The puzzle posed by COMMD1, a newly discovered protein binding Cu(II)

Copper is critically important for cellular metabolism. It plays essential roles in developmental processes, including angiogenesis. The liver is central to mammalian copper homeostasis: biliary excretion is the major route of excretion for ingested copper and serves to regulate the total amount of copper in the organism. An extensive network of proteins manipulates copper disposition in hepatocytes, but comparatively little is known about this protein system. Copper exists in two oxidation states: most extracellular copper is Cu(II) and most, if not all, intracellular copper is Cu(I). Typical intracellular copper-binding proteins, such as the Cu-transporting P-type ATPases ATP7B (Wilson ATPase) and ATP7A (Menkes ATPase), bind copper as Cu(I). Accordingly, the recent discovery that the ubiquitous protein COMMD1 binds Cu(II) exclusively raises the question as to what role Cu(II) may play in intracellular processes. This issue is particularly important in the liver and brain. In humans, Wilson's disease, due to mutations in ATP7B, exhibits progressive liver damage from copper accumulation; in some Bedlington terriers, mutations in COMMD1 are associated with chronic copper-overloaded liver disease, clinically distinct from Wilson's disease. It seems unlikely that Cu(II), which generates reactive oxygen species through the Fenton reaction, has a physiological role intracellularly; however, Cu(II) might be the preferred state of copper for elimination from the cell, such as by biliary excretion. We argue that COMMD1 participates in the normal disposition of copper within the hepatocyte and we speculate about that role. COMMD1 may contribute to the mechanism of biliary excretion of copper by virtue of binding Cu(II). Additionally, or alternatively, COMMD1 may be an important component of an intracellular system for utilizing Cu(II), or for detecting and detoxifying it.