trast, the cuticula of Cancer pagarus contains bisand tertyrosine as well as its bromo-derivative.
The coupling of protein-bound tyrosine residues
brings about the rapid (within a few minutes) hardening of the fertilization membrane of seaurchin eggs. In addition to bis- and tertyrosine,
the isomer of isotertyrosine, pulcherosine (see
Fig. 11.4, p. 380), is also formed [167]. Representatives of almost all animal phyla contain doparich proteins which either form networks by quinone tanning or adsorb on mineral or metallic
substrata via dopa side-chains and thereby function as adhesives [254].
12.6.3 Phenoloxidases
Apart from the pterin-dependent phenylalanine
and tyrosine hydroxylases and the haemcontaining peroxidases that have already been
mentioned, the biocatalysts of the oxidation of
aromatic compounds include in particular the
copper-containing phenoloxidases, of which two
types can be distinguished. One type, such as the
tyrosinase of fungi, is active against monophenols and o-diphenols, is inhibited by thiourea and
phenylthiourea, and has a neutral pH optimum.
The second type, such as the laccase of the fungi
and higher plants, attacks p- and o-diphenols,
but is inactive against monophenols. This type
mostly has a pH optimum in the weak acid region
and is insensitive to the inhibitors mentioned
above [4]. Many phenoloxidases occur as inactive
pro-enzymes which may be activated by proteolysis.
The sclerotizing cuticula of insects contains
non-extractable phenoloxidases which may be
brought into solution by trypsin. Little is known
about their structure. They oxidize 0- and pdiphenols but not monophenols, and thus belong
to the laccase group. Their pH optima usually lie
between 4.5 and 5.5 but are sometimes closer to
neutrality [4, 246].
The haemolymph of many insects contains
numerous pro-phenoloxidases which, after activation by cuticular enzymes, are active against
monophenols and o-diphenols, have neutral pH
optima, and are inhibited by thiourea, i.e. they
are tyrosinase-like. The cuticula also includes oxidases with a similar specificity which are not
identical to the haemolymph enzymes. Sclerotizing cuticular regions always contain laccases and
most also possess tyrosinases; in contrast, nonsclerotizing regions contain only the tyrosinases
[4, 12].
12.7 D-Amino Acids and Their Metabolism
433
The haemolymph of many insects and crustaceans contains multifactorial defence systems
which recognize foreign particles and encapsulate
them in melanin; at the same time, they function
opsonically and activate the synthesis of immune
proteins in the fat body. Amongst these systems
are the zymogens of tyrosinase-like phenoloxidases and both their activators and inhibitors. Some
of these components are synthesized and stored
in blood cells. The in vitro activation of prophenoloxidases can be brought about simply by
mild detergent treatment [232]. The natural activators are calcium-dependent serine proteases,
which also occur as zymogens. Corresponding
protease inhibitors have been detected in the lepidopteran Manduca sexta and the crayfish [9, 11,
13]. The phenoloxidase system of the arthropods
is activated by bacterial substances such as peptidoglucans and ~(1,3)-glucans. Corresponding
~(1,3)-glucan recognition proteins have been isolated from the haemolymph of the silkworm Bombyx mori and the cockroach Blaberus cranifer
[168, 233].
U.7 D-Amino Acids
and Their Metabolism
For methodological reasons, there are very few
data on the occurrence in animals of amino acids
with the D-configuration. Until recently, the only
stereospecific evidence was that obtained by
enzymic cleavage with D-amino acid oxidases;
because of the specificity of this enzyme, this
method only provided information about particular amino acids. Nowadays, there are chromatographic methods of broad application, but these
have not yet been used for the systematic investigation of D-amino acids in animals. Using the
purchasable amino acid oxidase from mammals,
18 out of 43 examined marine animals from eight
phyla were found to contain D-amino acids at concentrations of 1-2 flmoVg, and in some cases up to
44 flmoVg (coelomocytes of Glycera dibranchiata) [190]. Up to now, D-amino acids in animals
have been found only in the free form or as constituents of low molecular weight compounds,
and never as components of proteins.
n-Serine is found in the earthworm Lumbricus
in the free form as well as in the phosphagen
lombricin and its precursor serine-ethanolaminephosphoric acid diester; it also occurs as the free
amino acid in the larvae and pupae of the silk-
The coupling of protein-bound tyrosine residues
brings about the rapid (within a few minutes) hardening of the fertilization membrane of seaurchin eggs. In addition to bis- and tertyrosine,
the isomer of isotertyrosine, pulcherosine (see
Fig. 11.4, p. 380), is also formed [167]. Representatives of almost all animal phyla contain doparich proteins which either form networks by quinone tanning or adsorb on mineral or metallic
substrata via dopa side-chains and thereby function as adhesives [254].
