Streptokinase-dependent potentiating factor (SK-potentiator) for plasminogen activation from human plasma: its identification as a fibrinogen degradation product.

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A factor, named SK-potentiator, which is known to potentiate the activation of plasminogen by streptokinase (SK), was isolated from human plasma. The procedures consisted of column chromatographies on DEAE-Sephadex A-50 and heparin-agarose, followed with gel-filtration on a Sepharose 6B column and affinity chromatography on a lysine-Sepharose 4B column. The isolated SK-potentiator markedly potentiated the rate of activation of plasminogen by streptokinase. However, it did not show a potentiating effect on the activation of plasminogen by urokinase or on the plasmin activity. SK-potentiator showed a similar mobility to that of fibrinogen on both SDS-polyacrylamide gel and agarose gel electrophoresis, and cross-reacted with anti-fibrinogen antiserum. The amino acid composition of SK-potentiator was very close to that of human fibrinogen, although the content of serine and threonine was significantly lower. SK-potentiator showed a single band with a molecular weight of 300,000 on SDS-gel electrophoresis in the absence of 2-mercaptoethanol. In the presence of 2-mercaptoethanol, it showed two major bands with molecular weights of 53,000 and 48,000, respectively, which corresponded to the B beta chain and gamma chain of fibrinogen. To establish further that the isolated SK-potentiator may be one of the fibrinogen degradation products (FDP), human fibrinogen was digested with plasmin and the SK-potentiator activity generated in the course of the digestion was measured. As a result, the SK-potentiator activity was found to initially increase and then decrease with incubation time, suggesting that an early FDP has the ability to potentiate the SK-mediated activation of plasminogen. The early FDP was then isolated by gel-filtration on a column of Sepharose 6B and it was found that the SK-potentiator activity was associated with the early FDP. Moreover, the early FDP showed the same electrophoretic mobility as the isolated SK-potentiator on SDS-gel electrophoresis and their amino acid compositions were quite similar to each other. From these results, the SK-potentiator was identified as the early FDP.