Summary
An analysis was made of the various possible activators of single-chain urokinase-type
plasminogen activator (scu-PA) in the dextran sulphate euglobulin fraction (DEF) of
human plasma. scu-PA activators were detected in an assay system in which the substrate
scu-PA, in physiological concentration (50 pM), was immuno-immobilized. After activation
of the immobilized scu-PA for a certain period of time the activity of the generated
amount of immuno-immobilized two-chain u-PA was determined with plasminogen and the
chromogenic substrate S-2251. The scu-PA activator activity (scuPA-AA) in the DEF
of plasmas deficient in factor XII or prekallikrein was about half of that in the
DEF of normal plasma. Separation of scuPA-AA in the DEF by gel chromatography showed
to major peaks, one eluting with an apparent Mr of 500,000 and the other around Mr 100,000. The former peak, which coincided with the activity peak of the kallikrein-kininogen
complex, was absent in the DEF of plasma depleted of prekallikrein and therefore was
identified as kalli-krein. The latter peak was still present in the depleted plasma
and most likely represents plasmin, because its scuPA-AA coincided with the activity
peak of plasmin and could be fully inhibited by antibodies raised against human plasminogen.
It is concluded that plasmin and the contact-activation factor kallikrein each contribute
for about 50% to the scuPA-AA in the DEF. Compared on a molar basis, however, plasmin
was found to be almost 1,000 times more effective than kallikrein, and we conclude,
therefore, that in vivo plasmin is the primary activator of scu-PA and the role of
the contact system is of secondary importance.
