Background: Myeloperoxidase (MPO) neutrophils have been considered an important pathophysiological factor in oxidative stress. Mainly through generation of hypochlorous acid in the phagosome, unchecked activity may lead to inactivation of important proteins through modification of tyrosine and other residues. This has been shown for low-density lipoprotein, apolipoprotein AI and paraoxonase 1 (PON-1). Notably, plasminogen has 29 tyrosine residues and we previously demonstrated that it can be inactivated by nitration of these residues.
Methods: We hypothesized that plasminogen can also be inactivated by HOCl/OCl(-) in a similar manner to PON-1, and that this inhibition can be counteracted by cysteine or taurine. In the present study we compared the effects of HOCl/OCl(-) on these two plasma proteins and explored the effects of inhibitors.
Results: Our study revealed that HOCl/OCl(-) inhibits streptokinase-induced plasmin activity at low micromolar concentrations that may well occur in vivo at sites of inflammation (IC(50)=40 micromol/L). This inhibitory effect occurs at much lower concentrations than those that inhibit PON-1 activity (IC(50) 120 micromol/L). The inhibition is paralleled by an increase in 3-chlorotyrosine adducts in the protein. HOCl/OCl(-) inhibition of plasminogen and 3-chlorotyrosine formation are blocked by cysteine; taurine shows much lower protection. HOCl/OCl(-) does not modify the activation of plasminogen by streptokinase, leaving inactivation of active site Tyr 614 (possibly directed by Lys 615) as a very attractive hypothesis to explain the effect and the high sensitivity found.
Conclusions: Since neutrophils have been shown to secrete HOCl/OCl(-) up to 100 micromol/L, inactivation of plasmin molecules by neutrophil-generated HOCl/OCl(-) could partly explain the increased thrombo-genicity observed in inflammatory conditions, in smokers and in other diseases. Thus, dietary intervention or the use of thiols or large doses of taurine compounds may be useful as coadjuvant therapeutic measures.