AKH-RPCH family

Based upon similarity of structure and activity, Pea-HGH-I and II are classified as members of the AKH/RPCH family of neuropeptides. 

The octapeptide leucopyrokinin (LPK) was the eleventh and final myotropic peptide isolated from L. maderae (11). Unlike the leucokinins and LSK s, LPK exhibits a myotropic activity on the muscles of the cockroach foregut and oviduct (Holman, G. M. and Nachman, R. J., unpublished observation.). LPK was the highest titered of the Leucophaea myotropins (1.36 pmol/head) but was the least potent on the isolated hindgut preparation (threshold concentration -0.6 nM). LPK shares a 50% sequence identity with Pea-HTH-II at the 1, 4, 5, and 6 positions and is amidated at the C-terminus. Yet LPK does not contain Trp, which is present at position 8 in every AKH/RPCH peptide. Furthermore, LPK is positively charged with a residue of Arg at position 8. These structural observations show that LPK is not a member of the AKH/RPCH family. A study of LPK analogs supports this view (20). 

Since these initial reports, numerous peptides have been described for what is now recognized as the insect adipokinetic/red pigment-concentrating hormone (AKH/RPCH) family. The first representative of this family isolated and characterized from insects was the L, migratoria adipokinetic hormone (I m-AKH-I)... 

In addition to the AKH/RPCH family, neuroparsins A and B constitute a recently described class of neuropeptides that stimulate fat body metabolism. The neuroparsins are 14 IdDa proteins produced in PAF-positive, me al neurosecretory cells and transported to the storage lobes of L. migratoria CC for storage-secretion... 

We have discovered that Bld-HrTH has two additional physiological actions in B. discoidalis that have not been reported previously for members of the AKH/RPCH family. Bld-HrTH stimulates botii the synthesis of cytochrome hemes during fat body mitochondriogenesis and the rate of synthesis of export proteins by the female fat body. 

Other neurohormones of the AKH/RPCH family also stimulate the synthesis of cytohemes a+b in the fat body of B. discoidalis. At equivalent doses, Pea-CAH-I has as much activity as Bld-HrTH, but Pea-CAH-II and Lom-AKH-I are inactive (Figure 3). Therefore, the ability to stimulate developmentally-related heme synthesis in tile fat body appears to be a property of many of the AKH/HGH peptides. The inability of Pea-CAH-II and Lom-AKH-I to stimitiate heme synthesis in B. discoidalis does not indicate that these peptides do not have this effect in their host insect. Rather, these two peptides may ffer so much in structure from Bld-HrTH that they fail to interact adequately with the fat body receptors of B, discoidalis to promote a response, but could be effective at higher doses. 

Procedure. One chromatographic mode was used throughout, but a change in uffer and/or column at each step was used to effect purification. Alternatively, Hez-PBAN was purified by a sequence of three HPLC (RP-HPLC = HP-SEC => RP-HPLC) steps. In this case, changes in chromatographic mode, in addition to changes in buffer and/or column, were used to achieve purification in only three steps. The purity of the product isolated by either procedure was comparable. Similar multi-step procedures have been employed in the isolation of insect neuropeptides in the AKH/RPCH-family (3, 10, 11) and in the isolation of mammalian neu eptides from brain and hippocampus (12). 

Such structural homology is shown by the AKH/red pigment concentrating hormone (RPCH) family (S) and the myotropic neuropeptide family (S). As separation and characterization occur, e factors are assigned chemical and/or structural identity and, in the case of peptides, become identified by amino acid sequences. It is not suprising that a nomenclature standard has been proposed (M) to catalog the burgeoning number of reported sequences. 

Like the AKH/RPCH peptides, insect PDFs and crustacean PDHs are now an established family of neuropeptides common to arthropods. Whereas the fimctions of AKHs are varied and well defined in insects, the function of PDFs in insects remains unknown. The immunocytochemical distribution of PDF in the cephalic nervous tissues of orthopteran insects points to a neuromodulatory role, with potential functions in circadian rhythms this would be an exciting area for further research. 

Colour change versus metabolite mobilization the AKH/RPCH and PDH/PDF families of neuropeptides... 

The similarity of the crustacean blanching substance (Factor A) found in crustacean sinus glands and insect corpora cardiaca has been confirmed by the structural identification of a family of neuropeptides common to arthropods. The first identified member of this family is RPCH (red pigment concentrating hormone), an octapeptide (pGlu-Leu-Asn-Phe-Ser-Pro-Gly-Trp-amide) isolated from eyestalks of the shrimp Pandalus borealis (28). Shortly thereafter, a related peptide--designated as adipokinetic hormone (AKH)--was purified from locust... 

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