Biotransformation of endorphins by a synaptosomal plasma membrane preparation of rat brain and by human serum

β-Endorphin (β-LPH 61-91), γ-endorphin (61-77), des-tyrosine-γ-endorphin (62-77), α-endorphin (61-76), and β-LPH 61-69 either labeled with [¹²⁵I] at the N-terminal 61-tyrosine residue or unlabeled were incubated with a crude synaptosomal plasma membrane fraction of rat brain or in human serum. At different time intervals the release of [¹²⁵I]-tyrosine or the change in immunoreactivity of the endorphins was determined. The cSPM preparation displayed both high aminopeptidase and endopeptidase activities. In contrast, human serum mainly contained aminopeptidase activity. The data suggest that functional endorphin metabolism may occur at the synaptosomal plasma membrane. These membranes may potentially be involved in the formation of behaviorally active endorphin fragments.