Plasmin-catalysed cleavage of single chain tissue-type plasminogen activator in fibrin clots

The activity of single chain t-PA in the presence of fibrin was investigated and compared to that of its twochain counterpart. A plasmin resistant t-PA analogue in which Arg-275 is replaced by Gly was included in this study in order to avoid the complications caused by concomitant two chain generation during plasminogen activation experiments. Substantial plasminogen activation of single chain t-PA was observed during fibrin clot dissolution, still the clot lysis activity of two chain t-PA was found to be about 20% higher than that of the single chain form. Plasmin-catalysed cleavage of single chain t 25I-t-PA was studied in the presence and absence of fibrin. This reaction was not enhanced by fibrin, rather a small inhibition was observed. In addition to the primary cleavage site at Arg-275 Ile-276 secondary plasmin-catalysed cleavage resulting in a 30 000 Da fragment was observed. Appearance of this fragment took place roughly at the same time as a decline in clot lysis activity (but not in amidolytic activity) was observed. Secondary plasmin cleavage was not observed when fibrin was present.