Copper is a redox active
metal that is essential for
biological function.
Copper is potentially toxic; thus, its homeostasis is carefully regulated through a system of
protein transporters.
Copper is taken up across the lumen surface of the small intestinal microvilli as
cuprous ion by Ctr1. Cupric ion may also be taken up, but those processes are less well understood. Within the cell, intestinal as well as others,
copper is escorted to specific compartments by
metallochaperones. One, CCS, donates
copper to
superoxide dismutase. Another, COX17, delivers
copper to additional chaperones within the mitochondria for synthesis of
cytochrome c oxidase. A third chaperone, Atox1, delivers
copper to the secretory pathway by docking with 2
P-type ATPases. One, ATP7A, is the
protein nonfunctional in
Menkes disease. This
protein is required for cuproenzyme biosynthesis, and in the enterocyte it is required for
copper efflux to portal blood. The second, ATP7B, predominantly expressed in liver, is required for
copper metallation of
ceruloplasmin and biliary
copper excretion. Mutations in ATP7B lead to
Wilson disease. Additional intracellular hepatic
copper-
binding proteins COMMD1 (
copper metabolism MURR1 domain) and XIAP (
X-linked inhibitor of apoptosis protein) may also be required for excretion. Other
proteins involved in
copper homeostasis may include
metallothionein and
amyloid precursor
protein.
Plasma protein transport of
copper from the intestine to liver and in systemic circulation probably includes both
albumin and
alpha2-macroglobulin. Changes in the expression of
copper "transporters" may be useful to monitor
copper status of humans, provided a suitable cell type can be sampled.