Copper Homeostasis in Mammals, with Emphasis on Secretion and Excretion. A Review

Abstract

One of the hallmarks of Cu metabolism in mammals is that tissue and fluid levels are normally maintained within a very narrow range of concentrations. This results from the ability of the organism to respond to variations in intake from food and drink by balancing excretion, which occurs mainly via the bile and feces. Although this sounds straightforward and we have already learned a great deal about aspects of this process, the balance between overall intake and excretion occurs over a high background of Cu recycling, which has generally been ignored. In fact, most of the Cu absorbed from the GI tract actually comes from digestive fluids and is constantly "re-used". A great deal more recycling of Cu probably occurs in the interior, between cells of individual tissues and the fluid of the blood and interstitium. This review presents what is known that is pertinent to understanding these complexities of mammalian Cu homeostasis and indicates where further studies are needed.

Keywords: ATP7A; ATP7B; bile; ceruloplasmin; copper; excretion; gastric fluid; kidney; liver; pancreatic fluid; saliva; secretion; small copper carriers; urine.

Figure 1

Time course of loss (A) and incorporation (B) of excess extrinsic ⁶⁵Cu from mouse organs after the injection of this stable isotope. Mice were preloaded with 3 times more Cu than the total estimated to be in their bodies, over 14 h, after which they were sacrificed at various times, as indicated, and the percent dose was determined. Data for plasma and brain were multiplied 10× and 100×, respectively, to make them visible. Annotated, and reprinted with permission from Springer, Biometals, Copper binding components of blood plasma and organs and their response to influx of.

Figure 2

Distribution of ⁶⁷Cu in rat organs and blood plasma at various times after tracer administration. Data for each time point are mean values for mostly 5–7 rats, given as % of ⁶⁷Cu dose. Data for brain and heart have been multiplied 10× to make them more visible. Reprinted with permission from reference Weiss, K.C.; et al. [14].

Figure 3

Effect of bile duct ligation on turnover of whole body Cu in rats. Tabulated data on rates of ⁶⁷Cu loss in rats where the common bile duct was and was not (sham) ligated prior to ⁶⁷Cu administration. Reprinted with permission from Linder, M.C. et al. [2].

Figure 4

Summary of proposed steps involved in the movement of cu into bile and secreted by hepatocytes. Based mainly on the data, reviews and figures provided by Polishchuk and Polishchuk [26] and Stewart et al. [27]. Cu ions enter hepatocytes from plasma proteins via copper transporter 1 (CTR1) and at least one other as yet unidentified transporter, and are carried via the chaperone ATOX1 to ATP7B (blue triangles) in the transGolgi network (TGN). In the lumen of the TGN, Cu will be incorporated into apoceruloplasmin (apoCp) (violet circles), forming holoCp, and be released into the blood plasma by exocytosis (center right of the figure) across the “basolateral” hepatocyte membrane. In the absence of excessive Cu not otherwise incorporated into Cp and endogenous Cu-dependent proteins and mitochondria, TGN Cu in the lumen will be exported to the bile canaliculi that are formed between hepatocyte apical membranes, leading to the gall bladder and bile duct. This happens through the budding of vesicles from the TGN that contain Cu and may also contain ATP7B. These fuse with late endosomes also containing ATP7B as well as lysosomes (to form the endo-lysosomal compartment), which then fuses with the apical hepatocyte membrane at a canaliculus (left side of figure). This releases Cu that was present in the vesicular bodies and also can lead to ATP7B being part of the apical membrane. The latter is particularly important in the presence of excess Cu, which can then flow from ATOX1 directly to ATP7B that pumps it into the bile canaliculi. Small amounts of the Cu efflux “pump”, ATP7A (blue squares), are also expressed in hepatocytes (far right of the figure), and may also contribute to Cu that exits hepatocytes and enters the blood plasma, to bind to proteins like albumin in the exchangeable plasma Cu pool. Based on what is known in other cells, the much less abundant ATP7A in hepatocytes would either transfer some Cu in the TGN into secretory vesicles (for exocytosis), and/or traffic it in vesicles to the basolateral membrane to pump Cu in the cytosol (on ATOX1) into the blood plasma, following which ATP7A would recycle back to the TGN.

