308
8 Peptide Hormones
cardioactive peptide) has also been found in
Locusta. In contrast, the cardioactive peptide
corazonin, from Periplaneta americanus, has a
very different structure [208, 221].
The first-described insect neuropeptide, which
was sequenced in 1976, was the 10-amino-acid
adipokinetic hormone (AKH-I) from the corpora
cardiaca of the migratory locusts Locusta migratoria and Schistocera gregaria. The hormone acts
at the start of flight to ensure that diacylglycerol,
necessary for metabolism in the flight muscles, is
released from the fat bodies into the haemoiymph. As in all Orthoptera, there is in addition
to the normalAKH-I a shorter AKH-II with eight
amino acids and a somewhat different sequence
(Fig.8.6g). Surprisingly, the red-pigment concentrating hormone (RPCH) from the crustacean
Pandalus borealis has a completely different function but differs from the AKH-II of Locusta in
only one amino acid (Fig. 8.6h), and thus belongs
to the same peptide family. In fact, the AKHs
have effects on the pigment cells of crustaceans
and the RPCHs mobilize lipids in insect fat
bodies. To date, there are 18 peptides known
which belong to the AKHlRPCH family, most of
which are found in more than one species. They
are made up of 8-10 neutral amino acids, carry
N-terminal a pyroglutamate residue, and are
amidated at the C-terminus (Fig.8.6g &h). The
AKH of the dipterans Phormia terraenovae and
Drosophila melanogaster is the only one known
to carry a charged amino acid (Asp) [66, 67]. The
prepro-AKH of Manduca sexta has only one
AKH sequence behind the signal sequence, and
the precursors of both AKHs of the locust Schistocera nitans also have this organization. In the
latter case, the sequence of the prepro-AKH-I
(63 amino acids) agrees only 55 % with that of
prepro-AKH-II (25, 155]. Regulation of the
blood-sugar level in the crustaceans is not via
peptides of the AKHlRPCH family, but by a special class of protein hormone which also inhibits
moulting. The hyperglycaemic hormones (CHHs)
from the sinus glands of Carcinus maenas and
Homarus american us agree by 61 % in their
71-72 amino acids. Lying between the signal peptide of prepro-CHH of C. maenas and the CHH
sequence is a 28-residue peptide of unknown
function [33, 230].
In addition to the RPCH, two further pigmentdispersing chromatophorotropins have been
sequenced in crustaceans: the a-PDH (pigmentdispersing hormone) from Pandalus borealis,
which is also known as DRPH (distal retinal pigment hormone) (Fig. 8.6i), and the ~-PDH from
Uca pugilator. These hormones also appear to be
widely distributed amongst the arthropods. Thus,
a peptide of 18 amino acids isolated from the
locust Romalea microptera (Fig. 8.6 j) agrees at
14 positions with ~-PDH and at 7 positions with
a-PDH of the crustaceans [179].
8.9 Hormone Receptors
The functional links between chemical signals
arriving at the cell surface and the intracellular
processes take the form of specific signal transduction pathways. These begin with a membranespanning receptor whose extracellular domain
carries a hormone-binding site. The resulting conformational change affects an intracellular effector which brings about the cellular response via a
consecutive series of intermediate processes. For
one group of receptors, a GTP-binding protein
intervenes between the receptor and the primary
effector (G-protein-linked receptors). Most
receptors are present as subtypes which have differing pharmacological and physiological properties, and are even sometimes linked to different
effectors. The SUbtypes are either encoded by different genes or arise by alternative splicing of the
same transcript. The presence of a very specific
pattern of receptors on the cell surface leads to
differential reactions of organs to the same hormone. Post-translational modification, e.g. phosphorylation, may facilitate adaptation of the
receptor properties to specific requirements of
the system. There are two quite different receptor
types for the neurotransmitter acetylcholine. The
muscarinic acetylcholine receptors are stimulated
by muscarin, a poison from the fungus Amanita
muscaria. They are found mainly in the central
nervous system (CNS) , the sympathetic ganglia
and the heart, and belong to the G-protein-linked
receptors. The nicotinic acetylcholine receptors
are stimulated by nicotine and are inhibited by abungarotoxin; because of their rather unusual
molecular structure they are placed in a special
receptor family. They are found in the neuromuscular junctions of vertebrates and in the electric
organs of the electric rays Torpedo cali/ornica and
T. marmorata and of the electric eel Electrophus
electric us ; these organs are constructed from specialized muscle plates. Receptors from this family
are also found in the CNS. In addition to the Gprotein-linked receptors and the nicotinic acetylcholine receptors, the comparative biochemistry
8 Peptide Hormones
cardioactive peptide) has also been found in
Locusta. In contrast, the cardioactive peptide
corazonin, from Periplaneta americanus, has a
very different structure [208, 221].
