European Journal of Pharmacology, 167 (1989) 229-236 229 Elsevier EJP 50909 Degradation of [3H]fl-endorphin in rat plasma is increased with chronic stress Elizabeth A. Young *, Richard A. Houghten 1 and Huda Akil Mental Health Research Institute, University of Michigan, 205 Washtenaw Place, Ann Arbor, MI 48109-0720, and l Scripps Clinic and Research Institute, Department of Immunopathology, 10666 N. Torrey Pines Road, La Jolla, CA 92036, U.S.A. Received 29 March 1989, revised MS received 8 May 1989, accepted 16 May 1989 With a number of acute stressors fl-endorphin is released into plasma. It is unclear if fl-endorphin is converted into any other biologically active products, nor is it clear if the rate or pathways of degradation are changed during chronic stress. To explore these issues, we incubated [3H]fl-endorphinh labeled in positions 1 and 27 with plasma from normal and chronically footshocked rats and measured the rate of conversion of the label from fl-endorphin size material to smaller size material. Initial separations were done using a G-50 molecular sieving column, with subsequent characterization and identification on HPLC. By G-50 sieving, there is a time dependent formation of only one radioactive peak. HPLC identification demonstrates 3,-endorphin and another unidentified peak. This enzymatic activity is increased in the plasma of chronically stressed rats. ),-Endorphin; fl-Endorphin; Stress (chronic); Opioid peptides 1. Introduction The encoding of multiple active sequences and products within one polypeptide is a general principle in peptide biology, particularly in the opioid peptide families. Thus, most peptides are synthesized as part of a large molecular weight precursor protein which is then processed to yield smaller peptide fragments. In the case of pro- opiomelanocortin (POMC), one of the most widely studied opioid peptide precursors, there are several instances of one active sequence contained within another active sequence (Mains et al., 1977; Ro- berts and Herbert, 1977). In addition, there is evidence of tissue-specific processing of these se- quences, so that, in general, some tissues such as the intermediate lobe process POMC to primarily small peptides such as fl-endorphin and a-MSH * To whom all correspondence should be addressed. while other tissues such as the anterior lobe pro- cess POMC to primarily ACTH and fl-lipotropin (fl-LPH) (Mains and Eipper, 1979; Crine et al., 1978; Eipper and Mains, 1980). In addition to these classically studied POMC products, other biologically active peptide fragments, particularly a-endorphin, y-endorphin and 7-MSH are con- tained in the POMC sequence (Burbach et al., 1980a; Guillemin et al., 1976; Nakanishi et al., 1979). A number of acute stressors release fl-en- dorphin-IR from the anterior lobe of the pituitary. Of the fl-endorphin-IR released from anterior lobe, 2/3 of it is fl-endorphin size by molecular sieving (Young et al., 1986; Watson et al., 1988). The rate of disappearance of fl-endorphin released in plasma is rapid, with estimates of the half-fife of 45 rain or less (Houghten et al., 1980). It is unclear if fl-endorphin released into plasma is converted into any other biologically active fragments such as fl-endorphin-(1-16)(a-endorphin) or fl-en- dorphin-(1-17) (7-endorphin). Many peptides are 0014-2999/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)