434
12 Small Nitrogenous Compounds
worm Bombyx mori and almost all species of
Lepidoptera. Some moth larvae also contain Dalanine, as do the nymphs and adults of the bug
Oncopeltus fasciatus. In contrast, neither D-serine
nor D-alanine were detected in the honey bee or
several species of beetle [36]. D-alanine is found
in the muscles of all crustaceans examined, and in
some species it also occurs in the hepatopancreas
or the heart, but not in the haemolymph or the
nervous system. D-Alanine has also been found in
sea-urchin eggs and embryos [61]. The cysteine
residue in the luciferin of all fireflies examined
has the D-configuration but free n-cysteine was
not found [36]. D-2,3-diaminopropionic acid
occurs in the haemolymph and in the gut of the
larvae of Bombyx mori and in several other caterpillars before pupation [36].
L-Aspartic acid converts spontaneously to the
D-isomer at a temperature-dependent rate of
about 0.1-0.25 % per year. Long-lived proteins,
e.g. in human teeth or eye lenses, therefore contain increasing proportions of D-aspartate with
age. However, spontaneous racemization does
not explain, for example, why the cerebrum of
young rats has 8.4 % of total free aspartate as the
D-isomer, or why there is 4.9 % D-aspartate in
human foetal blood and the value decreases with
age. It is not clear whether the D-aspartate in
these cases originates in the diet or from bacterial
symbionts, or whether it is synthesized de novo
during cell metabolism. Perhaps D-aspartate
really has a transmitter function, as was claimed
several years ago, although the idea was largely
rejected [71]. In agreement with this concept is
the fact that D-asparaginase is found only in the
nervous system of the octopus and other cephalopods and not in other organs. The presence of
considerable quantities of D-aspartate in the gills
and foot of the mussel Solenomya reidi [78] is
probably the result of uptake of the D-amino acid
from seawater, which contains D-aspartate and
other D-amino acids.
So far, there is little evidence for the production of D-amino acids in metabolism. In the silkworm, D-serine is produced from 14C-Iabelled glucose or from L-serine. This reaction also takes
place in homogenates and has a pH optimum of
9. The production ofD-2,3-diaminopropionic acid
from glucose has been demonstrated using the
same technique [36]. The peptides dermorphin,
dermencephalin and deltorphin, which are highly
potent opioids in the skin of frogs from the subfamily Phyllomedusinae, contain a D-alanine residue which is formed from an L-alanine by posttranslational modification (p. 297). The D-amino
acids detected in the blood and various organs of
man and other mammals have been thought to
have a microbial origin; however, they also are
found in axenic ally reared mice [161].
More is known about the catabolism of the Damino acids than about their synthesis. As
already mentioned, the D-isomers can be used to
satisfy the requirements for several essential
amino acids. There are two types of D-amino acid
oxidases in animals, both of which contain FAD
as the coenzyme but otherwise differ in their specificities. The first type oxidizes a series of aminomonocarbonic acids, the exact spectrum of which
varies with the species. Enzymes of this type have
been found in the kidney and liver of almost all
vertebrates examined, although with widely different activities; they are localized in the peroxisomes. They have also been found in certain regions of the brain and in neutral leucocytes. The
345-347-amino-acid sequence of the kidney
enzyme agrees between different mammals at
about 77 % of positions and shows no relationship
to other proteins [241, 252]. Enzymes with similar
properties have also been found in the Malpighian tubules and fat bodies of insects and in the
hepatopancreas of molluscs; they are probably
widely distributed. Their biological significance is
unclear; the main function of the enzymes from
mammalian liver and kidney is that of a glycine
oxidase, although this would account for less than
1 % of the total glycine turnover [36]. The second
type of D-amino acid oxidase, which is highly specific for D-aspartic acid and D-glutamic acid, has
so far been found only in the kidney of some
mammals, the hepatopancreas of the cephalopod
Octopus vulgaris and the antennal gland of the
crayfish Orconectes limos us [164]. This enzyme
also has FAD as the coenzyme but, in contrast to
the enzymes of the first type, is not inhibited by
benzoates. Comparative investigations have
shown that D-aspartate oxidase is present in all
eight-armed cephalopods (order Octopoda), but
is consistently absent from the ten-armed species
(order Decapoda).
