Cathepsin G inactivates human protein S in vitro

Human polymorphonuclear leukocyte cathepsin G was used in vitro to digest human protein S. While clotting assays indicated that the proteinase induced a rapid decrease in activity, polyacrylamide gel electrophoresis-sodium dodecyl sulphate indicated the removal of a peptide of low molecular mass product protein S (des 1-40). The loss in activity was the result of cathepsin G cleaving position Phe40-Tyr41 and removing the calcium-binding region. Calcium ions almost totally protected the cofactor from the action of cathepsin G in vitro.