β -Lipotropin is the Major Opioid-Like Peptide of Human Pituitary and Rat Pars Distalis: Lack of Significant β -Endorphin

β -Lipotropin is the predominant opioid peptide of the human pituitary and rat pars distalis and is present in concentrations essentially equimolar with corticotropin. When freshly obtained nonfrozen rat anterior pituitaries were homogenized with 0.2 M HCl, approximately 98% of the immunoreactivity detected utilizing an antiserum that crossreacts equally with β -lipotropin and β -endorphin coeluted with125I-labeled human β -lipotropin upon molecular sieve chromatography. The remainder of the activity eluted with synthetic human β -endorphin. Similar results were obtained for human pituitary. HCl homogenization of thawed tissue or homogenization of fresh tissue with acetic acid yielded substantially greater concentrations of β -endorphin and decreased concentrations of β -lipotropin. In human subjects, acute anterior pituitary stimulation using either insulin-induced hypoglycemia or vasopressin administration was associated with increased plasma β -lipotropin and corticotropin levels. At the time of peak concentrations, no significant levels of β -endorphin were detectable. These data indicate the lack of significant amounts of β -endorphin in human pituitary. Additionally, there appears to be no specific intrapituitary conversion of β -lipotropin to β -endorphin.