EFFECT OF OXIDANTS ON PROTEASES OF THE FIBRINOLYTIC SYSTEM - POSSIBLE ROLE FOR METHIONINE RESIDUES IN THE INTERACTION BETWEEN TISSUE-TYPE PLASMINOGEN-ACTIVATOR AND FIBRIN

Evidence has recently been presented that activated macrophages (M-phi) express both urinary (u-PA) and tissue type (t-PA) plasminogen activator. Major cell products of M-phi and polymorphonuclear neutrophils (PMN) are reactive oxidants of the HOCl/ chloramine type. Since PMN and M-phi are involved in inflammatory and fibrinolytic processes, we were interested in the interaction of u-PA, t-PA, and plasmin with oxidants of the leukocyte type. The enzymes were treated with chloramine-T, which at pH 8.5 is a selective oxidant for methionine residues. Oxidation by chloramine-T of t-PA abolished about 40% of both stimulation susceptibility of t-PA by fibrinogen degradation products (FDP) and affinity of t-PA to FDP. However, the plasminogenolytic and amidolytic activity of unstimulated t-PA as well as the plasminogenolytic activity of u-PA and the amidolytic activity of plasmin are not impaired. Identification of the amino acid residues in the t-PA responsible for the interaction with fibrin might be of great importance in order to understand the mechanism of the clot- selectivity of t-PA. The present study gives evidence that fibrin specificity of t-PA partly depends on chloramine oxidizable amino acids, presumably methionine residues. Hence, experimental data on the interaction between t-PA and fibrin, using oxidized and labelled t-PA should be interpreted with caution. It may be suggested that oxidants of the leukocyte type might regulate t-PA activity and selectivity for fibrin.