KINETIC EVALUATION OF BETA-NEOENDORPHIN HYDROLYSIS BY THE SOMATIC ANDTESTICULAR ISOZYMES OF HUMAN ANGIOTENSIN-CONVERTING ENZYME

Angiotensin-converting enzyme (ACE) has both somatic and testicular is ozymes, the former possessing two catalytically active domains, amino- terminal and carboxyl-terminal, while the latter has only the carboxyl -terminal one. We compared hydrolysis processes of the nonapeptide bet a-neoendorphin by the two isozymes of human ACE. Both isozymes hydroly zed the peptide to Tyr(1)-Gly(2)-Gly(3) by the sequential removal of c arboxyl-terminal dipeptides in three consecutive steps. The rate const ant values for the second step, conversion of beta-neoendorphin(1-7) t o Leu-enkephalin, by the somatic isozyme in the presence of 10 or 200 mM NaCl were 4-fold higher than those for the first step, conversion o f beta-neoendorphin(1-9) to beta-neoendorphin(1-7). The k(cat) values of the somatic isozyme for beta-neoendorphin(1-7) were 2-fold higher t han those for beta-neoendorphin(1-9), indicating that beta-neoendorphi n(1-7) is more rapidly hydrolyzed than beta-neoendorphin(1-9). The rat e constant value for the second step at 10 mM NaCl was 5-fold higher t han that for the testicular isozyme. Similar extent of difference was also observed in k(cat) values for beta-neoendorphin(1-7) between the two isozymes. These results suggest that the amino-terminal domain of the somatic isozyme mainly contributes to the conversion of beta-neoen dorphin(1-7) to Leu-enkephalin at a low NaCl concentration. Optimal ch loride concentrations for the individual steps of beta-neoendorphin(1- 9) hydrolysis differed between the two isozymes.