Morphine-like activity of sheep β-lipotropin and of its tryptic fragments

Sheep β-lipotropin (β-LPH) (sequence 1–91) was selectively cleaved with trypsin after blocking the ε-amino groups of lysine with citraconic anhydride. The resulting peptides were purified by a combination of cation-exchange chromatography and high-voltage electrophoresis. The purified fragments were then tested for their morphine-like activity in the mouse vas deferens bioassay. The active peptides were 61–91 and 61–80. All other regions of the molecule investigated were not active. Moreover, the peptides 61–91 and 61–80 were about as active as the synthetic methionine-enkephalin, and in turn these were about 100 times more active than β-LPH itself. The inhibition of electrically stimulated mouse vas deferens by these peptides is reversed by naloxone, and suggests a competitive character of interaction. It is thus concluded that the active core for the morphine-like activity in the mouse vas deferens bioassay is the fragment 61–65 of β-LPH.