Characterization of glycosylphoshatidylinositol-anchored ceruloplasmin in multiple redent organs following dietary copper deficiency.

Abstract

Copper is a necessary metal cofactor in many enzymes that catalyze key reactions in both prokaryotes and eukaryotes. Ceruloplasmin (Cp) is a copper-dependent enzyme that acts as a ferroxidase, oxidizing FeII to FeIII for systemic iron mobilization. Cp is expressed as both a secreted plasma (sCp) enzyme and a membrane-bound glycosylphosphatidylinositol- anchored (GPI-Cp) splice variant enzyme. sCp is the most abundant copper-binding protein in mammalian plasma. The ferroxidase activity of Cp is essential for iron mobilization, as Cp null humans and mice exhibit selective tissue-specific iron overload. Dietary copper deficient (CuD) rodents have near total loss of Cp activity, severe loss of Cp protein, and anemia. The impacts of dietary copper deficiency on GPICp has not been previously evaluated. Studies were conducted in Holtzman and Sprague- Dawley rats, albino mice, and Cp -/- mice, to investigate the copper-iron interaction and further characterize GPI-Cp. Purified membrane extracts of these rodent tissues detected immunoreactive Cp protein, especially enriched in spleen and kidney, but not in membranes from Cp -/- mice. Immunoreactive Cp protein was released with phosphotidylinositol-specific phospholipase C treatment and released protein exhibited ferroxidase activity. These data suggest that the membrane-bound Cp immunoreactivity detected is GPI-anchored. Following perinatal and postnatal copper restriction, GPI-Cp was markedly lower in spleen and modestly lower in liver of CuD rats and mice, compared to copper-adequate (CuA) rodents. Livers of CuD mice contained elevated liver non-heme iron (NHI), while spleen NHI was lower in CuD than CuA rats, and not different in CuD mice, implying that lower GPI-Cp was not correlated with augmented NHI levels in CuD rodent spleens. Spleen and liver membranes of CuD rats expressed augmented levels of ferroportin, the iron efflux transporter, which may compensate for the loss of GPI-Cp in iron efflux. Copper deficient rats and mice both develop severe anemia but only in rats is plasma iron lower than normal, consistent with impaired Cp function. As multicopper oxidases like Cp are thought to be the major metabolic link between copper and iron, additional research is needed to determine the impact, if any, of lower GPI-Cp on iron flux and the development of anemia when copper is limiting.