12.1.2. Concentration and Spectrum of the Free Amino Acids
405
fuses out of the eye fluid into the lens, where it is
acetylated; it then diffuses back into the eye fluid
and is cleaved by a deacetylase. All freshwater
animals have large amounts of acetylhistidine in
the heart; marine teleosts, in contrast, have much
less. In fish that undertake spawning migration,
the concentration of acetylhistidine in the heart
correlates to the habitat in which embryo development occurs, e.g. it is tenfold higher in the salmon than in the eel [82].
Methylated amino acids are formed posttranslationally in numerous proteins and are
released by hydrolysis. They include, in particular, N-mono-, di- and trimethyllysine, 3methylhistidine, and mono- and dimethylarginine. 1-Methylhistidine, on the other hand, arises
by methylation of the free amino acid. Thus, for
example, the silkworm Bombyx mori contains
eight different methyl derivatives of lysine, histidine and arginine, often at high concentrations.
Trimethyllysine exceeds free lysine during the
fifth larval stage, where it enters the ovary of the
pupa and accumulates in the eggs; adult males
contain only trace amounts. In this case, the trimethyllysine apparently has a specific role, perhaps as the starting point for carnitine synthesis
[174]. Mammalian erythrocytes also contain high
concentrations of trimethyllysine [153]. The marked, species-specific differences in the concentration of methylated amino acids are exploited to
identify certain sorts of meat in foodstuff chemistry. For example, the presence of 3methylhistidine is an indicator of pork in meat
products. A large number of N-trimethyl amino
acids (betaines) are known in plants, whereas animals contain mainly glycinebetaine (Fig. 12.1),
which arises by oxidation of choline and is an
important donor of methyl groups. It is of great
importance for intracellular osmoregulation in
many marine invertebrates. Thus, its concentration in the tissues of marine molluscs and crustaceans can reach very high levels of 80-120 !J.mol/g
[10].
The sulphur-containing amino acid taurine
(Fig. 12.8) has various biological functions: it is
an important osmoregulatory substance in many
marine animals; it is conjugated with bile acids,
retinoic acid or certain foreign substances in vertebrates; it is bound in peptide form in the synapto somes of calf brain; and it is a constituent of the
phosphagen taurocyamine. It is therefore not
surprising to find marked species- and tissuespecific variations in taurine content. The concentration in skeletal muscle is usually higher than
in other tissues of the same animal [262].
The highest recorded concentration of about
100 !J.mol/g fresh weight (1.3 % of the total fresh
weight) is found in the giant nerve fibres of the
cephalopod Loligo sp. Values of around 10 !J.mol/g
are found in the tissues of many marine invertebrates and teleosts, but values are much lower in
the freshwater fish, crayfish and insects. Deamination of the taurines produces isethionic acid
(Fig. 12.8), which is also found at its highest level
in cephalopod nerves: the giant axons of Loligo
contain more than 150 !J.mol/g fresh weight, i.e.
no less than 20 % of the dialysable anions. Much
lower concentrations have been recorded in the
ganglions of the snail Ap/ysia and the mussel Spisu/a, and in various mammalian organs; about
40-200 !J.mol pen day are excreted in human
urine [82, 271]. Free L-Ianthionine (Fig. 12.1) has
been found in most insects examined, but not in
crustaceans, annelids, molluscs, echinoderms,
tunicates or vertebrates. In contrast, the structurally related amino acid cystathionine is widely
found as an intermediate in the metabolism of
methionine to cysteine (Fig. 12.7). An enzyme
discovered in the fat body of Bombyx mori produces lanthionine from cysteine and is apparently
very similar, or even identical, to cystathionine
synthase, which produces cystathionine from
homocysteine and serine. The fat body also contains an enzyme that cleaves lanthionine to cysteine, ammonia and pyruvate [225]. The sulphurcontaining amino acid feIinine (Fig. 12.1) has an
especially interesting distribution: it is excreted in
amounts of up to 120 mg per day in the urine of
the domestic cat and somewhat less in the urine
of the ocelot; it is absent from other cat species,
such as the lion, tiger and puma, and is found
nowhere else, except in some bacteria [62, 82].
12.1.2. Concentration and Spectrum
of the Free Amino Acids
The ninhydrin reagent normally used for the
determination of free amino acids also reacts to
varying degrees, depending on the conditions,
with unsubstituted amino groups such as amines,
amino-sugars and ammonia. In contrast, peptides
react less strongly than their constituents released
by hydrolysis. Therefore, when considering concentration data it is necessary to distinguish
between free amino acids, non-protein-bound
amino acids (after hydrolysis of the oligopeptides) and total ninhydrin-positive substances.
