430
12 Small Nitrogenous Compounds
tion of tyrosine to dihydroxyphenylalanine
(dopa). This reaction in insects, unlike the situation in mammals and molluscs [113], does not
involve a tyrosine hydroxylase, but is catalysed by
tyrosinase or phenoloxidase; it is not clear in that
case why dopa is not further oxidized to 0quinone by the same enzyme [4, 128]. Dopa does
not accumulate in the tissues but is converted
immediately to dopamine or N-acetyldopamine.
The metabolism of dopa to dopamine is brought
about by dopa decarboxylase, which is present
not only in the haemolymph cells but also especially in the epidermis. It is induced by the moulting enzyme 20-hydroxyecdysone. Dopa decarboxylase of various dipteran larvae, like that of the
mammals, is a dimer of about 100 kDa. The
insect enzyme is more specific than that of the
mammals and is inactive against tyrosine and
phenylalanine; it is competitively inhibited by Nacetyldopamine [146]. The enzyme responsible
for the acetylation of dopamine has been isolated
from Drosophila and Aedes [230]. In addition to
N-acetyldopamine (NADA) , the cuticula of many
insects contains N-~-alanyldopamine (NBAD) ,
which is synthesized from ~-alanine and dopamine by the action of NBAD synthase [130].
Hydroxylation at position 2 (~) of the side-chain
gives rise to N-acetylnorepinephrine (NANE)
from NADA, and to N-~-alanylnorepinephrine
(NBANE) from NBAD [157]. The corresponding
o-quinone is then produced by phenoloxidase;
this is converted to the unstable o-quinone
methide, which reacts spontaneously with water
to give ~-hydroxylated products, or becomes
involved in ~-sclerotization [5, 6, 238]. The cuticula of the Coleoptera is unusual in that it contains 3,4-dihydroxyphenylacetic acid (DOPAC) in
large amounts. According to the results of tracer
experiments, this arises from tyrosine via phydroxyphenylpyruvate and 3,4-dihydroxyphenylpyruvate as intermediates [19]. The spectrum
of catecholamine derivatives varies with the species, the developmental stage, and the exact function of the cuticula [60].
The tanning of non-cuticular protein structures
involves other phenolic substances, e.g. 3,4dihydroxybenzoic acid (protocatechuic acid;
Fig. 12.11) and the corresponding alcohol in the
egg capsule of the cockroaches, NADA, Nmalonyldopamine (NMAD) and other Nacyldopamines in the egg capsule of the praying
mantids (Mantodea) [128, 239], 3,6-dihydroxybenzoic acid (gentisinic acid) in the silk of
Antheraea pernyi, and 2-amino-3-hydroxybenzoic acid (3-hydroxaanthranilic acid) in the silk of
Hyalophora cecropia [4]. The synthesis of these
compounds is still somewhat obscure. There are
most likely several pathways, which probably
begin with the transamination of tyrosine to phydroxyphenylpyruvate, decarboxylation to tyramine, or hydroxylation to dopa.
Whilst the cuticula contains predominantly
catechols, the haemolymph has mainly conjugates such as ~-glucosides, sulphates, or phosphates; these are less subject to oxidation. For example, NBAD, NADA and dopamine are present in
the haemolymph of newly hatched individuals of
Manduca sexta almost exclusively as glucosides;
dopamine and NADA are found as sulphates in
the haemolymph of Periplaneta americana [128].
Sulphatases with the appropriate specificity are
synthesized in the skin glands of cockroaches. ~Glucosides of protocatechuic acid, the corresponding aldehydes and alcohols, and various Nacyldopamines are found in the left colleterial
gland of the cockroaches and the praying mantids
(Mantodea); this gland is involved in the production of the egg capsule [276].
12.6.2 The Process of Sclerotization
The newly formed insect cuticula is plastic and
flexible; its tensile strength is mainly due to chitin
fibrils which can slide within the protein matrix.
Greater mechanical strength of the cuticula is
achieved by the process of sclerotization, which is
initiated immediately following moulting. This
involves cross-linking of cuticular proteins to each
other and to chitin fibrils, reorientation of the
chitin fibrils, reduction of protein solubility,
reduction in water content, and often the appearance of brown pigmentation. The extent of sclerotization and the resulting reinforcement varies
in different regions of the cuticula. For example,
the mandibles of the locust Schistocera gregaria
have 20-fold higher cross-linking than the cuticula
of the abdominal terga. In insect larvae, only the
outer layer of the cuticula (exocuticu1a) is usually
sclerotized, but in adults the endocuticula is also
included. Those parts of the cuticula which also
contain resilin (an ideal rubbery material; see
p.388) have special mechanical properties.
Two types of model have been designed to
describe the phenomenon of sclerotization.
