Differential inhibition of serine proteinases by rabbit alpha 1-proteinase inhibitors F and S.

Inhibition of six serine proteinases (bovine trypsin and chymotrypsin, equine leucocyte proteinases type 1 and 2A, porcine pancreatic elastase type III and rabbit plasmin) by rabbit alpha 1-proteinase inhibitors F and S was studied. In each case examined, the F form reacted more rapidly. The number of moles of an enzyme inhibited by one mole of alpha 1-proteinase inhibitor in a complete reaction (molar inhibitory capacity) ranged from 0.26 (leucocyte proteinase type 1) to 1.01 (trypsin). More significantly, however, the molar inhibitory capacities of both alpha 1-proteinase inhibitors differed for the same enzymes. The highest F/S inhibitory ratio was recorded with chymotrypsin (1.88), and the lowest with elastase (0.69). These differences in molar inhibitory capacities are likely to reflect the dual nature of the reaction between the inhibitor and a proteinase, that is, either complex formation or inactivation of alpha 1-proteinase inhibitor without enzyme inhibition. No evidence was obtained to suggest that differential reactivity and differential inhibitory capacity are interdependent. The observations are consistent with the view that rabbit alpha 1-proteinase inhibitors F and S are closely related yet functionally distinct proteins.