12.6.3 Phenoloxidases
Apart from the pterin-dependent phenylalanine
and tyrosine hydroxylases and the haemcontaining peroxidases that have already been
mentioned, the biocatalysts of the oxidation of
aromatic compounds include in particular the
copper-containing phenoloxidases, of which two
types can be distinguished. One type, such as the
tyrosinase of fungi, is active against monophenols and o-diphenols, is inhibited by thiourea and
phenylthiourea, and has a neutral pH optimum.
The second type, such as the laccase of the fungi
and higher plants, attacks p- and o-diphenols,
but is inactive against monophenols. This type
mostly has a pH optimum in the weak acid region
and is insensitive to the inhibitors mentioned
above [4]. Many phenoloxidases occur as inactive
pro-enzymes which may be activated by proteolysis.
The sclerotizing cuticula of insects contains
non-extractable phenoloxidases which may be
brought into solution by trypsin. Little is known
about their structure. They oxidize 0- and pdiphenols but not monophenols, and thus belong
to the laccase group. Their pH optima usually lie
between 4.5 and 5.5 but are sometimes closer to
neutrality [4, 246].
The haemolymph of many insects contains
numerous pro-phenoloxidases which, after activation by cuticular enzymes, are active against
monophenols and o-diphenols, have neutral pH
optima, and are inhibited by thiourea, i.e. they
are tyrosinase-like. The cuticula also includes oxidases with a similar specificity which are not
identical to the haemolymph enzymes. Sclerotizing cuticular regions always contain laccases and
most also possess tyrosinases; in contrast, nonsclerotizing regions contain only the tyrosinases
[4, 12].
12.7 D-Amino Acids and Their Metabolism
433
The haemolymph of many insects and crustaceans contains multifactorial defence systems
which recognize foreign particles and encapsulate
them in melanin; at the same time, they function
opsonically and activate the synthesis of immune
proteins in the fat body. Amongst these systems
are the zymogens of tyrosinase-like phenoloxidases and both their activators and inhibitors. Some
of these components are synthesized and stored
in blood cells. The in vitro activation of prophenoloxidases can be brought about simply by
mild detergent treatment [232]. The natural activators are calcium-dependent serine proteases,
which also occur as zymogens. Corresponding
protease inhibitors have been detected in the lepidopteran Manduca sexta and the crayfish [9, 11,
13]. The phenoloxidase system of the arthropods
is activated by bacterial substances such as peptidoglucans and ~(1,3)-glucans. Corresponding
~(1,3)-glucan recognition proteins have been isolated from the haemolymph of the silkworm Bombyx mori and the cockroach Blaberus cranifer
[168, 233].
U.7 D-Amino Acids
and Their Metabolism
For methodological reasons, there are very few
data on the occurrence in animals of amino acids
with the D-configuration. Until recently, the only
stereospecific evidence was that obtained by
enzymic cleavage with D-amino acid oxidases;
because of the specificity of this enzyme, this
method only provided information about particular amino acids. Nowadays, there are chromatographic methods of broad application, but these
have not yet been used for the systematic investigation of D-amino acids in animals. Using the
purchasable amino acid oxidase from mammals,
18 out of 43 examined marine animals from eight
phyla were found to contain D-amino acids at concentrations of 1-2 flmoVg, and in some cases up to
44 flmoVg (coelomocytes of Glycera dibranchiata) [190]. Up to now, D-amino acids in animals
have been found only in the free form or as constituents of low molecular weight compounds,
and never as components of proteins.
n-Serine is found in the earthworm Lumbricus
in the free form as well as in the phosphagen
lombricin and its precursor serine-ethanolaminephosphoric acid diester; it also occurs as the free
amino acid in the larvae and pupae of the silk-