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TABLE 1-12 Amino Acid Requirements of Juvenile Gilthead Sea Bream
in tryptophan-deficient diets greatly reduces the incidence of scoliosis (Akiyama et al., 1986).
Changes in mineral metabolism were observed in tryptophan-deficient rainbow trout (Walton et al., 1984b). Significantly greater concentrations of calcium (Ca) (a fourfold increase over control trout), sodium (Na), and potassium (K) were found in the kidneys of tryptophan-deficient trout. Concentrations of Ca, magnesium (Mg), Na, and K in the livers of tryptophan-deficient trout were also significantly greater than in normal trout. The metabolic lesion(s) responsible for these changes have not been resolved.
Relationships Among Amino Acids
Cystine can be formed metabolically from dietary methionine at a rate sufficient to meet the requirements of fish. The reverse sequence of reactions does not occur, however, and fish have an absolute requirement for methionine. Methionine can thus meet the total sulfur amino acid requirement of fish, although some of this requirement may be met by cystine.
Rainbow trout can use D-methionine to replace L-methionine on an equimolar basis (Kim et al., 1992). D-methionine is deaminated by D-amino acid oxidase and subsequently reaminated to L-methionine. This metabolic capacity is probably also characteristic of other fish.
A similar relationship exists between aromatic amino acids. Fish readily convert phenylalanine to tyrosine so that phenylalanine alone can meet requirements for aromatic
TABLE 1-13 Amino Acid Requirements of Juvenile Lake Trout
Amino Acid
Protein in Diet (%)
Requirement as Percentage of Dietary Protein
Requirement as Percentage of Dry Diet
Type of Diet
Reference
Isoleucine
27
2.0-2.6
0.5-0.7
Purified
Hughes et al. (1983)
Leucine
27
3.5-4.6
1.0-1.3
Purified
Hughes et al. (1983)
Valine
27
2.6-3.3
0.6-0.8
Purified
Hughes et al. (1983)
amino acids. However, the presence of tyrosine in the diet will reduce some of the requirement for phenylalanine.
Some adverse interactions may occur between amino acids that are structurally related when their concentrations in the diet are imbalanced. Well-known examples in homeotherms are antagonisms arising from dietary imbalances of lysine-arginine and of leucine-valine. No convincing evidence exists, however, for lysine-arginine antagonism in fish. Robinson et al. (1981) could not demonstrate any effects when diets with excess lysine in the presence of adequate or marginal arginine were fed to channel catfish; diets containing excess arginine in the presence of adequate or marginal lysine similarly failed to show any antagonistic effect. Nor did excess lysine affect the growth rates of rainbow trout fed low concentrations of arginine (Kim et al., 1983).
Antagonism between branched-chain amino acids generally arises in mammals from an excess of leucine over isoleucine and valine; the first two steps of the catabolic breakdown of all three branched-chain amino acids are catalyzed by the same enzymes. Data on antagonisms among branched-chain amino acids in fish are not clear-cut and are inconsistent between species. Thus the isoleucine requirement of chinook salmon (Oncorhynchus tshawytscha) increased slightly with increasing concentrations of dietary leucine (Chance et al., 1964). Hughes et al. (1983) observed changes in concentrations of branched-chain amino acids in lake trout given diets containing increasing amounts of valine. Plasma isoleucine and leucine were both elevated in valine-deficient fish, and their concentrations decreased as
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