Figure 5

Metallothionein (MT) and other Cu components in fish bile separated in size exclusion chromatography (SEC) coupled to ICP-MS. Bile samples were first heated to remove most other proteins by precipitation, and the supernatants (containing MT) were applied to SEC. Elution of Cu and some other metal ions is shown for bile obtained from fish in uncontaminated waters (A), in Cu contaminated waters (B), and in waters containing excess Zn (C). X-axis is time (from 5 to 40 min); Y-axis is metal isotope intensity for Cu (black), Zn (light grey), as well as Pb (dark grey) and Hg (very pale). Reprinted with permission from Hauser-Davis et al. [51].

Figure 5

Metallothionein (MT) and other Cu components in fish bile separated in size exclusion chromatography (SEC) coupled to ICP-MS. Bile samples were first heated to remove most other proteins by precipitation, and the supernatants (containing MT) were applied to SEC. Elution of Cu and some other metal ions is shown for bile obtained from fish in uncontaminated waters (A), in Cu contaminated waters (B), and in waters containing excess Zn (C). X-axis is time (from 5 to 40 min); Y-axis is metal isotope intensity for Cu (black), Zn (light grey), as well as Pb (dark grey) and Hg (very pale). Reprinted with permission from Hauser-Davis et al. [51].

Figure 6

Separation of Cu components of blood plasma from various species showing some Cu eluting at the end of the column volume, where the small Cu carrier (SCC) elutes. Whole plasma samples (100 uL) from humans (red), pigs (blue), sheep (green), and cows (black) were applied to a 25 mL column of Superdex 200, and the Cu content of 0.5 mL fractions collected was measured by furnace atomic absorption spectrometry (Linder et al., unpublished.) The peak of ceruloplasmin elution is in the region of fraction 29–30, and albumin is right after. Alpha-2-macro-globulin elution peaks at about fraction 22.

Figure 7

Cu in livers, urine and blood plasma of wild type (Control) and Wilson disease (Atp7b-/-) model mice. Elevations of urinary Cu (A) and liver Cu (B) (blue vs. grey bars) in the Atp7b-/- mice vs. wild type, and evidence for the presence of large amounts of low molecular weight Cu in the blood plasma (C) seen in Superdex 200 size exclusion chromatography (SCC, fractions 40–50). Figure A reprinted with permission, from Gray et al. [93]; data in B replotted from those in Table 1 of Huster et al. [94]; data in C are from Miguel Tellez, Abigael Muchenditsi, Svetlana Lutsenko, and Maria C. Linder, unpublished.

Figure 8

Levels of Cu associated with the small copper carrier (SCC) in the plasma (left) and urine (right) of Labrador retrievers that do and do not have defects in Atp7B, relative to the wild type. Values are for ng/mL Cu in 3 kDa ultrafiltrates (Means + SD) for blood plasma (left) and urine (right) of wild type controls (blue), and for retrievers with hetero (green) and homozygous (red) defects in Atp7b (5–6 animals/group). Pooled ultrafiltrate samples eluted as a single peak of ~1.7 kDa by size exclusion chromatography from a small pore gel column (Superdex 30 Increase) (not shown). From Sai Vallabhaneni, Kaitlynne Kim, Maria C. Linder and Hille Fieten, unpublished. * p < 0.01 vs. WT.

Figure 9

Overview and Summary of the homeostatic processes described in this review for the normal human adult. This figure diagrams amounts of copper entering the digestive tract from various sources and how they travel from the intestine to key organs (like liver and kidney) and the rest of the organism, as well as how copper is excreted. Values indicated are average mg Cu per day. Dietary Cu is indicated by purple lines; absorption and distribution to liver, kidney and other cells is in darker blue; involvement of ceruloplasmin (Cp) is in lighter blue; involvement of Cp and hephaestin (Hp) in Fe efflux from liver and intestine, respectively, is in the light blue and orange; secretions that lead to Cu efflux into the GI tract, feces and urine are in red. Modified and updated from Linder [9].