Synthesis of porcine leumorphin and some of its biological activities

The carboxy-terminal nonacosapeptide sequence of porcine preproenkephalin B contains the sequence of Leu-enkephalin at its amino terminus. The endogenous existence of this peptide, leumorphin, has not yet been proved. Synthesis of leumorphin was carried out by a solid-phase technique and the purity and structure of the synthetic peptide were confirmed. Synthetic porcine leumorphin exhibited a dose-dependent opiate effect (ED₅₀ 4.70 · 10^?9 M) on electrically stimulated contraction of the guinea pig ileum preparation. The potency was about 100 times as high as that of Leu-enkephalin. Leumorphin was less potent than dynorphin(1–13) (ED₅₀ 0.38 · 10^?9 M) but it was more active than $\text{β}{}_{\mathrm{<mtext>h</mtext>}}\text{-}\text{endorphin}$ (ED₅₀ 18 · 10^?9 M). The opiate activity was only partially reversed by naloxone. Intracisternal injection of synthetic leumorphin caused significant analgesia in mice (ED₅₀ 7.31 nmol/mouse). The potency was lower than that of $\text{β}{}_{\mathrm{<mtext>h</mtext>}}\text{-}\text{endorphin}$ (ED₅₀ 0.60 nmol/mouse) but higher than that of dynorphin(1–13) (ED₅₀ 16.10 nmol/mouse). Intracisternally injected leumorphin did not produce such a violent behavioral effect as did dynorphin(1–13), and it exhibited a mild sedative effect. The data supports the concept that leumorphin is a new type of opioid peptide and that the synthetic preparation will be useful for further biological and immunological studies on this peptide.     

Leumorphin is a novel endogenous opioid peptide derived from preproenkephalin B

Using synthetic leumorphin, we obtained antisera for leumorphin and set up two radioimmunoassays (RIAs) with different specificities. Gel exclusion chromatography coupled with the two RIAs showed the existence of a considerable amount of leumorphin-like peptide in water extracts from porcine neurointermediate pituitaries. Reverse phase high performance liquid chromatography revealed that leumorphin-like peptide in the water extracts was indistinguishable from synthetic leumorphin. These results along with potent opioid activity of leumorphin indicate that leumorphin is a novel endogenous opioid peptide derived from preproenkephalin B.     

Leumorphin has an anti-apoptotic effect by activating epidermal growth factor receptor kinase in rat pheochromocytoma PC12 cells

Endogenous opioid peptides, found in the central and peripheral nervous systems, perform neuromodulatory roles, and display a wide range of functional and pharmacological properties in vitro and in vivo. In this study, we investigated the effects of prodynorphin gene products on intracellular signaling events and cell survival in rat pheochromocytoma PC12 cells. Leumorphin, but not other prodynorphin gene products including dynorphin A, beta-neoendorphin and rimorphin (dynorphin B), increased cell viability in PC12 cells. The cytoprotective effect of leumorphin was dependent on the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways, but was insensitive to both naloxone, a general antagonist of the opioid receptor, and nor-binaltorphimine, a specific antagonist of the kappa opioid receptor. Moreover, a competition-binding assay clearly revealed that leumorphin had another binding site(s) in addition to that for the kappa opioid receptor. Interestingly, leumorphin induced activation of the epidermal growth factor receptor via a Src-dependent mechanism, which was proved to be responsible for the increased survival response. Flow cytometric and microscopic analysis showed that leumorphin rescued cells from serum deprivation-induced apoptosis. Collectively, we suggest that leumorphin prevents apoptosis via epidermal growth factor receptor-mediated activation of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways, which occur independent of the kappa opioid receptor.