Ferroxidase activity

Another condition involving ceruloplasmin is aceru-loplasminemia. in this genetic disorder, levels of ceruloplasmin are very low and consequently its ferroxidase activity is markedly deficient. This leads to failure of release of iron from cells, and iron accumulates in certain brain cells, hepatocytes, and pancreatic islet cells. Affected individuals show severe neurologic signs and have diabetes mellitus. Use of a chelating agent or administration of plasma or ceruloplasmin concentrate may be beneficial. 

Although channel mutations and chemical modifications reduce rates of iron oxidation and uptake, they do not completely abolish the ferroxidase activity of... 

Haephaestin (Vulpe et al., 1999) basolateral membrane ferroxidase activity Vulpe et al, 1999) sex-linked anemia transfer... 

Ceruloplasmin Ferroxidase activity Cp-/- mice (Harris et al, 1999) Aceruloplasminaemia (Logan et al, 1994 Takahashi et al, 1996) deficient iron mobilization low serum iron tissue iron overload... 

Plants contain phytoferritins, which accumulate in non-green plastids1 in conditions of iron loading. They are targeted to the plastids by a putative-transit peptide at their N-terminal extremity, and possess the specific residues for ferroxidase activity and iron nucleation, found in mammalian H-type or L-type ferritin subunits. 

About a quarter of the total body iron is stored in macrophages and hepatocytes as a reserve, which can be readily mobilized for red blood cell formation (erythropoiesis). This storage iron is mostly in the form of ferritin, like bacterioferritin a 24-subunit protein in the form of a spherical protein shell enclosing a cavity within which up to 4500 atoms of iron can be stored, essentially as the mineral ferrihydrite. Despite the water insolubility of ferrihydrite, it is kept in a solution within the protein shell, such that one can easily prepare mammalian ferritin solutions that contain 1 M ferric iron (i.e. 56 mg/ml). Mammalian ferritins, unlike most bacterial and plant ferritins, have the particularity that they are heteropolymers, made up of two subunit types, H and L. Whereas H-subunits have a ferroxidase activity, catalysing the oxidation of two Fe2+ atoms to Fe3+, L-subunits appear to be involved in the nucleation of the mineral iron core once this has formed an initial critical mass, further iron oxidation and deposition in the biomineral takes place on the surface of the ferrihydrite crystallite itself (see a further discussion in Chapter 19). 

The multi-copper oxidases include laccase, ceruloplasmin, and ascorbate oxidase. Laccase can be found in tree sap and in fungi ascorbate oxidase, in cucumber and related plants and ceruloplasmin, in vertebrate blood serum. Laccases catalyze oxidation of phenolic compounds to radicals with a concomitant 4e reduction of O2 to water, and it is thought that this process may be important in the breakdown of lignin. Ceruloplasmin, whose real biological function is either quite varied or unknown, also catalyzes oxidation of a variety of substrates, again via a 4e reduction of O2 to water. Ferroxidase activity has been demonstrated for it, as has SOD activity. Ascorbate oxidase catalyzes the oxidation of ascorbate, again via a 4e reduction of O2 to water. Excellent reviews of these three systems can be found in Volume 111 of Copper Proteins and Copper Enzymes (Lontie, 1984). 

Several site-directed mutations were carried out with a new member of the blue oxidases , the Fet3p protein from Saccharomyces cerevisiae which is involved in iron trafficking. The protein oxidizes Fe2+ to Fe3+ as first step in the iron uptake chain. The mutations were targeted to block this ferroxidase activity and thus to identify the residues involved in iron binding. The respective mutants showed no perturbed type 1 sites.91... 

Caeruloplasmin (0.2-0.4 mg/ml) This copper-containing protein is regarded as a physiological inhibitor of lipid peroxidation. In this one of its many roles, it acts as an antioxidant by virtue of its ferroxidase activity, converting iron(II) to iron-(III) by electron transfer. 

The L chain subunit appears to be involved mostly in mineralization of the core but modulates the ferroxidase activity of the H chain subunit as well. 

Cartwright and Wintrobe and their co-workers suggested a link between copper deficiency and anemia in mammals 50 years ago (see Lahey et al., 1952). Cartwright subsequently demonstrated that this copper-dependent anemia was unresponsive to iron supplementation but was corrected on administration of ceruloplasmin (see Lee et al., 1968). The molecular basis of this link was indicated by Osaki and Friedan, who characterized the ferroxidase activity of ceruloplasmin and kinetically demonstrated that Cp could play a critical role in catalyzing trafficking of the potentially cytotoxic Fe(II) in the plasma to apoA f (see Frieden and... 

Osaki, 1974 Osaki, 1966 Osaki et al., 1966). Using the kinetic values given above, they estimated that without the ferroxidase activity of Cp in the plasma 80% of the iron released from erythrocyte turnover would accumulate as non-Tf-bound Fe(ll) and thereby would be unavailable for reabsorption by the reticuloendothelial system. Furthermore, this free Fe(II) could catalyze the formation of reactive oxygen species via the Fenton reaction. This, in turn, could lead to a subsequent organismal pathophysiology (Miyajima et al., 1996 Nakano, 1993). This inference has been strikingly confirmed by research over the past 6 years in both yeasts and mammals this research has directly tested the hypothesis that multicopper oxidase-dependent ferroxidase activity is essential to eukaryotic iron homeostasis (Askwith et al., 1996 Harris et al., 1995 1998 Wessling-Resnick, 1999). 

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