrointestinal effects (nausea, diarrhea, cramps, abdominal pain), and, in some cases, liver damage (O'Donohue et al., 1993; Pizarro et al., 1999).

Wilson disease results from defects in ATP7B, the Cu-ATPase responsible for trans­ferring copper from the liver to the bile or to the sites of ceruloplasmin synthesis. The defect results in copper overload in the liver and the brain. Patients suffering from Wilson's disease exhibit symptoms of copper toxicity such as nausea, vomiting, diarrhea, acute hemolytic anemia, hepatic necrosis, hepatic central vein dilation, and jaundice (Zelkowitz et al., 1980). In Wilson's disease, copper accumulates in the liver because the copper in the liver cells can neither enter the bile nor participate in cerulo­plasmin synthesis (Linder and Hazegh-Azam, 1996; Suzuki and Gitlin 1999). See Czlonkowska et al., 2019 for a com­pre­hensive review. Idio­pathic copper toxicosis has been described in an Austrian population and termed Tyrolean copper toxicosis (Muller et al., 1998). It is hypoth­esized that the disorder arises as a combination of an inherited genetic mutation and high coper intake. A similar disease has been described by Tanner and colleagues in Indian children (Tanner 1998), although the situation is probably more complex than a simple copper overload (Pankit and Bhave 2002).There is no evidence that high dietary copper intakes are a public health problem. The U.S Tolerable Upper Intake Level (UL) for copper for adults > 19y is 10mg/d. For pregnant and lactating women and adolescents 14–18y, the UL is set at 8mg/d (IOM, 2001). Currently in Europe (2020), EFSA is examining upper levels of copper to harmonise permitted pesticide levels with nutritional guidelines.

• Indices of copper status

The methods used to assess copper status in humans include static tests, as well as the measurement of the activities of certain copper enzymes. These methods are discussed below. Other approaches use cate­gorical regression to determine the optimal levels of copper in the diet (Chambers et al., 2010). There is still no consensus on the best bio­marker to use, in part, because of the efficiency of the homeo­static control that maintains tissue copper concen­trations within a narrow range (Harvey and McArdle 2008; Harvey et al., 2009; Danzeisen et al., 2007). As a result, measures of copper status tend to be resistant to change, except when dietary intakes are very low or very high. Serum copper or serum cerulo­plasmin are most often used to assess copper status, but the sensitivity and specificity of both these measures are poor. Hence, they should be combined with a more sensitive and specific functional measure such as cyto­chrome c oxi­dase in platelets or super­oxide dismu­tase in erythro­cytes. It is likely that in the future functional indicators, perhaps including anti­oxidant status, changes in immune function, and the lysyl oxidase activity in skin, will be used in conjunction with other biochemical markers.