Some effects of selenium deficiency on glutathione peroxidase (EC 1.11.1.9) activity and tissue pathology in rainbow trout (Salmo gairdneri)

1. Two duplicate groups of rainbow trout (Salmo gairdneri; mean weight 27 g) were given diets of differing selenium content (deficient 0.025 mg Se/kg; supplemented 1.022 mg Se/kg) for 30 weeks. 2. There were no significant differences between treatments in weight gain but packed cell volume, liver vitamin E and liver and plasma Se concentrations were all significantly lower in the Se-deficient trout. 3. Ataxia occurred in about 10% of the Se-deficient trout and histopathologies were evident in nerve cord (damage to axon sheath) and liver (loss of integrity in endoplasmic reticulum and mitochondria with appearance of increased vesiculation). 4. Glutathione peroxidase (EC 1.11.1.9) activity was significantly reduced in liver and plasma of Se-deficient fish but there was no indication, from differential assay, of any non-Se-dependent glutathione peroxidase activity. Glutathione transferase (EC 2.5.1.18) activity was significantly increased in Se-deficient trout.     

The effects of dietary tryptophan levels on growth and metabolism of rainbow trout (Salmo gairdneri)

Groups of rainbow trout (Salmo gairdneri) (mean weight 14 g) were given diets containing 0.8, 1.3, 2, 3, 4 or 6 g tryptophan/kg diet for 12 weeks. By analysis of the growth results, the dietary requirement of tryptophan was found to be 2.5 g/kg diet (equivalent to 50 mg/kg biomass per d). Carbon dioxide expired by trout following intraperitoneal injection of [14COOH]tryptophan contained little radioactivity when dietary tryptophan level was low but, above 2.0 g/kg diet, it increased rapidly with increasing dietary tryptophan level. The break point in the dose-response curve did not, however, coincide with that from the growth results. Changes in concentrations of free tryptophan in blood and liver and activity of hepatic tryptophan pyrrolase (EC 1. 13. 11. 11) in response to changes in dietary tryptophan concentration did not provide reliable indicators for quantifying dietary requirement. Unlike the situation in mammals, blood tryptophan was not protein-bound to any appreciable extent. Tryptophan pyrrolase of trout has properties which suggest it has no apoenzyme form. In fish given adequate levels of tryptophan injected intraperitoneally with a tracer dose of [14COOH]tryptophan, 60% of the dose was incorporated into body protein within 1 d. The turnover of the label in this protein is very slow. Those trout given diets deficient in tryptophan suffered from severe scoliosis and lordosis as well as having increased liver and kidney levels of calcium, magnesium, sodium and potassium.     

Effect of increased dietary carbohydrate on selenium metabolism and toxicity in rainbow trout (Salmo gairdneri)

Juvenile trout were reared on either a high available carbohydrate (HC) or low available carbohydrate (LC) diet supplemented with from 0 to 10 micrograms selenium per gram of diet for 16 weeks, to determine if excess liver glycogen deposition affected the metabolism and toxicity of dietary selenium. Trout reared on the HC diet with 10 micrograms selenium per gram diet first demonstrated signs of selenosis and had significantly higher (P less than 0.05) liver selenium levels than trout reared on the LC diet with 10 micrograms selenium per gram diet after 16 weeks, indicating that excess dietary carbohydrate enhances dietary selenium toxicity in trout. The mechanism of the interaction is unclear since neither selenium elimination rates nor carcass and kidney selenium levels were affected by the dietary carbohydrate level. Trout reared on high dietary selenium diets (10 micrograms/g) had an increased incidence of renal calcinosis. In addition, liver copper levels were significantly affected by both dietary selenium and liver glycogen content indicating a significant copper-selenium and copper-glycogen interaction in trout. The development of renal calcinosis and the copper interactions suggest a variety of toxic effects of selenium on trout that may all be responsible for the observed changes in growth and feed efficiency.     

The effect of dietary lysine levels on growth and metabolism of rainbow trout (Salmo gairdneri)

Groups of rainbow trout (Salmo gairdneri; mean weight 5 g) were given diets containing 10, 12, 14, 17, 21, 24 and 26 g lysine/kg diet for 12 weeks. By analysis of the growth values the dietary requirement of lysine in this experiment was found to be 19 g/kg diet. A similar requirement value was obtained from a dose-response curve of expired 14CO2 (following an intraperitoneal injection of L-[U-14C]lysine) v. dietary lysine concentration. Liver concentrations of total lipid and carnitine and activities of lysine-alpha-ketoglutarate reductase (saccharopine dehydrogenase (NADP+, lysine-forming), EC 1.5.1.8) in the liver were not significantly different in fish from the different dietary treatments. Hepatosomatic index, however, was higher in those fish given low levels of dietary lysine.     