With the exception of certain insect species, the
concentration of peptides in the tissue and body
405
fuses out of the eye fluid into the lens, where it is
acetylated; it then diffuses back into the eye fluid
and is cleaved by a deacetylase. All freshwater
animals have large amounts of acetylhistidine in
the heart; marine teleosts, in contrast, have much
less. In fish that undertake spawning migration,
the concentration of acetylhistidine in the heart
correlates to the habitat in which embryo development occurs, e.g. it is tenfold higher in the salmon than in the eel [82].
Methylated amino acids are formed posttranslationally in numerous proteins and are
released by hydrolysis. They include, in particular, N-mono-, di- and trimethyllysine, 3methylhistidine, and mono- and dimethylarginine. 1-Methylhistidine, on the other hand, arises
by methylation of the free amino acid. Thus, for
example, the silkworm Bombyx mori contains
eight different methyl derivatives of lysine, histidine and arginine, often at high concentrations.
Trimethyllysine exceeds free lysine during the
fifth larval stage, where it enters the ovary of the
pupa and accumulates in the eggs; adult males
contain only trace amounts. In this case, the trimethyllysine apparently has a specific role, perhaps as the starting point for carnitine synthesis
[174]. Mammalian erythrocytes also contain high
concentrations of trimethyllysine [153]. The marked, species-specific differences in the concentration of methylated amino acids are exploited to
identify certain sorts of meat in foodstuff chemistry. For example, the presence of 3methylhistidine is an indicator of pork in meat
products. A large number of N-trimethyl amino
acids (betaines) are known in plants, whereas animals contain mainly glycinebetaine (Fig. 12.1),
which arises by oxidation of choline and is an
important donor of methyl groups. It is of great
importance for intracellular osmoregulation in
many marine invertebrates. Thus, its concentration in the tissues of marine molluscs and crustaceans can reach very high levels of 80-120 !J.mol/g
[10].
The sulphur-containing amino acid taurine
(Fig. 12.8) has various biological functions: it is
an important osmoregulatory substance in many
marine animals; it is conjugated with bile acids,
retinoic acid or certain foreign substances in vertebrates; it is bound in peptide form in the synapto somes of calf brain; and it is a constituent of the
phosphagen taurocyamine. It is therefore not
surprising to find marked species- and tissuespecific variations in taurine content. The concentration in skeletal muscle is usually higher than
in other tissues of the same animal [262].
The highest recorded concentration of about
100 !J.mol/g fresh weight (1.3 % of the total fresh
weight) is found in the giant nerve fibres of the
cephalopod Loligo sp. Values of around 10 !J.mol/g
are found in the tissues of many marine invertebrates and teleosts, but values are much lower in
the freshwater fish, crayfish and insects. Deamination of the taurines produces isethionic acid
(Fig. 12.8), which is also found at its highest level
in cephalopod nerves: the giant axons of Loligo
contain more than 150 !J.mol/g fresh weight, i.e.
no less than 20 % of the dialysable anions. Much
lower concentrations have been recorded in the
ganglions of the snail Ap/ysia and the mussel Spisu/a, and in various mammalian organs; about
40-200 !J.mol pen day are excreted in human
urine [82, 271]. Free L-Ianthionine (Fig. 12.1) has
been found in most insects examined, but not in
crustaceans, annelids, molluscs, echinoderms,
tunicates or vertebrates. In contrast, the structurally related amino acid cystathionine is widely
found as an intermediate in the metabolism of
methionine to cysteine (Fig. 12.7). An enzyme
discovered in the fat body of Bombyx mori produces lanthionine from cysteine and is apparently
very similar, or even identical, to cystathionine
synthase, which produces cystathionine from
homocysteine and serine. The fat body also contains an enzyme that cleaves lanthionine to cysteine, ammonia and pyruvate [225]. The sulphurcontaining amino acid feIinine (Fig. 12.1) has an
especially interesting distribution: it is excreted in
amounts of up to 120 mg per day in the urine of
the domestic cat and somewhat less in the urine
of the ocelot; it is absent from other cat species,
such as the lion, tiger and puma, and is found
nowhere else, except in some bacteria [62, 82].
12.1.2. Concentration and Spectrum
of the Free Amino Acids
The ninhydrin reagent normally used for the
determination of free amino acids also reacts to
varying degrees, depending on the conditions,
with unsubstituted amino groups such as amines,
amino-sugars and ammonia. In contrast, peptides
react less strongly than their constituents released
by hydrolysis. Therefore, when considering concentration data it is necessary to distinguish
between free amino acids, non-protein-bound
amino acids (after hydrolysis of the oligopeptides) and total ninhydrin-positive substances.
With the exception of certain insect species, the
concentration of peptides in the tissue and body