STRUCTURE-ACTIVITY STUDIES REVEAL 2 ALLATOSTATIN-RECEPTOR TYPES IN CORPORA-ALLATA OF DIPLOPTERA-PUNCTATA

Synthetic variants of the octadecapeptide amide ASB2 (AYSYVSEYKRLPVYNF GL-NH2), a cockroach allatostatin, were assayed in vibro on corpora al lata (CA) from 2-day-old (vitellogenic) and 10-day-old (post-vitelloge nic) female Diploptera punctata, The analogs [(17)psi(18),CH2-S]ASB2, [D-Trp(17)]ASB2 and [Ile(18)]ASB2 inhibited juvenile hormone (JH) synt hesis with simple dose-response curves on sensitive CA from 10-day-old females, These analogs were fully effective but less potent than ASB2 , When tested on CA from 2-day-old mated females, which are only parti ally (65-70%) sensitive to ASB2, the three analogs gave biphasic dose- response curves and elicited a maximal effect only at higher concentra tions, The dose-response curve for ASB2 on CA from 2-day-old females h ad a Hill plot slope of only 0.78 +/- 0.03, These findings suggested t hat the observed CA sensitivity to ASB2 may be the result of two parti al responses having an IC50 of approximately 0.35 and 3 nM respectivel y, lane partial response, or receptor type, appeared more sensitive th an the other to adverse modification of the ''message'' segment of the peptide, The activity of shorter allatostatins was also studied, indi cating that pentapeptides of the YXFGL-amide structure are fully effec tive, albeit at low potency, as inhibitors of JH biosynthesis. (C) 199 7 Elsevier Science Ltd.