Biosynthesis of adipokinetic hormones (AKHs): further characterization of precursors and identification of novel products of processing

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Abstract
Two adipokinetic hormones (AKH I: pGlu-Leu-Asn-Phe-Thr-Pro-Asn-Trp-Gly- Thr-NH2, and AKH II: pGlu-Leu-Asn-Phe-Ser-Thr-Gly-Trp-NH2) are synthesized by the neurosecretory cells of the corpora cardiaca (CC) of the locust Schistocerca gregaria. Both AKHs are released into the blood during flight and serve to regulate lipid metabolism and other physiological processes involved in flight. By in vitro “pulse-chase” experiments we show that 2 precursors (P1 and P2) are involved in AKH biosynthesis. These are about 8.4 kDa polypeptides which are AKH immunoreactive but also contain an amino acid (tyrosine) not present in the AKH peptides. By following the fate of 14C-tyrosine incorporated into P1 and P2, we have identified 2 novel products of precursor processing. These are called AKH-Precursor Related Peptides or APRP 1 and APRP 2. Size exclusion chromatography shows that the APRPs are about 6.5 kDa large and therefore represent major fragments of the precursors. The optical density peaks corresponding to P1, P2, APRP 1, and APRP 2 on a reverse-phase chromatogram are identified. The precursors being metabolic intermediates are represented by minor optical density peaks that disappear when de novo protein synthesis is blocked by cycloheximide. In contrast, the APRPs are represented by major optical density peaks consistent with their being accumulating end products of AKH precursor processing. The function of the APRPs is as yet unknown. They are, however, co-synthesized and also co-released with the AKHs, and may therefore also have hormonal functions related to flight.