| Abstract: | Alpha-bag cell peptide α-BCP (Ala-Pro-Arg-Leu-Arg-Phe-Tyr-Ser-Leu)] is a neurotransmitter that mediates bag cell-induced inhibition of left-upper-quadrant (LUQ) neurons L2, L3, L4, and L6 in the abdominal ganglion of Aplysia. Our recent biochemical studies have shown that α-BCP1–9] is cleaved into α-BCP1–2], 3–9], 1–5], 6–9], and 7–9] by a combination of three distinct peptidase activities located within the extracellular spaces of the CNS: A diaminopeptidase-IV (DAP-IV)-like enzyme cleaves α-BCP1–9] at the 2–3 peptide bond; a neutral metalloendopeptidase (NEP)-like enzyme cleaves either α-BCP1–9] or α-BCP3–9] at the 5–6 bond; an aminopeptidase M-II (APM-II)-like enzyme cleaves α-BCP6–9] at the 6–7 bond, but cleaves neither α-BCP1–9], nor the other ganglionic peptidase products. To further understand the manner in which α-BCP is inactivated after release, that is loses its electro-physiological activity, we studied its structure-activity relationship by recording intracellularly from LUQ neurons in isolated abdominal ganglia that were arterially perfused with peptides dissolved in artificial sea water. The effects of α-BCP1–9] and 15 of its fragments (1–8], 1–7], 1–6], 1–5], 2–9], 3–9], 3–8], 6–9], 7–9], 8–9], 6–7], 6–8], 1–2], Phe, Tyr) indicated that the sequence Phe6-Tyr7 was both necessary and sufficient to produce LUQ inhibitory activity. The combined results of our electrophysiological and biochemical studies strongly suggest that α-BCP1–9] is inactivated by the serial actions of the NEP-like and APM-II-like peptidases; that is, the NEP-like enzyme yields an electro-physiologically active product, α-BCP6–9], that is cleaved by the APM-II-like enzyme to yield inactive α-BCP7–9]. Furthermore, because α-BCP6–9] is more active than α-BCP1–9], cleavage by the NEP-like enzyme potentiates α-BCP's activity. © 1992 John Wiley & Sons, Inc. |