According to one model, sclerotization involves
the covalent linking of single amino acid residues
on different polypeptide chains by aromatic or
quinoid bridges. The aryl-substituted amino acids
in cuticular hydrolysates predicted by this bridge
12 Small Nitrogenous Compounds
tion of tyrosine to dihydroxyphenylalanine
(dopa). This reaction in insects, unlike the situation in mammals and molluscs [113], does not
involve a tyrosine hydroxylase, but is catalysed by
tyrosinase or phenoloxidase; it is not clear in that
case why dopa is not further oxidized to 0quinone by the same enzyme [4, 128]. Dopa does
not accumulate in the tissues but is converted
immediately to dopamine or N-acetyldopamine.
The metabolism of dopa to dopamine is brought
about by dopa decarboxylase, which is present
not only in the haemolymph cells but also especially in the epidermis. It is induced by the moulting enzyme 20-hydroxyecdysone. Dopa decarboxylase of various dipteran larvae, like that of the
mammals, is a dimer of about 100 kDa. The
insect enzyme is more specific than that of the
mammals and is inactive against tyrosine and
phenylalanine; it is competitively inhibited by Nacetyldopamine [146]. The enzyme responsible
for the acetylation of dopamine has been isolated
from Drosophila and Aedes [230]. In addition to
N-acetyldopamine (NADA) , the cuticula of many
insects contains N-~-alanyldopamine (NBAD) ,
which is synthesized from ~-alanine and dopamine by the action of NBAD synthase [130].
Hydroxylation at position 2 (~) of the side-chain
gives rise to N-acetylnorepinephrine (NANE)
from NADA, and to N-~-alanylnorepinephrine
(NBANE) from NBAD [157]. The corresponding
o-quinone is then produced by phenoloxidase;
this is converted to the unstable o-quinone
methide, which reacts spontaneously with water
to give ~-hydroxylated products, or becomes
involved in ~-sclerotization [5, 6, 238]. The cuticula of the Coleoptera is unusual in that it contains 3,4-dihydroxyphenylacetic acid (DOPAC) in
large amounts. According to the results of tracer
experiments, this arises from tyrosine via phydroxyphenylpyruvate and 3,4-dihydroxyphenylpyruvate as intermediates [19]. The spectrum
of catecholamine derivatives varies with the species, the developmental stage, and the exact function of the cuticula [60].
The tanning of non-cuticular protein structures
involves other phenolic substances, e.g. 3,4dihydroxybenzoic acid (protocatechuic acid;
Fig. 12.11) and the corresponding alcohol in the
egg capsule of the cockroaches, NADA, Nmalonyldopamine (NMAD) and other Nacyldopamines in the egg capsule of the praying
mantids (Mantodea) [128, 239], 3,6-dihydroxybenzoic acid (gentisinic acid) in the silk of
Antheraea pernyi, and 2-amino-3-hydroxybenzoic acid (3-hydroxaanthranilic acid) in the silk of
Hyalophora cecropia [4]. The synthesis of these
compounds is still somewhat obscure. There are
most likely several pathways, which probably
begin with the transamination of tyrosine to phydroxyphenylpyruvate, decarboxylation to tyramine, or hydroxylation to dopa.
Whilst the cuticula contains predominantly
catechols, the haemolymph has mainly conjugates such as ~-glucosides, sulphates, or phosphates; these are less subject to oxidation. For example, NBAD, NADA and dopamine are present in
the haemolymph of newly hatched individuals of
Manduca sexta almost exclusively as glucosides;
dopamine and NADA are found as sulphates in
the haemolymph of Periplaneta americana [128].
Sulphatases with the appropriate specificity are
synthesized in the skin glands of cockroaches. ~Glucosides of protocatechuic acid, the corresponding aldehydes and alcohols, and various Nacyldopamines are found in the left colleterial
gland of the cockroaches and the praying mantids
(Mantodea); this gland is involved in the production of the egg capsule [276].
12.6.2 The Process of Sclerotization
The newly formed insect cuticula is plastic and
flexible; its tensile strength is mainly due to chitin
fibrils which can slide within the protein matrix.
Greater mechanical strength of the cuticula is
achieved by the process of sclerotization, which is
initiated immediately following moulting. This
involves cross-linking of cuticular proteins to each
other and to chitin fibrils, reorientation of the
chitin fibrils, reduction of protein solubility,
reduction in water content, and often the appearance of brown pigmentation. The extent of sclerotization and the resulting reinforcement varies
in different regions of the cuticula. For example,
the mandibles of the locust Schistocera gregaria
have 20-fold higher cross-linking than the cuticula
of the abdominal terga. In insect larvae, only the
outer layer of the cuticula (exocuticu1a) is usually
sclerotized, but in adults the endocuticula is also
included. Those parts of the cuticula which also
contain resilin (an ideal rubbery material; see
p.388) have special mechanical properties.
Two types of model have been designed to
describe the phenomenon of sclerotization.
According to one model, sclerotization involves
the covalent linking of single amino acid residues
on different polypeptide chains by aromatic or
quinoid bridges. The aryl-substituted amino acids
in cuticular hydrolysates predicted by this bridge