Elastin decreases the efficiency of neutrophil elastase inhibitors

Elastase inhibitors are potential drugs for the control of lung emphysema. Since neutrophils may release elastase in the lung interstitium, elastin and inhibitors may complete locally for the binding of enzyme. To better evaluate the potential activity of antielastases, we have run experiments that mimic this in vivo competition. Elastase was added to mixtures of human lung elastin and inhibitor, and the solubilization of the fibrous substrate was measured as a function of time. Controls in which a synthetic substrate was used instead of elastin were run under identical conditions. We show that the rate constants for the irreversible inhibition of elastase by methoxysuccinyl-Ala2-Pro-Val-chloromethylketone and L-657,229, a substituted beta lactam, are 28- and 63-fold lower with elastin than with a synthetic substrate, respectively. The rate constant decreases with increasing concentrations of elastin, indicating that the inhibition is competitive. Elastin also impairs the potency of the following reversible inhibitors: trifluoroacetyl-Lys-Ala-NH-C6H4-p-C6H11, trifluoroacetyl-Lys-Ala-NH-C6H4-pN(C2H5)2, methoxysuccinyl-Ala2-Pro-Boro-Val-OH, and mucus proteinase inhibitor whose Ki values are 29- to 127-fold higher with elastin than with a synthetic substrate. Again the inhibition is competitive. We conclude that association rate constants of irreversible inhibitors and Ki values of reversible ones may be measured accurately using elastin as a substrate. The kinetic constants measured with elastin and not those determined with synthetic substrates should be used to decide whether a given inhibitor is potent enough to be a physiologic antielastase or a potential antielastase drug.