In vitro lipolytic activity of beta-endorphin and its partial sequences.

beta-Endorphin stimulates glycerol release from adipose tissue in vitro in the rabbit. Thirty different amino acid sequences of this peptide were tested for lipolytic activity. Four turned out to be active: porcine and human beta-endorphin-(1-31), human beta-endorphin-(6-31), and human beta-endorphin-(1-5)-(16-31). Structure-activity investigations showed that for the lipolytic action of beta-endorphin the C-terminal part [longer than beta-endorphin-(27-31)] is relatively important. Of special importance seems to be the C-terminal amino acid residue, because none of the sequences lacking the last two amino acid residues was lipolytically active. Furthermore, a different lipolytic response to beta-endorphin was obtained in starved, ad libitum-fed, and starved-refed animals, showing that the regulation of the lipolytic potency is not only mediated by peptide concentrations in the medium.