Bacitracin-sensitive aminopeptidase(s) degradation of methionine(5)-enkephalin by human brain putamen and hippocampus preparations: inhibition by phenothiazine drugs.

Select phenothiazine drugs significantly decrease, in a dose-dependent manner, the rate of methione(5)-enkephalin (MET) degradation by discrete human brain areas, for example, putamen and hippocampus. This pentapeptide is rapidly, and essentially completely, hydrolyzed at the tyrosine-glycine bond by bacitracin-sensitive aminopeptidase(s) (AP); neither dipeptidyl peptidase(s) (N-carboxyphenylmethyl leucine and captopril) nor peptidyl dipeptidase(s) (thiorphan) inhibitors altered the kinetics of MET degradation. Half-life (t1/2) and initial velocity (Iv) of this reaction were significantly increased and decreased, respectively, by fluphenazine > prochlorpherazine > chlorpromazine > thioridazine > promethazine > ethopropazine; brain A hippocampus (t1/2, control 2.8 and 60.2, 22.9, 15.4, 10.0, 9.9, and 4.8 minutes; and Iv, control 59.0 and 3.1, 10.7, 21.3, 22.8, 23.8, and 36.9 pg MET/mg brain tissue/minute) and putamen (t1/2, control 2.5 and 52.4, 19.9, 12.4, 8.2, 8.4, and 4.1 minutes; and Iv, control 53.1 and 2.7, 9.3, 17.1, 18.6, 20.1, and 31.7 pg MET/mg brain tissue/minute). Bacitracin was used for comparison purposes; hippocampus (t1/2 and Iv of 64.2 minutes and 4.3 pg MET/mg brain tissue/minute, respectively). Results using brain B tissue followed a comparable pattern, providing similar results. Hippocampus and putamen samples from both brains showed similar Km (mean 25.2, mu M; range, 23.3-27.2 mu m), Vmax (mean 108 nmol/mg protein/minute; range, 103-115 nmol/mg), and IC50 values (bacitracin, mean 14.4 mu M; range, 13.8-14.7 mu m) for MET AP degradation. Neither the phenothiazines methotrimeprazine or trifluoperazine nor other commonly used nonphenothiazine antipsychotic tested, for example, clozapine, haloperidol, loxapine, molindone, sulpiride and thiothixine, significantly altered the kinetics of this reaction. The presence of the phenothiazine molecule appears to be necessary for AP inhibition; however, our results failed to show s correlation among chemical structure, pharmacologic profile, and tested compound ability to inhibit MET degradation. This research provides initial information that could lead to the rational design of agents capable of modulate the bioavailability of enkephalins and other AP-metabolized biologically active compounds. Whether their development could find useful pharmacologic applications remains to be explored.