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.