Urea cycle activity and arginine formation in rainbow trout (Salmo gairdneri)

Studies were conducted to determine whether rainbow trout fingerlings possess the ability to synthesize arginine via the urea cycle. Several urea cycle enzymes were detected in trout tissues. An experiment was conducted to determine whether the enzymes increase in response to starvation or in response to dietary protein level (0, 30, 40, 50% protein). Although some effects were observed, they did not appear to be consistent with the function of the urea cycle as a mechanism of detoxifying ammonia in the fish. The activities of kidney arginase and liver and muscle carbamoyl phosphate synthetase (CPS) were higher (P less than 0.05) when protein was omitted from the diet (P less than 0.05) than when it was present but were unaffected by protein level otherwise. The activities of liver arginase and kidney and muscle CPS and ornithine transcarbamoylase (OTC) were higher (P less than 0.05) in starved fish than in fish that received adequate levels of protein. Liver CPS and OTC were lower in starved fish than in fish fed 30% protein. L-[l-14C]ornithine hydrochloride and L-[carbamoyl-14C]citrulline, injected intraperitoneally, were incorporated into tissue arginine, a finding consistent with arginine biosynthesis via the urea cycle. When one-half of dietary arginine was replaced by equimolar amounts of glutamic acid, ornithine or citrulline, glutamic acid markedly reduced growth (P less than 0.05), whereas growth was depressed only slightly by ornithine (P less than 0.05) and not depressed by citrulline (P greater than 0.05). We conclude that trout have a urea cycle that provides for potential arginine biosynthesis.     

Effect of selenium deficiency on hydroperoxide-stimulated release of glutathione from isolated perfused liver of rainbow trout (Salmo gairdneri)

1. Duplicate groups of rainbow trout (Salmo gairdneri) were each given partially purified diets which were either adequate or depleted in selenium for 40 weeks. 2. Although there was no significant difference in weight gain, liver Se concentration was significantly lower in fish given the deficient diet. 3. Glutathione (GSH) peroxidase (EC 1.11.1.9) activity was significantly reduced in liver of Se-deficient fish but a differential assay did not indicate the presence of a non-Se-dependent GSH peroxidase activity, although liver GSH S-transferase (EC 2.5.1.18) was significantly increased. 4. Perfusion of livers from trout given Se-adequate diets with t-butyl hydroperoxide (BuOOH) or hydrogen peroxide caused an increase in the rate of release of glutathione disulphide (GSSG) into the perfusate. 5. Perfusion of livers from Se-deficient trout with BuOOH or H2O2 did not result in any change in rate of release of GSSG into the perfusate. 6. These findings confirm the absence of any compensatory non-Se-dependent peroxidase activity in Se-depleted trout.     

Riboflavin requirement for growth, tissue saturation and maximal flavin-dependent enzyme activity in young rainbow trout (Salmo gairdneri) at two temperatures

Two temperatures (10 degrees and 15 degrees C) and two fish stocks differing in growth potential were used to determine the dietary riboflavin requirement of young rainbow trout (Salmo gairdneri) on the basis of growth parameters, tissue saturation and a flavin-dependent biochemical function. Two experiments were conducted with purified diets based on vitamin-free casein. In experiment 1, fry (initially 2.0 g/fish) were fed, for 16 wk at 15 degrees C, diets containing 0.6, 2.6, 3.6, 4.6, 5.6 or 6.6 mg riboflavin per kilogram diet. In experiment 2, fry (initially 1.7-1.8 g/fish) were held at either 10 degrees or 15 degrees C and fed, for 10 wk, diets containing 0.7, 2.7, 3.7, 4.7, 6.7 or 8.7 mg riboflavin per kilogram. The riboflavin requirements for maximal growth rate, liver flavin saturation, spleen and head kidney flavin saturation and maximal hepatic D-amino-acid oxidase activity were 3.6, 4.6, 6.6 and 5.6 mg/kg diet, respectively, in a diet containing 40% crude protein and 15% ether extract. The requirements were not affected by temperature or by genetically determined differences in maximal growth rate. When expressed on a dietary energy basis, the riboflavin requirements of the trout for maximal growth rate and liver flavin saturation appear similar to those of several homeothermic species.     

Influence of dietary zinc on cataracts in rainbow trout (Salmo gairdneri).

The purpose of this study was to determine the cause of lens cataracts in hatchery trout fed diets containing white fish meal. After preliminary investigations, three experiments were conducted with fry of rainbow trout (Salmo gairdneri) that were fed practical-type diets containing either 40% herring meal (control diet) or 40% white fish meal with and without various mineral supplements. In all experiments, the trout fed herring meal grew well and had normal lenses. Those fed the diet containing white fish meal without supplement grew slowly and developed bilateral cataracts determined by using a slit-lamp biomicroscope. The severity of cataracts was increased by supplementing the diet with a mixture of minerals (phosphates and carbonates of calcium, sodium and potassium). Cataracts were prevented, however, with supplemental Na2EDTA or zinc but not by supplements of manganese, copper, iron or various other minerals. The metabolic alterations responsible for the zinc-deficiency cataract were not determined.