Inhibitor of serine proteases. Its primary target is elastase, but it also has a moderate affinity for plasmin and thrombin. Interacts with CELA2A (By similarity). Interacts with ERGIC3 and LMAN1/ERGIC53 (By similarity). Interacts with PRSS1/Trypsin (By similarity). Expressed not only in liver but also in kidney tubule cells, where it is regulated by androgens during development. The reactive center loop (RCL) extends out from the body of the protein and directs binding to the target protease. The protease cleaves the serpin at the reactive site within the RCL, establishing a covalent linkage between the carboxyl group of the serpin reactive site and the serine hydroxyl of the protease. The resulting inactive serpin-protease complex is highly stable (By similarity). Belongs to the serpin family.