LEYES REGLAMENTARIAS DE AMPARO

Ferritin

Ferritin is the principal protein for iron storage and has a half-hfe of about 60 hours. Almost all cells contain ferritin, which functions as a storage site for iron and as an accessible reserve for iron that has been acquired by the cell in excess of its metabolic

demands. As shown in Figure 1.2A-C, the protein is a spherical shell of MW about 450,0000 g/mol made up of 24 subunits that are designated as the H (heavy) and L (light) subunits. The greatest number of L subunits are found in cells of tissues that have specialized roles in iron storage, for instance liver and spleen whereas H subunit is abundant in heart muscle. The walls of the ferritin shell are approximately 10 °A thick and surround a spherical space approximately 80 °A in diameter (Theil, 1987). The amount of iron as a crystalhne core of ferric-oxide-phosphate is varied in the protein shell. Measurement of non-haem iron concentrations show relatively large amounts in the liver, spleen and bone marrow. The spherical space can contain a maximum of 4,500 iron atoms, which is equivalent to about 0.25 M, or about lO'^-fold more concentrated than Fe(III) ions in aqueous solution (Donlin et al., 1998).

■*%

*v:v r#-A

B

C

Figure 1 .2 Molecular models of ferritin: 3-fold channel with the three intersecting subunits (A), 4-fold channel with the four intersecting subunits (B) represented as ribbons and all 24 subunits (C) (From Donlin et al., 1998).

The concentration of ferritin in serum of healthy persons is directly related to the available storage iron in the body and is higher in men than women, with a range between 12 and 250 pg/1. In patients with iron-deficiency anaemia ferritin concentrations are below 12 pg/1; conversely, the concentration may be as high as 10,000 |ig/l in serum of patients with iron overload (Jacobs et al., 1978). The concentration of serum ferritin appears to reflect the storage iron in macrophages and hepatocytes so that it can be used to monitor therapeutic removal of excess storage iron.

H aem osiderin

Haemosiderin is the major iron storage protein present when excessive iron deposits in the tissues. The structure of haemosiderin is supposed to relate to ferritin (Fisbach et al., 1971). However the exact chemical nature of haemosiderin is not well understood. It is known to be a water insoluble protein possessing a high iron to protein ratio (Crichton and Ward, 1992), probably consisting of degraded ferritin protein and ferric hydroxide polymers or cores of varying size (Hoy and Jacobs, 1981). These could potentially be involved in the formation of hydroxyl radicals (O'Connell et al., 1986) and consequently liver tissue damage (Weir et al., 1984). Whereas ferritin is the major iron-storage protein in the liver in the absence of iron overload, haemosiderin is predominantly found in the iron overload conditions (Halliday et al., 1994) and in some liver diseases such as liver cirrhosis and acute hepatitis (Graudal et al., 1996).

1.2.2.3 Functional iron pools

A brief description of other key iron binding molecules in human metabolism is given below. A detailed account of the stmcture and function of these molecules is beyond the scope of this thesis. Nevertheless, a brief overview is required in order to set the context of NTBI.

Haem oglobin

Haemoglobin (Hb) is a spheroidal tetrameric haemoprotein composed of two pairs of a- and p-globin polypeptide chains with a haem, ferroprotoporphyrin IX, as prosthetic group bound covalently at a specific site in each chain (Bunn and Forget, 1986) (Figure 1.3).

A planar haem molecule consists of a ferrous ion atom located in the centre of a tetrapyrrole ring which is tightly bound to the protein subunits. Strong hydrophobic interactions and a single co-ordinate bond between the imidazole ring of the proximal histidine residue of globin and Fe(II) are responsible for binding haem to the globin.

Haemoglobin A (HbA) is the major adult haemoglobin comprising of two «-chains (each containing 141 amino acids) and two p-chains (each containing 146 amino acids) written as «2^2- Two billion erythrocytes are produced each day in the normal adult (Brittenham,

1994). Each red cell also contains over a billion iron atoms located in the tetrameric haem centre of Hb. With the binding of an oxygen molecule to haem iron, each unliganded a or p subunit undergoes a conformational change in tertiary stmcture and subsequently causes an increase in the oxygen affinity of the remaining unliganded subunits.

M yoglobin

Myoglobin consists of between 150 and 160 amino acid residues in a single polypeptide chain, called globin, being species dependent and associated with a haem group like haemoglobin. The folded globin chain forms a crevice that almost completely encloses a haem group (McKee T and McKee JR, 1999). Amino acid residues of the myoglobin molecule are appreciably different from those of haemoglobin (Maclean, 1978). Myoglobin, found in high concentrations in skeletal and cardiac muscle, gives these tissues their characteristic red colour and serves mainly as oxygen storage and as a facilitator of intracellular O2 diffusion.

The cytochromes

The cytochromes are named for their intense red-orange colour and are found in most aerobic life forms. They play important roles in dioxygen utilisation, mitochondrial respiration, and electron transport in photosystems I and II. The cytochromes involved in electron transport are classified as cytochromes a, b and c. The iron atom in all cytochromes can alternate between Fe^^ and Fe^^oxidation states. Comparatively, the microsomal cytochrome-P450s contain haem iron as a prosthetic group (Crichton, 1990). The major functions of the cytochrome-P450s are involved in the biosynthesis and catabolism of endogenous compounds such as steroids and are relevant to the oxidative metabolism of exogenous compounds such as drugs and chemicals.

Lactoferrin

Lactoferrin is a non-haem, single-chain bilobed 80-kD glycoprotein containing two N- linked complex biantennate oligosaccharides to bind two Fe^^ (Baker et ah, 1987). It was first characterized as a red iron-binding protein from bovine and human milk and has been subsequently found in nearly all body fluids and in the secondary granules of neutrophils (Masson and Heremans, 1968; Bennett and Kokinski, 1979). The iron-binding sites of lactoferrin retain their affinity for the metal under very acidic conditions (Groves, 1960). Human milk lactoferrin contains 0.10-0.11% iron. Blood lactoferrin originates in neutrophils which release lactoferrin during exocytosis of specific granules. It is present in the plasma of normal humans at concentrations of 0.1-2.6 mg/1 and at higher concentrations in lactating women and patients with chronic myeloid leukaemia (Rumke et al., 1971; Bennett and Mohla, 1976). Lactoferrin in blood may inhibit division of macrophage and granulocyte progenitor cells in the bone marrow (Hangoc et al., 1987) and may also regulate iron retrieval and processing during erythrocyte catabolism and biogenesis (Retegui et al., 1984). Lactoferrin is unlikely to play a significant role in plasma iron transport but has been proposed to mediate the hypoferraemia of acute inflammation by blocking iron release from macrophages (Van Snick et al., 1974). Hepatocytes take up iron from lactoferrin by endocytosis (McAbee and Esbensen, 1991); however, the role of lactoferrin in hepatic iron metabohsm remains unclear. Clinically, plasma lactoferrin has been used as a marker of neutrophil populations and activation (Suzuki et al., 1991).