In mammals, concentrations of copper in tissues and fluids are kept very constant. The main mechanism underlying this constancy is the ability of the liver to modulate excretion of copper by varying the amounts entering the bile – which end up in the feces. When biliary excretion of copper is hindered, as occurs when one of the proteins involved is defective, copper accumulates in the liver and later in other organs, with fatal consequences if untreated. Major examples are the copper “pump”, ATP7B, defective in Wilson Disease, and COMMD1, which also aids biliary excretion. With copper overload, there is increased copper in the urine, which normally is very low. Studies by Gray et al.[1] with the mouse Atp7b knockout model of Wilson disease determined that the increase in urine is due to a small copper carrier (SCC) of about 2 kDa, which our laboratory then determined was also in the blood plasma of these mice. Moreover, canines with defective ATP7B also had large increases in SCC-Cu in their blood and urine. We have been researching ways of purifying significant amounts of SCC, using commercially available porcine plasma, so it can be further characterized. Based on size exclusion chromatography (SEC) of 3 kDa ultrafiltrates in small pore gels, we know that usually most of the Cu is in a single component (SCC), which by ESI-MS appears to have a mass/charge ratio of 845, one atom of Cu(II) – detected in EPR as bound to N and O ligands, and a pI in the range of 3-4. New ways of purifying SCC beyond the ultrafiltration stage have been developed, involving concentration by lyophilization and special forms of dialysis using very small pore membranes. Hydrophilic interaction chromatography and additional SEC are providing much purer SCC, with more reasonable yields. Meanwhile, we have also been investigating the secretion and uptake of SCC-Cu, using cultured cells. SCC is secreted by hepatic cells as well as those modeling intestinal and kidney epithelium, particularly when preloading with Cu(II)-diHis. Intestinal (Caco2) cell monolayers with tight junctions secreted SCC mainly across the apical membrane (representing the gut lumen), but this was dramatically reversed when albumin was present in the basal medium (which is more physiological). Kidney epithelial cells also secreted SCC-Cu and some additional copper components mainly into the apical fluid. Other studies, with 67Cu-labeled SCC from hepatic secretions, confirmed not only that SCC was secreted, but showed that it was also taken up by all cell types examined. These findings suggest that metabolism of SCC and its copper is very active, and that SCC may play an important role in mammalian copper homeostasis. [1]Gray et al. PLoS ONE 7(6): e38327.