FISH NUTRITION
401
be the determination of their amino acid composition. It would then
seem a straightforward step to supplying the required amino acid
pattern from a mixture of proteins. Unfortunately the processing of
proteins frequently interferes with the availability of certain amino
acids to the body. While amino acids remain chemically measurable
a proportion of some of them becomes biologically unavailable. In
particular many proteins are considerably altered by heat processing,
during which terminal groups of certain amino acids are prone to combine with other compounds (carbohydrate and lipid) or groups. Other
amino acids such as methionine or tryptophan may be oxidized.
Consequently the proteins become resistant to proteolytic enzymes.
The best known example relates to the essential amino acid lysine.
In the intact protein the E-amino group of lysine residues in the protein
will react under conditions of moist heat with active groups of nonprotein materials to form chemical linkages which cannot be broken
by proteolytic enzymes. Carpenter and his colleagues (Carpenter and
Ellinger, 1955) have shown that such substituted lysine residues are
biologically unavailable ; only lysine residues with reactive E-amino
groups are apparently available. Carpenter (1 960) has described a
method of measuring the biologically available lysine in a food
protein by reaction of E-amino groups with l-fluoro-2,4-dinitrobenzene
and estimating the total e-dinitrophenyl lysine after acid hydrolysis.
Thus variations will occur both in the nutritive quality of different
samples of the same protein and in the nutritive value of different types
of proteins of similar amino acid content. Although a knowledge of the
chemically determined amino acid composition of proteins is extremely
useful, the nutritive value of proteins and of mixed protein diets can
only finally be assessed by testing them on the animal species for which
they are intended, and under the conditions in which they will be used.
The measurement of the nutritional value of proteins presents
difficulties and is inherently non-specific because the assay is concerned
with the qualitative and quantitative adequacy of ten essential amino
acids at one and the same time. This contrasts with the assay of other
essential nutrients , e.g, vitamins , where each compound is assayed
singly. Methods used for the assay of nutritive value of proteins have
been frequently and critically reviewed (Frost, 1959 ; McLaughlan and
Campbell, 1969; Morrison and Rao, 1966) but a brief description of the
principal procedures seems appropriate at this point.
Chemical more
Whole hen’s egg protein approximates to the dietary requirements
of several mammals in its essential amino acid composition and is
401
be the determination of their amino acid composition. It would then
seem a straightforward step to supplying the required amino acid
pattern from a mixture of proteins. Unfortunately the processing of
proteins frequently interferes with the availability of certain amino
acids to the body. While amino acids remain chemically measurable
a proportion of some of them becomes biologically unavailable. In
particular many proteins are considerably altered by heat processing,
during which terminal groups of certain amino acids are prone to combine with other compounds (carbohydrate and lipid) or groups. Other
amino acids such as methionine or tryptophan may be oxidized.
Consequently the proteins become resistant to proteolytic enzymes.
The best known example relates to the essential amino acid lysine.
In the intact protein the E-amino group of lysine residues in the protein
will react under conditions of moist heat with active groups of nonprotein materials to form chemical linkages which cannot be broken
by proteolytic enzymes. Carpenter and his colleagues (Carpenter and
Ellinger, 1955) have shown that such substituted lysine residues are
biologically unavailable ; only lysine residues with reactive E-amino
groups are apparently available. Carpenter (1 960) has described a
method of measuring the biologically available lysine in a food
protein by reaction of E-amino groups with l-fluoro-2,4-dinitrobenzene
and estimating the total e-dinitrophenyl lysine after acid hydrolysis.
Thus variations will occur both in the nutritive quality of different
samples of the same protein and in the nutritive value of different types
of proteins of similar amino acid content. Although a knowledge of the
chemically determined amino acid composition of proteins is extremely
useful, the nutritive value of proteins and of mixed protein diets can
only finally be assessed by testing them on the animal species for which
they are intended, and under the conditions in which they will be used.
The measurement of the nutritional value of proteins presents
difficulties and is inherently non-specific because the assay is concerned
with the qualitative and quantitative adequacy of ten essential amino
acids at one and the same time. This contrasts with the assay of other
essential nutrients , e.g, vitamins , where each compound is assayed
singly. Methods used for the assay of nutritive value of proteins have
been frequently and critically reviewed (Frost, 1959 ; McLaughlan and
Campbell, 1969; Morrison and Rao, 1966) but a brief description of the
principal procedures seems appropriate at this point.
Chemical more
Whole hen’s egg protein approximates to the dietary requirements
of several mammals in its essential amino acid composition and is