5.2 Essential Minerals for Finfish
5.2.4. Iron 1. Functions and Metabolism
Iron is an essential element in the cellular respiratory process through its oxidation–reduction activity and electron transfer. It is found in the body mainly in the complex form bound to proteins such as heme compounds (hemoglobin and myoglobin), heme enzymes (mitochondrial and micro- somal cytochromes, catalase, peroxidase, and so on), and nonheme com- pounds [transferrin, ferritin, and iron-containing flavoproteins (ferredox- ins, dehydrogenases)]. Iron metabolism has been reviewed extensively by Bernat (1983).
A great deal of information in the area of Fe absorption and transport has resulted from studies on humans. Myoglobin found in muscle differs from hemoglobin only in the nature of the protein component of the molecule.
Iron occurs in the blood as hemoglobin in erythrocytes and as transferrin in plasma. Small quantities of Fe as ferritin are also found in the serum.
Transferrin serves as the principal carrier of Fe in blood and therefore plays an important role in Fe metabolism. Storage Fe proteins, ferritin and hemosiderin, occur widely in liver, spleen, and bone marrow. The two pro- teins are chemically different although intimately related in function.
The absorption of Fe is influenced by the age, state of health, iron status, and conditions within the gastrointestinal tract of the fish. The amount and chemical form of Fe ingested and the amount and proportion of both inorganic and organic components present in the diet can also affect the absorption of Fe in fish. Iron occurs in foods mainly in the organic form in combination with proteins such as hemoglobin, myoglobin, and other
complexes. Iron present in inorganic forms or as iron–protein complexes must be reduced to the ferrous state to be available for absorption. These changes are accomplished by gastric juices and other digestive secretions.
The presence of reducing substances in the diet (e.g., ascorbic acid) can enhance the ability of fish to absorb iron. The mechanism of Fe absorption and transport is complex and is reviewed elsewhere (Wienket al.,1999).
There is relatively little information on the absorption and metabolism of iron in fish and other aquatic organisms. It appears that mechanisms of iron absorption from the digestive tract, and of storage and excretion, may be similar to those in other vertebrates. Some absorption of Fe takes place across the gill membrane, however, the intestinal mucosa is considered the major site of Fe absorption. Food is considered as the major source of Fe for metabolic purposes. However, the addition of ferrous sulfate to water may also enhance growth and hemoglobin level in certain warmwater fishes(Xiphophorus helleriandX.maculatus)(Roeder and Roeder, 1966). The absorbed Fe is probably transported in the blood by transferrin, which has been identified in several fish species. Aisenet al.(1972) found a protein in hagfish that closely resembles human transferrin. In rainbow trout Fe is absorbed from the peritoneal cavity and stored at higher concentrations in the liver, spleen, and head kidney (Walker and Fromm, 1976). There is little endogenous iron lost in the urine or feces.
5.2.4.2. Deficiency
Iron deficiency is not commonly observed in fish cultured under practical conditions, however, it may be readily produced experimentally in certain fish fed low-Fe diets (Table 5.2). Iron deficiency causes characteristic micro- cytic anemia or low hemoglobin levels in brook trout (Kawatsu, 1972), rain- bow trout (Desjardins, 1985), Atlantic salmon (Bjornvic and Maage, 1993;
Andersenet al., 1996; Lall, unpublished data), red sea bream (Sakamoto and Yone, 1976a, 1978b), yellowtail (Ikedaet al.,1973), eels (Nose and Arai, 1979), and carp (Sakamoto and Yone, 1978c). In most cases, the growth of fish was not influenced by the Fe deficiency. The normal liver color changes to yellowish-white during Fe deficiency in carp (Sakamoto and Yone, 1978c).
In catfish, Fe deficiency suppresses hemotocrit, hemoglobin, and plasma iron levels and causes transferrin saturation (Gatlin and Wilson, 1986b).
Sakamoto and Yone (1979c) found that ferrous chloride and ferric chloride were equally effective in prevention of anemia in red sea bream. However, a somewhat higher concentration of ferric citrate was required.
Dietary Fe toxicity signs develop in rainbow trout fed more that 1380 mg Fe kg−1(Desjardins, 1985). The major effects of Fe toxicity include reduced growth, poor feed utilization, feed refusal, increased mortality, diarrhea, and histopathological damage to liver cells.
5.2.4.3. Requirement
The Fe requirement of certain fish has been established quantitatively (Table 5.3). The Fe requirements for catfish (Gatlin and Wilson, 1986b), red sea bream (Sakamoto and Yone, 1976a, 1978b), and eel (Nose and Arai, 1979) are 30, 150, and 170 mg/kg of diet, respectively. The reported dietary Fe requirement of Atlantic salmon ranges from 33 to 100 mg/kg (Bjornvic and Maage, 1993; Andersenet al.,1996; Lall, unpublished). Iron supplied from a purified diet (39 mg of iron/kg of diet) may not be sufficient to avert Fe deficiency in rainbow trout (Desjardins, 1985), indicating a need for dietary inorganic Fe supplement.
5.2.4.4. Sources
The concentration of Fe in common feedstuff is highly variable and greatly influenced by the degree of contamination from ferrous metal dur- ing processing. Feeds of animal origin, other than milk by-products, are rich sources of iron. Fish meal and meat meal contain approximately 150 to 800 mg Fe/kg. The Fe content of cereal grain varies from 30 to 60 mg/kg, whereas oil seed protein may contain 100 to 200 mg Fe/kg.
Certain feed-grade calcium phosphates and limestones may contain 2000 mg Fe/kg.
Little is known about the form of Fe found in common feedstuffs and their bioavailability to fish. In animal protein supplements, Fe may be present as iron–porphyrin, myoglobin, and hemoglobin. In cereal grains, a small proportion of Fe occurs as an iron–phytin complex. The utilization of fer- rous sulfate and ferric chloride was essentially the same. In red sea bream, ferrous and ferric chloride are more efficiently utilized than ferric citrate (Sakamoto and Yone, 1979c). A hemoglobin regeneration assay in Atlantic salmon showed that the relative biological availability of Fe from ferric chloride, ferric oxide, blood meal, and herring meal was 98.8, 17.8, 52.3, and 47.1%, respectively (Naser and Lall, 1997). However, Andersonet al.
(1997) observed that heme iron from blood meal was more efficiently uti- lized than ferrous sulfate in Atlantic salmon using liver Fe concentration and hemoglobin slope ratio methods. It is well known that in animals the bioavailability of iron is affected by the composition of test diets, process- ing methods used to produce blood meal, Fe concentration in the diet, age and species of test animals, intake of iron relative to the need, chem- ical form in which iron is supplied from other dietary components, and amount and proportion of other dietary components with which Fe inter- acts metabolically (Hallberg, 1981; Forbes and Erdman, 1983; Wienket al., 1999).