ENZYMATIC DEGRADATION OF LUTEINIZING-HORMONE-RELEASING HORMONE (LHRH)[D-ALA(6)]-LHRH IN LUNG PNEUMOCYTES/

Purpose. To investigate the cellular proteolytic activities of various lung pneumocytes using luteinizing hormone releasing hormone (LHRH) a nd [D-Ala(6)]-LHRH as model peptide substrates. Methods. HPLC analysis was used to investigate the degradation kinetics of LHRH/[D-Ala(6)]-L HRH and to identify their degradation products in isolated lung pneumo cytes. Results. Pulmonary macrophages exhibited the strongest proteoly tic activity against LHRH)/[D-Ala6]-LHRH, followed by type II and type I-like pneumocytes. Three major degradation products of LHRH, namely LHRH 4-10. LHRH 6-10, and LHRH 7-10, were identified in macrophages an d type II pneumocytes, whereas in type I-like pneumocytes only the LHR H 7-10 was found. Co-incubation of the cells with known enzyme inhibit ors including captopril (an ACE inhibitor), thiorphan (an EP24.11 inhi bitor), and EDTA (an EP24.15 inhibitor) inhibited the formation of LHR H 4-10, LHRH 7-10, and LHRH 6-10 respectively. In all cell types, the degradation rate of [D-Ala6]-LHRH was about 3-8 times lower than that of LHRH. This peptide analog was resistant to degradation by EP24.15 a nd EP24.11, but was susceptible to ACE. Conclusions. ACE, EP24.11 appr oximate to ACE. No EP24.15 or ACE activity was observed in type-I like pneumocytes and only a weak EP24.11 activity was detected.