Interestingly, this peptide specically inhibits both gelatinase A and B and has no effect on several other members of the MMP family or serine proteases.Gelatinase B was thought to be synthesised and immediately secreted as a latent form.However, we have reported an accumulation of both the latent and active forms of gelatinase B in the cytosol of human endothelial microvascular cells in response to PMA. Immunogold electron microscopy revealed that gelatinase B was localised in secretory vesicles.Interestingly, cytosolic gelatinase B was free of TIMP, whereas gelatinase B secreted was bound to TIMP, suggesting that the formation of gelatinase B and TIMP occurred after their secretion.Gelatinase B was also localised to the endothelial cell membrane, possibly binding to the hyaluronan receptor, CD. A nearby free MTMMP then partially activates Betaine hydrochloride progelatinase A by initiating a cleavage in the propeptide domain of progelatinase A.In addition, the promoter region of gelatinase A has a unique noncanonical TATA box, which may be responsible for basal Trilostane secretion of the enzyme.A nearby free MTMMP then activates progelatinase A by initiating a cleavage in the propeptide domain of progelatinase A generating the kD intermediate form.The partially active species is further processed over time to the kD fully active form by autocatalysis. TIMP allows the activation to occur as long as its concentration does not exceed the binding capacity of MTMMP.Similarly, progelayinase A activation induced by PMA has been reported in neonatal foreskin microvascular endothelial cells demonstrated the presence of both the latent and the two active forms of gelatinase A in the membrane fraction of HUVE when stimulated with PMA.Activation of gelatinase A was sensitive to TIMP, but not TIMP.Recently, we have demonstrated a similar effect in human endothelial cells and conrmed that collagen activates gelatinase A via the MTMMP pathway. Furthermore, activation of gelatinase A by collagen is delayed, but sustained for long periods of time in culture.The ability of type collagen to activate gelatinase A suggests that endothelial cells may use active gelatinase A to move through the type I collagenrich interstitium during angiogenesis.We recently reported that thrombininduced activation differs from that of type collagen. Whereas collagen exhibits a delayed effect, thrombin activates gelatinase A rapidly, within h.The thrombin proteaseactivated receptors are not required for the activation.This is evidenced by the nding that the synthetic thrombin receptor activating peptide is unable to activate gelatinase A. Activation by thrombin, however, requires an endothelial membrane molecule, but interestingly, it does not involve MTMMP.Thus, gelatinase A activation induced by thrombin operates via a novel pathway that is independent of the thrombin receptor and proteolytic activity of MTMMP.During the anticoagulation process, thrombin triggers the protein C pathway by interacting with thrombomodulin which is constitutively expressed on the surface of endothelial cells.The thrombin TM complex converts protein C to the anticoagulant serine protease, activated protein C. APC activation of gelatinase A occurs in the absence of cells, indicating that it acts directly.Recent data from our laboratory suggests that thrombin activates gelatinase A through APC, by binding to thrombomodulin and activating protein C.