Granzymes (lymphocyte serine proteases): characterization with natural andsynthetic substrates and inhibitors

Natural killer (NK) and cytotoxic T-lymphocytes (CTLs) kill cells within an organism to defend it against viral infections and the growth of tumors. O ne mechanism of killing involves exocytosis of lymphocyte granules which ca uses pores to form in the membranes of the attacked cells, fragments nuclea r DNA and leads to cell death. The cytotoxic granules contain perforin, a p ore-forming protein, and a family of at least 11 serine proteases termed gr anzymes. Both perforin and granzymes are involved in the lytic activity. Al though the biological functions of most granzymes remain to be resolved, gr anzyme B clearly promotes DNA fragmentation and is directly involved in cel l death. Potential natural substrates for Gr B include procaspases and othe r proteins involved in cell death. Activated caspases are involved in apopt osis. The search continues for natural substrates for the other granzymes. The first granzyme crystal structure remains to be resolved, but in the int erim, molecular models of granzymes have provided valuable structural infor mation about their substrate binding sites. The information has been useful to predict the amino acid sequences that immediately flank each side of th e scissile peptide bond of peptide and protein substrates. Synthetic substr ates, such as peptide thioesters, nitroanilides and aminomethylcoumarins, h ave also been used to study the substrate specificity of granzymes. The dif ferent granzymes have one of four primary substrate specificities: tryptase (cleaving after Arg or Lys), Asp-ase (cleaving after Asp), Met-ase (cleavi ng after Met or Leu), and chymase (cleaving after Phe, Tyr, or Trp). Natura l serpins and synthetic inhibitors (including isocoumarins, peptide chlorom ethyl ketones, and peptide phosphonates) inhibit granzymes. Studies of subs trate and inhibitor kinetics are providing valuable information to identify the most likely natural granzyme substrates and provide tools for the stud y of key reactions in the cytolytic mechanism. (C) 2000 Elsevier Science B. V. All rights reserved.