The slow and tight binding of MR-387A to aminopeptidase N

Abstract

MR-387A [(2S,3-R)-2-hydroxy-3-amino-4-phenylbutanoyl-L-valyl-L-prolyl-(2,4-trans)-L-4- hydroxyproline] reversibly inhibits aminopeptidase N (EC 3.4.11.2) in a process that is remarkable for its unusual degree of time dependence. The time required to inactivate the enzyme by 50% (t1/2) for establishing steady-state levels of EI* complex was approximately 5 minutes. This indicates that the inhibition is a slow-binding process. In dissociation experiments of EI* complex, enzymic activity was regained slowly in a quadratic equation, indicating that the inhibition of aminopeptidase N by MR-387A is tight-binding and reversible. Thus, the binding of MR-387A by aminopeptidase N is slow and tight, with Ki (for initial collision complex, EI) and K1* (for final tightened complex, EI*) of 2.2 × 10-8 M (from Lineweaver-Burk plot) and 4.4×10-10 M (from rate constants), respectively. These data indicate that MR-387 A and aminopeptidase N are bound approximately 200-fold more tightly in the final EI* complex than in the initial collision EI complex.