In biology and biochemistry, protease inhibitors are molecules that inhibit the function of peptidases (old name: protease, hence the term protease inhibitor). Many naturally occurring protease inhibitors are proteins.
In medicine, protease inhibitor is often used interchangeably with alpha 1-antitrypsin (A1AT, which is abbreviated Pi for this reason).[1] A1AT is indeed the protease inhibitor most often involved in disease, namely in alpha 1-antitrypsin deficiency.