Porphyromonas gingivalis is closely associated with the development of some forms of periodontitis. The major cysteine proteinases released by this bacterium hydrolyze peptide bonds only after arginyl (gingipain R) or lysyl residues (gingipain K). No target protein inhibitors have been identified for either enzyme, leading us to investigate their inhibition by human plasma alpha 2-macroglobulin (alpha 2M). Both 50- and 95 kDa gingipain R were efficiently inhibited by alpha 2M, whereas the catalytic activity of gingipain K could not be eliminated. All 3 enzymes were, however, inhibited by a homologous macroglobulin from rat plasma, alpha 1-inhibitor-3 (alpha 1I3). alpha-Macroglobulins must be cleaved in the so-called "bait region" in order to inhibit proteinases by a mechanism involving physical entrapment of the enzyme. A comparison of the amino acid sequences of the 2 macroglobulins indicates that the lack of lysyl residues within the bait region of alpha 2M protects Lys-specific proteinases from being trapped. On this basis, other highly specific proteinases might also not be inhibited by alpha 2M, possibly explaining the inability of the inhibitor to control proteolytic activity in some bacterially induced inflammatory states, despite its abundance (2-5 mg/ml) in vascular fluids.