Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

III. Copper
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acid on Cu absorption are modest and probably occur only
at the extremes of ascorbate intake ( Jacob et al. , 1987 ;
Lonnerdal, 1998 ; Stern et al. , 2007 ).
From a conceptual perspective, studies in yeast have
shed light on proteins involved in the process of Cu transport.
For example, in S. cerevisiae , high-affinity Cu ion
uptake has been characterized as temperature and ATP
dependent. Cu ion uptake appears to be coupled with K
efflux with a 1:2 stoichiometry, suggesting that the process
takes place via a Cu /2 K antiport mechanism. In yeast,
the gene for Cu reductase activity, designated FRE1 , is
regulated by activation of a transcription factor in response
to cellular Cu levels ( Pena et al. , 1999 ; Puig and Theile,
2002; Stern et al. , 2007 ; Theile, 2003). In mammalian
cells, the entry of Cu into cells is first orchestrated by the
action of a reductase and then contact with a high-affinity
Cu transporter, currently designated as Ctr1 and Ctr3 (Ctr2
is a low-affinity transporter). High-affinity Cu uptake is
saturable with a K m of 1 4 μ mol/l. Under Cu-limiting
conditions, there is evidence that the transporters and proteins
involved in Cu redox are up-regulated, whereas under
Cu-replete conditions, they are down-regulated (Theile,
2003). Of some significance, Ag and Zn ions can compete
for Cu by using the Crt transporters Zn and Ag ions have
similar chemical characteristics as Cu 1 . Effective competition
does not occur except at intakes 5 to 10 or more
fold the Zn requirement and when Cu intake is marginal
( Committee on Cu in Drinking Water, 2000 ; Stern et al. ,
2007 ).
In addition to the transporters, cellular chaperones specific
for Cu deliver Cu to specific cellular proteins ( Fig. 22-4 ).
Other important features of Cu regulation include the role
of metallothionein, a divalent metal-binding protein for Cu,
Zn, Hg, and Cd, which acts to buffer shifts in the cellular
concentrations of Cu (and Zn). Cu egress or transport out
of cells is controlled by P-ATPase Cu-transporters that are
located on the surface of vesicles that arise from Golgi processing.
Although a change in Cu status does not appear
to alter Cu-transporting P-ATPase gene expression, it can
affect the movement of copper-containing vesicles to and
from that outer cell membrane. Cu homeostasis must be
coordinated, as the release of free Cu ions can cause damage
to cellular components by catalyzing the generation of
reactive oxidant species (ROS).
Free cuprous ions (and ferric) react readily with
hydrogen peroxide to yield deleterious hydroxyl radicals.
Accordingly, Cu homeostasis is regulated tightly, and
unbound Cu is extremely low in concentration ( one atom/
cell). For example, Atox1, a chaperone that delivers Cu into
egress or efflux pathways, docks with a Cu-transporting
ATPase (ATP7B in the liver or ATP7A in other cells).
ATP7B directs Cu to plasma ceruloplasmin or to biliary
excretion in concert with another chaperone, Murr1, the protein
missing in certain types of canine Cu toxicosis. ATP7A
directs Cu within the trans Golgi network to the proteins
dopamine β -monooxygenase, peptidylglycine α-amidating
monooxygenase, lysyl oxidase, and tyrosinase, depending
on the cell type ( Stern et al. , 2007 ; Theile, 2003).
FIGURE 22-4 (Continued) nearly all Cu uptake under low copper conditions. It is one of a family of proteins involved in Cu transport that is transcriptionally
induced at low copper levels and degraded at high copper levels. Associated with this transporter is a copper reductase that maintains Cu
in the 1 state (its most soluble form) while in the vicinity of the transporter. Next, Cu is transferred to chaperones whose functions are to carry copper
to specific proteins within the cell (e.g., COX-17 → cytochromes, ATOX → vesicular P-ATPases, CCS → SOD). Copper egress (efflux) is accomplished
by a novel process, the transport of copper into secretory vesicles via post-Golgi processing. This occurs coincidently with efflux of specific
apocuproproteins (see text) that are localized to the same vesicles. On the membrane of the vesicles, two membrane-bound Cu-transporting ATPase
enzymes, ATP7A and ATP7B, catalyze an ATP-dependent transfer of Cu to intracellular compartments or expel Cu from the cell (from the Golgi to and
from the cell membrane). In response to a high level of cellular Cu, there is recycling of the vesicles at higher rates to more effectively remove copper.
Within the vesicles, apocuproproteins can also become activated. Consequently, secreted cuproproteins with enzyme activity, such as lysyl oxidase or
ceruloplasmin, often reflect Cu status or dietary intake. Some evidence also suggests DMT or Nramp transporters important to iron transport can play a
minor role in copper uptake. (b) Intestinal and systemic cellular manganese transport is mediated mostly by divalent metal transporter 1 (DMT1) and is
up-regulated in iron deficiency. Within the body, Mn bound to transferrin is taken up by transferrin receptors. Unlike other transition metals, which are
not found in “free” ion forms in cells, the behavior of Mn is analogous to Mg (i.e., dissociable ion exists). Less is know currently about specific chaperones
than for copper or zinc. Excess Mn in cells is sequestered on ferritin. A Golgi-derived ATPase has been described to facilitate the movement of Mn
from and to the nucleus and cis- and trans-Golgi compartments. Although the evidence is indirect that other types of ion channels and vesicular egress
play a role in Mn cellular transport, given that MnO 4 anion can be transported in addition to Mn 2 and Mn 3 , a role for oxyanion transport is indicated.
(c) Selenium is delivered to cells via amino acid and oxyanion transporters and when present in plasma via processes that recognize selenoprotein P. The
selenite and selenate forms must first be reduced (via a glutathione reduction system) to HSe- before Se can be utilized as a cofactor. Selenomethionine,
if not incorporated into protein, can also be eventually converted to HSe-. Next, for incorporation into specific Se-proteins (e.g., GPx, 5-ID, or
Se-protein P), HSe- is phosphorylated (requires ATP). Then following transfer to Ser-tRNA UGA to form Se-Cys-tRNA UGA , the stage is set for translation
of Se-containing proteins. Regarding cellular efflux, Se is lost from cells as secreted Se-proteins, such as selenoprotein P, Se-cystathionine, or as
volatile forms of methylated Se (e.g., CH 3 -Se-CH 3 ). (d) Zinc uptake and cellular translocation are controlled by two large families of metal transporters
for which there more than two dozen variants. More specifically, the two solute-linked carrier (SLC) gene families encode the zinc transporters: ZnT
(SLC30) and Zip (SLC39). The ZnT transporters reduce intracellular zinc availability by promoting zinc efflux from cells or into intracellular vesicles,
whereas Zip transporters increase intracellular zinc and promote extracellular zinc uptake. The ZnT and Zip transporter families exhibit unique tissuespecific
expression and differential responsiveness to dietary Zn intake and to physiological stimuli. Temporary influxes of Zn are buffered by the induction
of metallothionein. DMT1 and over-ion channels can play minor roles in Zn transport.