STRUCTURE ACTIVITY STUDIES OF FLUOROKETONE INHIBITORS OF ALPHA-LYTIC PROTEASE AND HUMAN-LEUKOCYTE ELASTASE

We have synthesized a series of peptidyl fluoroketones that reversibly inhibit the serine proteases human leukocyte elastase (HLE) and α-lytic protease (α-LP). Ac-ambo-AlaCF3 (1) inhibits HLE and α-LP with Ki's of 2.4 and 15 mm, respectively. The effects of structural variations on this parent compound on Ki and the kinetics of inhibition were studied. The acetyl group was replaced by the tripeptide Z-l-Ala-l-Ala-l-Pro to yield the tetrapeptide trifluoroketone (TFK) Z-l-Ala-l-Ala-l-Pro-ambo-AlaCF3 (2). This extension reduced Ki 3500-fold for HLE and 3000-fold for α-LP. Removal of a fluorine atom from a TFK decreases Ki about 15- to 30-fold with both enzymes. Replacement of one fluorine atom of 2 by a residue (CH2CH2COLeuOMe) (6) which can interact with the S′1 and S′2 subsites decreased Ki 30-fold for HLE and 150-fold for α-LP compared to Z-l-Ala-l-Ala-l-Pro-ambo-AlaCF2H (3). The Ki of 6 for HLE is approximately equal to that of trifluoroketone 2. For α-LP Ki of 6 is 10-fold lower than that for the trifluoroketone 2. Inhibitors with Ki values <10-7 m exhibit slow binding kinetics. By analogy to cholinesterases and chymotrypsin, it is likely that these enzymes combine with the keto form of the inhibitor to form the enzyme-inhibitor complex. Therefore, kon and Ki were corrected for the ketone concentration. The corrected kon values for the slow binding inhibitors are in most cases less than diffusion controlled, ranging between 8.2 × 104 and 4.68 × 106 m-1 s-1. An exception is Z-l-Ala-l-Ala-l-Pro-ambo-ValCF3 (8) where kon = 9 × 107 m-1 s-1, which is nearly diffusion controlled. © 1990.