COPPER TRANSPORT

In adult humans, the net absorption of dietary copper is approximate t o 1 mg/d. Dietary copper joins some 4-5 mg of endogenous copper flowin g into the gastrointestinal tract through various digestive juices. Mo st of this copper returns to the circulation and to the tissues (inclu ding liver) that formed them. Much lower amounts of copper flow into a nd out of other major parts of the body (including heart, skeletal mus cle, and brain). Newly absorbed copper is transported to body tissues in two phases, borne primarily by plasma protein carriers (albumin, tr anscuprein, and ceruloplasmin). In the first phase, copper goes from t he intestine to the liver and kidney; in the second phase, copper usua lly goes from the liver land perhaps also the kidney) to other organs. Ceruloplasmin plays a role in this second phase. Alternatively, liver copper can also exit via the bile, and in a form that is less easily reabsorbed. Copper is also present in and transported by other body fl uids, including those bathing the brain and central nervous system and surrounding the fetus in the amniotic sac. Ceruloplasmin is present i n these fluids and may also be involved in copper transport there. The concentrations of copper and ceruloplasmin in milk vary with lactatio nal stage. Parallel changes occur in ceruloplasmin messenger RNA expre ssion in the mammary gland las determined in pigs). Copper in milk cer uloplasmin appears to be particularly available for absorption, at lea st in rats.