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

Sheep beta-lipotropin (beta-LPH) (sequence 1-91) was selectively cleaved with trypsin after blocking the epsilon-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 were about as active as the synthetic methionine-enkephalin, and in turn these were about 100 times more active than beta-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 beta-LPH.