12.8 Amines
Many amines have specific biological functions as
neurotransmitters, neuromodulators or neurohormones. Biogenic amines are therefore found
especially in the nervous system, although in very
different, species-specific concentrations and
combinations. As components of toxins, they are
12 Small Nitrogenous Compounds
worm Bombyx mori and almost all species of
Lepidoptera. Some moth larvae also contain Dalanine, as do the nymphs and adults of the bug
Oncopeltus fasciatus. In contrast, neither D-serine
nor D-alanine were detected in the honey bee or
several species of beetle [36]. D-alanine is found
in the muscles of all crustaceans examined, and in
some species it also occurs in the hepatopancreas
or the heart, but not in the haemolymph or the
nervous system. D-Alanine has also been found in
sea-urchin eggs and embryos [61]. The cysteine
residue in the luciferin of all fireflies examined
has the D-configuration but free n-cysteine was
not found [36]. D-2,3-diaminopropionic acid
occurs in the haemolymph and in the gut of the
larvae of Bombyx mori and in several other caterpillars before pupation [36].
L-Aspartic acid converts spontaneously to the
D-isomer at a temperature-dependent rate of
about 0.1-0.25 % per year. Long-lived proteins,
e.g. in human teeth or eye lenses, therefore contain increasing proportions of D-aspartate with
age. However, spontaneous racemization does
not explain, for example, why the cerebrum of
young rats has 8.4 % of total free aspartate as the
D-isomer, or why there is 4.9 % D-aspartate in
human foetal blood and the value decreases with
age. It is not clear whether the D-aspartate in
these cases originates in the diet or from bacterial
symbionts, or whether it is synthesized de novo
during cell metabolism. Perhaps D-aspartate
really has a transmitter function, as was claimed
several years ago, although the idea was largely
rejected [71]. In agreement with this concept is
the fact that D-asparaginase is found only in the
nervous system of the octopus and other cephalopods and not in other organs. The presence of
considerable quantities of D-aspartate in the gills
and foot of the mussel Solenomya reidi [78] is
probably the result of uptake of the D-amino acid
from seawater, which contains D-aspartate and
other D-amino acids.
So far, there is little evidence for the production of D-amino acids in metabolism. In the silkworm, D-serine is produced from 14C-Iabelled glucose or from L-serine. This reaction also takes
place in homogenates and has a pH optimum of
9. The production ofD-2,3-diaminopropionic acid
from glucose has been demonstrated using the
same technique [36]. The peptides dermorphin,
dermencephalin and deltorphin, which are highly
potent opioids in the skin of frogs from the subfamily Phyllomedusinae, contain a D-alanine residue which is formed from an L-alanine by posttranslational modification (p. 297). The D-amino
acids detected in the blood and various organs of
man and other mammals have been thought to
have a microbial origin; however, they also are
found in axenic ally reared mice [161].
More is known about the catabolism of the Damino acids than about their synthesis. As
already mentioned, the D-isomers can be used to
satisfy the requirements for several essential
amino acids. There are two types of D-amino acid
oxidases in animals, both of which contain FAD
as the coenzyme but otherwise differ in their specificities. The first type oxidizes a series of aminomonocarbonic acids, the exact spectrum of which
varies with the species. Enzymes of this type have
been found in the kidney and liver of almost all
vertebrates examined, although with widely different activities; they are localized in the peroxisomes. They have also been found in certain regions of the brain and in neutral leucocytes. The
345-347-amino-acid sequence of the kidney
enzyme agrees between different mammals at
about 77 % of positions and shows no relationship
to other proteins [241, 252]. Enzymes with similar
properties have also been found in the Malpighian tubules and fat bodies of insects and in the
hepatopancreas of molluscs; they are probably
widely distributed. Their biological significance is
unclear; the main function of the enzymes from
mammalian liver and kidney is that of a glycine
oxidase, although this would account for less than
1 % of the total glycine turnover [36]. The second
type of D-amino acid oxidase, which is highly specific for D-aspartic acid and D-glutamic acid, has
so far been found only in the kidney of some
mammals, the hepatopancreas of the cephalopod
Octopus vulgaris and the antennal gland of the
crayfish Orconectes limos us [164]. This enzyme
also has FAD as the coenzyme but, in contrast to
the enzymes of the first type, is not inhibited by
benzoates. Comparative investigations have
shown that D-aspartate oxidase is present in all
eight-armed cephalopods (order Octopoda), but
is consistently absent from the ten-armed species
(order Decapoda).
12.8 Amines
Many amines have specific biological functions as
neurotransmitters, neuromodulators or neurohormones. Biogenic amines are therefore found
especially in the nervous system, although in very
different, species-specific concentrations and
combinations. As components of toxins, they are