Degradation of luteinizing hormone-releasing hormone by neuroblastoma cells and their membrane: evidence for the involvement of a thiol protease and angiotensin-converting enzyme.

Luteinizing hormone-releasing hormone was degraded by cells of the N-18 line of mouse neuroblastoma and their membrane. Cleavage products were separated by HPLC and identified by amino acid analysis. Fragments (1-3), (4-5), and (6-10) were major cleavage products. All the products increased in level as a function of time except for fragment (1-5), which increased in amount only during a short incubation time and then decreased. The accumulation of fragment (1-5) was increased in the presence of captopril or EDTA, whereas that of fragments (1-3) and (4-5) decreased inversely. On the other hand, the generation of either fragment (1-3) or (4-5) was stimulated by the presence of Cl-. The results suggest that the conversion of fragment (1-5) into fragments (1-3) and (4-5) is catalyzed by angiotensin-converting enzyme. p-Chloromercuribenzoate inhibited the formation of fragment (1-5), a result suggesting the involvement of a thiol protease in this formation. Thus, the degradation of luteinizing hormone-releasing hormone by neuroblastoma cells and their membrane seems to take place mainly through the cleavage of the Tyr5-Gly6 bond by a thiol protease, followed by the release of the dipeptide Ser-Tyr from the liberated fragment (1-5) by angiotensin-converting enzyme. It is further suggested that the thiol protease and angiotensin-converting enzyme are also responsible for the initial minor cleavages of the Gly6-Leu7 bond and the Trp3-Ser4 bond, respectively.