Chemistry Everyday for Everyone
Iron As Nutrient and Poison N. M. Senozan and M. P. Christiano Department of Chemistry and Biochemistry, California State University, Long Beach, CA 90840 Abstract Iron containing compounds of the body and the ingestion and elimination of iron are discussed. The function and transport of this metal among different sites and substances of the body are emphasized. Biochemical defects and nutritional habits that lead to excessive accumulation of iron and some unexpected consequences of this accumulation are described. Recent work implicate iron, even when its levels are only moderately elevated, in the onset of cardiovascular illness. Keywords Biochemistry Inorganic Chemistry Bioorganic/Bioinorganic Food Science Metals Iron Supplementary Materials No supplementary material available.
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Journal of Chemical Education • Vol. 74 No. 9 September 1997
Chemistry Everyday for Everyone
Iron As Nutrient and Poison N. M. Senozan and M. P. Christiano Department of Chemistry and Biochemistry, California State University, Long Beach, CA 90840 70% of the body’s iron is in hemoglobin (12–14). Myoglobin, which consists of a single polypeptide chain and resembles the β chain of hemoglobin, accounts for 10% of the body’s iron content. Myoglobin has a high affinity for oxygen: at 37 °C under an O2 pressure of 2.5 mm Hg, it is 50% saturated—half of the binding sites in the sample (i.e., the ferrous irons) are occupied by O2 (15). The high O2 affinity of myoglobin creates an environment of low oxygen pressure and facilitates the flow of this gas from blood into the cell. As expected, muscle cells with high oxygen demand are rich in myoglobin; skeletal muscle of an active porpoise, for example, contains ten times more myoglobin than that of the sluggish, shallow-diving sea cow. Myoglobin also serves as a cellular reservoir of oxygen: deep-diving mammals have such large quantities of this protein that their muscles appear almost black (16). Ferritin and hemosiderin together contain the bulk of the remaining 25% of the iron in the body. Unlike other iron-containing compounds, ferritin and hemosiderin do not Distribution of Iron in the Human Body have a specific function. They serve only as storage facilities, removing unneeded iron from the plasma or the intraThere is about 4.0 g of iron in an adult man weighing cellular environment or releasing it when the need arises. 75 kg. Women have on average 20% less iron for each kiloBoth compounds have very high iron content—up to 28% of gram of body mass, about 40 mg/kg (11). Nearly all the iron the ferritin mass can be iron. In hemosiderin this percentin the body, over 98%, is found in four compounds: hemoage may be even higher. In both, the metal is in the ferric globin, myoglobin, ferritin, and hemosiderin. The rest is disstate. Ferritin is a nearly spherical molecule made of a polytributed among a number of enzymes and the iron-transmeric ferric–hydroxide–phosphate core and a protein shell port protein transferrin. The abundance, iron content, moencapsulating this core. The shell protein has a molecular lecular characteristics, and function of iron-containing commass of 480,000 and is made of 24 subunits. Its outside dipounds of the human body are given in Table 1. ameter is 130 Å, about 2.5 times that of the hemoglobin tetHemoglobin is a tetrameric protein made of two α and ramer. The core is not well defined. An approximate empiritwo β polypeptide chains, each with a ferrous iron. It transcal formula is (FeOOH)8(FeOOPO3H 2). A single ferritin molports oxygen from the lungs to the tissues; on the average, 100 mL of blood contains 15 g of hemoglobin (5.2 × 10-2 g of ecule may contain anywhere from zero to 4500 ferric ions (5, 17–19). Fe) and is capable of holding 3.0 × 10{2 g of oxygen. Sixty to Hemosiderin is like ferritin whose protein capsule has been parTable 1. Iron-Containing Compounds of the Human Bodya tially or wholly denatured. Unlike % of Total Molecular Iron Content Heme or ferritin, hemosiderin is water inCompound Function b c c Body Iron Mass per Molecule Nonheme soluble. It is an amorphous material 4 Fe2+ Heme Oxygen transport Hemoglobin 60–85 6.4 × 104 of indefinite composition, made of mostly ferric hydroxide and phosMyoglobin 10 1.7 × 104 1 Fe2+ Heme Oxygen transport phate and some protein. It is beFerritin 6–15 8.9 × 105 4500 Fe3+ Nonheme Iron storage lieved that hemosiderin is made Hemosiderin 4–10 Ill defined Ill defined Nonheme Iron storage from ferritin by the oxidative denaTransferrin
