Adipokinetic Hormones

Definition
Adipokinetic hormones (AKH) are a family of structurally related peptides that occur widely in insects, it belongs to member of a large arthropod neuropeptide family. It exerts a wide range of functions, many of which are analogous to those of vertebrate glucagons.

Discovery
It was discovered in the desert locust Schistocerca gregaria and it was first purified in 1976 by stone and team. It was isolated from corpora cardiaca of the painted lady butterfly, Vanessa cardui1.

Classification
There are 3 isoforms of AKH viz. AKH-1, AKH-II and AKH-III. AKH-I is a decapeptide, AKH –II and AKH-III are octapeptide the molecular proportion of these 3 peptides vary (14:2:1) respectively. All these hormones are derived from 63- and 61-amino-acid peptide precursors.

Structural Characteristics

The molecular weight of adipokinetic hormone is 1008.1 KDa. The amino acid sequence of the peptide is pGlu -leu-Asn-Phe-Thr-Pro-Asn-Trp-Gly-Thr-NH2. Insect AKHs are post-translationally modified at the N-terminus by pyroglutamic acid and at the C-terminus by carboxyamide2. They contain aromatic amino acids at position 4 (mostly phenylalanine, in a few cases tyrosine) and at position 8 (tryptophan). Position 9 is always glycine which is used in the octapeptides for the amidation, and the majority of the peptides do not have net charge.

Mode of action

AKH family regulate mobilisation of substrates from stores in the fat body of insects during episodes of flight. During the period of flight, AKHs bind to a G (q)-protein-coupled receptor, activates phospholipase C resulting in formation of inositol trisphosphate that releases Ca(2+) from internal stores. In addition, influx of extracellular Ca2+ is increased and via a kinase cascade glycogen phosphorylase is activated, glucose-1-phosphate produced, and transformed to trehalose which is released into the haemolymph3.

Functions

AKHs are multifunctional; they exert various physiological functions like cardio acceleration in cockroaches and migration of tegumentary and retinal distal pigments in crustaceans. It induces transcription of the cytochrome P450 gene in the fat body of cockroaches and expression of a gene encoding fatty acid binding protein in the flight muscle of locusts. These peptides have excitatory effects on motor neurons in moths4. There is enough evidence to show that AKH regulates the balance of carbohydrate and lipid oxidation in flight muscles.

References

1. Köllisch GV, Lorenz MW, Kellner R, Verhaert PD and Hoffmann KH (2004). Structure elucidation and biological activity of an unusual adipokinetic hormone from corpora cardiaca of the butterfly, Vanessa cardui. European Journal of Biochemistry, 267(17)5502 – 5508.
2. Gäde G, Simek P, Clark KD, Auerswald L (2006). Unique translational modification of an invertebrate neuropeptide: a phosphorylated member of the adipokinetic hormone peptide family. Biochem. J, 393: 705–713.
3. Gäde G, Auerswald L (2003).Mode of action of neuropeptides from the adipokinetic hormone family. Gen Comp Endocrinol, 132 (1): 10-20.
4. Van der Horst DJ, Van Marrewijk WJ, Diederen JH (2001). Adipokinetic hormones of insect release, signal transduction, and responses. Int Rev Cytol, 211: 179-240.