Nonisotropic enzyme-inhibitor interactions: A novel nonoxidative mechanism for quantum proteolysis by human neutrophils

Traditional theories of enzyme kinetics do not model the influences of rapidly changing and nonisotropic enzyme concentrations in real-world systems. We have modeled local enzyme concentrations in space and time following quantal release of human leukocyte elastase (HLE) from cytoplasmic granules of polymorphanuclear neutrophils (PMN). Calculations from first principles indicate that similar to 67000 molecules of HLE are stored in each azurophil granule at a mean concentration of 5.33 mM, which exceeds pericellular inhibitor concentrations in vivo by nearly 3 orders of magnitude. Diffusion analysis predicts obligate catalytic activity (excess of local enzyme over inhibitor concentration) that extends to 1.33 mu m from the site of granule extrusion (7.8-fold larger than the mean radius of the granule), with a duration of 12.4 ms, when the pericellular concentration of alpha(1)-antitrypsin equals that of normal plasma. In contrast, when PMN are bathed in alpha(1)-antitrypsin concentrations found in plasma from individuals with alpha(1)-antitrypsin deficiency, the radius and duration of obligate catalytic activity are increased 2.5-fold and 6.2-fold, respectively. These simulations agree remarkably well with our recent direct observations and provide a novel, nonoxidative mechanism by which quantum bursts of extracellular proteolytic activity occur despite proteinase inhibitors in the bathing medium. Titration of local enzyme-inhibitor concentration is the dominant determinant of the size and duration of such events. This construct provides new insights into the pathogenesis of tissue injury in alpha(1)-antitrypsin deficiency. The nonisotropic analyses presented herein supplement Michaelis-Menten theory and have implications for the pathogenesis and therapy of alpha(1)-antitrypsin deficiency and other inflammatory diseases associated with considerable morbidity and mortality.