Inhibition of endopeptidase 24.15 slows the in vivo degradation of luteinizing hormone-releasing hormone

Endopeptidase (EP) 24.15 cleaves the Tyr⁵-Gly⁶ bond of luteinizing hormone-releasing hormone (LHRH) (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH₂), and is the primary LHRH degrading enzyme in pituitary and hypothalamic membrane preparations. Potent and specific inhibitors were used to identify the enzymes involved in the in vivo degradation of LHRH. After i.c.v. administration of LHRH, only about 1% of the peptide was recovered from brain after 1 hr. Concurrent administration of LHRH and N-[1-(RS)-carboxy-3-phenylpropyl]-Ala-Ala-Phe-p-aminobenzoate (cFP-AAF-pAB), a specific inhibitor of EP 24.15, led to a more than 10-fold increase in LHRH recovery. Administration of N-[1-(RS)-carboxy-3-phenylpropyl]-Phe-pAB(cFP-F-pAB) or captopril, inhibitors of 'enkephalinase' (EP 24.11) and angiotensin converting enzyme, respectively, did not significantly increase LHRH recovery. Intravenous administration of LHRH and either cFP-F-pAB or cFP-AAF-pAB but not captopril, led to an increase in the half-life of LHRH from 10 min to 15 and 20 min, respectively. Concurrent administration of both inhibitors resulted in a dramatic 8-fold increase in the half-life of LHRH, similar to values reported for 'superactive' analogs of LHRH which are rendered resistant to enzymatic degradation by introduction of a D-amino acid in position 6. Concentrations of plasma LHRH 65 to 80 min after administration of inhibitors were 100- to 200-fold higher than those in controls. The potentiating effect of cFP-F-pAB resulted from inhibition of the in vivo degradation of cFP-AAF-pAB by EP 24.11. It is concluded that EP 24.15 is the dominant factor determining the in vivo LHRH degradation, suggesting a role of the enzyme in the regulation of the amount of LHRH reaching pituitary gonadotropes.