The first-described insect neuropeptide, which
was sequenced in 1976, was the 10-amino-acid
adipokinetic hormone (AKH-I) from the corpora
cardiaca of the migratory locusts Locusta migratoria and Schistocera gregaria. The hormone acts
at the start of flight to ensure that diacylglycerol,
necessary for metabolism in the flight muscles, is
released from the fat bodies into the haemoiymph. As in all Orthoptera, there is in addition
to the normalAKH-I a shorter AKH-II with eight
amino acids and a somewhat different sequence
(Fig.8.6g). Surprisingly, the red-pigment concentrating hormone (RPCH) from the crustacean
Pandalus borealis has a completely different function but differs from the AKH-II of Locusta in
only one amino acid (Fig. 8.6h), and thus belongs
to the same peptide family. In fact, the AKHs
have effects on the pigment cells of crustaceans
and the RPCHs mobilize lipids in insect fat
bodies. To date, there are 18 peptides known
which belong to the AKHlRPCH family, most of
which are found in more than one species. They
are made up of 8-10 neutral amino acids, carry
N-terminal a pyroglutamate residue, and are
amidated at the C-terminus (Fig.8.6g &h). The
AKH of the dipterans Phormia terraenovae and
Drosophila melanogaster is the only one known
to carry a charged amino acid (Asp) [66, 67]. The
prepro-AKH of Manduca sexta has only one
AKH sequence behind the signal sequence, and
the precursors of both AKHs of the locust Schistocera nitans also have this organization. In the
latter case, the sequence of the prepro-AKH-I
(63 amino acids) agrees only 55 % with that of
prepro-AKH-II (25, 155]. Regulation of the
blood-sugar level in the crustaceans is not via
peptides of the AKHlRPCH family, but by a special class of protein hormone which also inhibits
moulting. The hyperglycaemic hormones (CHHs)
from the sinus glands of Carcinus maenas and
Homarus american us agree by 61 % in their
71-72 amino acids. Lying between the signal peptide of prepro-CHH of C. maenas and the CHH
sequence is a 28-residue peptide of unknown
function [33, 230].
In addition to the RPCH, two further pigmentdispersing chromatophorotropins have been
sequenced in crustaceans: the a-PDH (pigmentdispersing hormone) from Pandalus borealis,
which is also known as DRPH (distal retinal pigment hormone) (Fig. 8.6i), and the ~-PDH from
Uca pugilator. These hormones also appear to be
widely distributed amongst the arthropods. Thus,
a peptide of 18 amino acids isolated from the
locust Romalea microptera (Fig. 8.6 j) agrees at
14 positions with ~-PDH and at 7 positions with
a-PDH of the crustaceans [179].
8.9 Hormone Receptors
The functional links between chemical signals
arriving at the cell surface and the intracellular
processes take the form of specific signal transduction pathways. These begin with a membranespanning receptor whose extracellular domain
carries a hormone-binding site. The resulting conformational change affects an intracellular effector which brings about the cellular response via a
consecutive series of intermediate processes. For
one group of receptors, a GTP-binding protein
intervenes between the receptor and the primary
effector (G-protein-linked receptors). Most
receptors are present as subtypes which have differing pharmacological and physiological properties, and are even sometimes linked to different
effectors. The SUbtypes are either encoded by different genes or arise by alternative splicing of the
same transcript. The presence of a very specific
pattern of receptors on the cell surface leads to
differential reactions of organs to the same hormone. Post-translational modification, e.g. phosphorylation, may facilitate adaptation of the
receptor properties to specific requirements of
the system. There are two quite different receptor
types for the neurotransmitter acetylcholine. The
muscarinic acetylcholine receptors are stimulated
by muscarin, a poison from the fungus Amanita
muscaria. They are found mainly in the central
nervous system (CNS) , the sympathetic ganglia
and the heart, and belong to the G-protein-linked
receptors. The nicotinic acetylcholine receptors
are stimulated by nicotine and are inhibited by abungarotoxin; because of their rather unusual
molecular structure they are placed in a special
receptor family. They are found in the neuromuscular junctions of vertebrates and in the electric
organs of the electric rays Torpedo cali/ornica and
T. marmorata and of the electric eel Electrophus
electric us ; these organs are constructed from specialized muscle plates. Receptors from this family
are also found in the CNS. In addition to the Gprotein-linked receptors and the nicotinic acetylcholine receptors, the comparative biochemistry