Apart from tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), a third PA appears to occur in human plasma. Its activity is initiated when appropriate triggers of the contact system are added, and the activation depends on the presence of factor XII and prekallikrein in plasma. The activity of this, so-called, contact-system dependent PA accounts for 30% of the PA activity in the dextran sulphate euglobulin fraction of plasma and was shown not to be an intrinsic property of one of the contact-system components, nor could it be inhibited by inhibitory antibodies against t-PA or u-PA. We have succeeded in identifying this third PA in dextran sulphate euglobulin fractions of human plasma. Its smallest unit (SDS-PAGE) is an inactive 110 kDa single-chain polypeptide which upon activation of the contact system is converted to a cleaved, disulphide-bridged molecule with PA activity. The native form, presumably, is an oligomer, since the apparent Mr on gel-chromatography is 600,000. The IEP is 4.8, much lower than that of t-PA and u-PA. Although the active 110 kDa polypeptide cannot be inhibited by anti-u-PA, it yet comprises a 37 kDa piece with some u-PA related antigenic determinants. However, these determinants are in a latent or cryptic form, only detectable after denaturation by SDS. The 110 kDa polypeptide is evidently not a dimer of 55 kDa u-PA or a complex of u-PA with an inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)
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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 kallikrein. 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.
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Endothelial cells were isolated from arteries and veins obtained from elderly people at autopsy and propagated for 37 to 69 population doublings. The cells secreted tissue-type plasminogen activator (t-PA) and PA inhibitor-1, and, after subculturing, urokinase-type PA (u-PA) antigen. The following differences between endothelial cells from adult arteries and veins were observed: 1) The cells had the potential to be propagated as a healthy monolayer. The diameter of aortic endothelial cells increased after 8 to 19 population doublings, while a homogeneous population of small diameter vena cava cells was retained for 35 population doublings. 2) The amount of secreted t-PA varied. Vena cava cells produced four times more t-PA than aorta cells, and 20-fold more than umbilical artery or vein endothelial cells. The t-PA mRNA content of vena cava cells did not exceed that of aorta cells, but was fourfold greater than that of umbilical cord endothelial cells. 3) The release of u-PA antigen varied. No u-PA antigen was detectable in conditioned medium of primary cultures of human aorta and vena cava endothelial cells or of early passage vena cava cells. After prolonged subculturing, vena cava cells started to secrete u-PA. Endothelial cells from aorta and other adult arteries, however, started secreting u-PA after one to four passages, parallel to the occurrence of enlarged endothelial cells. u-PA was present as a u-PA/inhibitor complex and as a single-chain u-PA. These differences may be developmentally related to their artery or vein origin or may reflect differences acquired during the "life history" of these blood vessels in vivo. Our data suggest that the release of u-PA antigen by human macrovascular endothelial cells can be used as an indicator of cell senescence.
