Kinetic evaluation of β-neoendorphin hydrolysis by the somatic and testicular isozymes of human angiotensin-converting enzyme

Abstract Angiotensin-converting enzyme (ACE) has both somatic and testicular isozymes, 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 β -neoendorphin by the two isozymes of human ACE. Both isozymes hydrolyzed the peptide to Tyr 1 -Gly 2 -Gly 3 by the sequential removal of carboxyl-terminal dipeptides in three consecutive steps. The rate constant values for the second step, conversion of β -neoendorphin 1–7 to 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 of β -neoendorphin 1–9 to β -neoendorphin 1–7 . The k cat values of the somatic isozyme for β -neoendorphin 1–7 were 2-fold higher than those for β -neoendorphin 1–9 , indicating that β -neoendorphin 1–7 is more rapidly hydrolyzed than β -neoendorphin 1–9 . The rate constant value for the second step at 10 mM NaCl was 5-fold higher than that for the testicular isozyme. Similar extent of difference was also observed in k cat values for β -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 β -neoendorphin 1–7 to Leu-enkephalin at a low NaCl concentration. Optimal chloride concentrations for the individual steps of β -neoendorphin 1–9 hydrolysis differed between the two isozymes.