We have examined in detail the kinetics of binding of the serpin alpha-2-antiplasmin to the serine proteases alpha-chymotrypsin and plasmin. These represent model systems for serpin binding. We find, in contrast to earlier published results with alpha-2-antiplasmin and plasmin, that binding is reversible, and slow binding kinetics can be observed, under appropriate conditions. Binding follows a two-step process with both enzymes, with the formation of an initial loose complex which then proceeds to a tightly bound complex. In the absence of lysine and analogues, equilibrium between alpha-2-antiplasmin and plasmin is achieved rapidly, with an overall inhibition constant (K(i)') of 0.3 pM. In the presence of tranexamic acid or 6-aminohexanoic acid, lysine analogues that mimic the effects of fibrin, plasmin binding kinetics are changed such that equilibrium is reached slowly following a lag phase after mixing of enzyme and inhibitor. The K(i)' is also affected, rising to 2 pM in the presence of 6-aminohexanoic acid concentrations above 15 mM. Thus extrapolation to the in vivo situation indicates that complex formation in the presence of fibrin will be delayed, allowing a burst of enzyme activity following plasmin generation, but a tight, pseudoirreversible complex will result eventually. Chymotrypsin is more weakly inhibited by alpha-2-antiplasmin, exhibiting an overall K(i)' of 0.1 nM, after two-stage complex formation. The inhibition constant for the initial loose complex (K(i)) is very similar for both enzymes. The difference in binding strength between the two enzymes is accounted for by the dissociation rate constant of the second step of complex formation. To our knowledge, this rate constant has previously not been measured for serpin interactions. The reversibility of binding argues against a complex having a covalent bond between enzyme and inhibitor. This mechanism of binding has important consequences for the measurement of serpin activity. Structure/function relationships are discussed to explain inhibitory action.
