5.2 Essential Minerals for Finfish
5.2.7. Zinc 1. Functions and Metabolism
The essential function of zinc for living organisms is based on its role as an integral constituent of a number of metalloenzymes and as a catalyst for regulating the activity of specific Zn-dependent enzymes. Approximately 20 Zn metalloenzymes have been identified, including carbonic anhydrase, alkaline phosphatase, carboxypeptidases A and related peptidases, alcohol dehydrogenases, and cytsolic superoxide dismutase (Table 5.1). Thus Zn regulates many metabolic processes of carbohydrate, lipid, and protein metabolism. In addition to its role in enzyme function, Zn may have a structural role in nucleoproteins and is also involved in the metabolism of prostaglandins. Although the role of Zn in metabolic processes is evi- dent, little is known of the relationship between the biochemical function and the pathological signs. Some of the clinical features of Zn deficiency may arise from disturbances of nucleic acid and protein metabolism.
Fish accumulate Zn from both water and dietary sources. Interest in en- vironmental pollution and heavy metal contamination of human food has resulted in many investigations on the accumulation and distribution of Zn in aquatic organisms (Hogstrand and Wood, 1996; Alsop and Wood, 1999). In fish, the main sites of Zn uptake are the gills and the gastrointesti- nal tract (Pentreath, 1973; Part and Svanbert, 1981; Lovegrove and Eddy, 1982). However, dietary Zn is more efficiently utilized. In freshwater, even when dietary Zn levels are adequate, there is an active uptake of waterborne Zn (Spryet al.,1988). Zinc absorption is relatively higher in rainbow trout fed a Zn-deficient diet (Spryet al.,1988). The gills in rainbow trout also play a major role in excretion of dietary Zn (Hardyet al.,1987).
Very little information is available on Zn absorption and mechanisms involved in its regulation. In mammals, Zn homeostasis is thought to be maintained by mechanisms operating at the sites of absorption and secre- tion in the gastrointestinal tract. In winter flounder, the entire digestive tract is capable of absorbing Zn, but the uppermost portion of the intestine has the highest capacity and the stomach has the lowest (Shears and Fletcher, 1983). Waterborne Ca is a competitive inhibitor of branchial Zn influx (Alsop and Wood, 1999). Zinc inhibits the influx of Ca across gills thus Ca in water protects fish against Zn toxicity. Rainbow trout and carp can tolerate 1700 to 1900 mg zinc/kg without any apparent signs of toxicity (Wekellet al., 1983; Jeng and Sun, 1981). Common carp absorb and accumulate higher amounts of Zn in viscera, whereas the ability of trout to tolerate Zn in water is very limited (McKee and Wolf, 1963). It appears that excretory mechanisms and the control of gastrointestinal tract uptake may play important roles in maintaining Zn homeostasis. The zinc content of scales reflects the environ- mental metal concentrations. Zinc is normally excreted via the kidneys or by chloride cells of the gills (Bryan, 1976).
5.2.7.2. Deficiency
Although the essentiality of Zn and various deficiency signs for terrestrial animals and humans have been well recognized (Underwood, 1977; Prasad, 1985), its ubiquity in the environment and in feed ingredients makes it seem unlikely that a deficiency of Zn could cause a significant problem in fish.
Between 1973 and 1974, a widespread occurrence of cataracts in rainbow trout fed practical diets in the United States ultimately led to the realization that Zn was unavailable in white fish meal (Ketola, 1979). Other signs of Zn deficiency have also been characterized (Table 5.2). In rainbow trout, Zn deficiency causes growth depression, high mortality, lens cataracts, ero- sion of fins and skin (Ogino and Yang, 1978a), and short body dwarfism (Satohet al.,1983a). Excess minerals (total ash) present in white fish meal may affect Zn absorption and retention, resulting in lens cataract (Ketola, 1979). Caudal fin Zn concentration is a good indicator of Zn status in rain- bow trout (Wekell et al., 1986). In catfish, diets low in Zn cause reduced growth, appetite, bone Zn and Ca levels, and serum Zn concentrations (Gatlin and Wilson, 1983). Broodstock diets low in Zn reduced the egg production and hatchability of eggs (Takeuchiet al.,1981).
Elevated levels of dietary Zn (500 to 1000 mg Zn/kg) caused reduced hemoglobin, hematocrit, and hepatic copper concentrations in rainbow trout (Knoxet al.,1982, 1984). However, the Cu status of catfish was not im- paired by diets containing 200 mg Zn/kg (Gatlinet al.,1989). Common carp accumulate much higher concentrations of Zn in their tissues, particularly
in the viscera, than other fish studied, without any apparent toxicity signs ( Jeng and Sun, 1981).
5.2.7.3. Requirement
A dietary requirement for Zn (mg/kg) has been reported for several freshwater fish fed semipurified diets (Table 5.2): rainbow trout and com- mon carp, 15–30 (Ogino and Yang, 1978, 1979); Atlantic salmon, 37–67 (Maage and Julshamn, 1993); channel catfish, 20 (Gatlin and Wilson, 1983);
blue tilapia, 20 (McClain and Gatlin, 1988); red drum, 20–25 (Gatlinet al., 1991); and guppy, 80 (Shim and Lee, 1993). The minimum Zn requirement varies with age, sexual maturity, composition of diet, water temperature, and water quality. Among dietary factors, Ca and P levels, phytic acid, pro- tein source, form of Zn, and Ca content affect Zn absorption and retention in fish (Takeda and Shimma, 1977; Gatlin and Wilson, 1984a; Hardy and Shearer, 1985; Richardsonet al.,1985; Wekellet al.,1986; Satohet al.,1987, 1989; McClain and Gatlin, 1988).
5.2.7.4. Sources
The concentration of Zn in numerous marine invertebrates, vertebrates, and plants from several geographic locations worldwide is summarized by Eisler (1980). The richest source of Zn are filter feeding bivalve mollusks, especially oysters (>1200 mg Zn/kg). Most animal and fish tissues, when un- contaminated, contain approximately 30 mg Zn/kg of dry matter. Among feedstuffs, the common cereal grains contain 15 to 30 mg zinc/kg. Most of the Zn is found in the bran and germ fraction of grain. Typical veg- etable protein concentrates may contain 40–80 mg Zn/kg. Levels of 80 to 100 mg Zn/kg are more common in fish meal. Egg albumin, because of its low Zn content (<3 mg/kg), is generally used in low-Zn experimental diets.
Zinc sulfate (ZnSO4) and ZnNO3are effectively utilized by rainbow trout as Zn supplements (40 mg/kg) in diets containing white fish meal, and they also alleviate dwarfism and cataract problems (Satoh et al., 1987).
Some differences exist in Zn bioavailability from feedstuffs of plant and animal origin. As mentioned previously, plant protein contains phytates.
Soluble phytates added to animal protein decrease Zn bioavailability and account for a large part of the low availability in oil seed protein (Oberleas, 1973). The bioavailability of Zn in fish meal is greatly affected by the tri- calcium phosphate content (Satoh et al., 1987). Higher levels of supple- mental Zn should be included in the practical feeds to compensate for the reduced Zn bioavailability caused by dietary phytate, calcium, and phosphorus.