Evidence is presented for the occurrence of a plasminogen-like proenzyme in human tissues.

When homogenates of human connective tissue are incubated with SK they dissolve fibrin clots far more easily than control homogenates without SK. This dissolution of fibrin clots by tissue extracts is a proteolytic reaction similar to the dissolution of fibrin clots by plasma enzyme.

The chemical features of tissue "plasminogen" are the same as of blood plasminogen. It can be extracted at very high and very low PH, precipitated at pH 5.3 (isoelectric point), and inhibited by ions of heavy metals in exactly the same manner as blood plasminogen.

A similar behavior was found in the case of tissue plasminogen in accordance with recent views on the complex nature of the blood fibrinolytic system. The occurrence of tissue inhibitors of plasmin was proven, and it was shown, on comparing the activity of human and bovine tissues, that there is the possibility that in tissues there also exist a "proactivator" and "plasminogen" in accordance with Müllertz and Astrup.

Tissue "plasminogen" occurs chiefly in organs rich in connective tissue, like the aorta wall, fascia, etc. In other organs the "plasminogen" activity is negligible.

It is suggested that tissue "plasminogen" may be of significance in the pathogenesis of certain diseases of connective tissue, e.g. rheumatoid arthritis, in which proteolysis is postulated to be an important pathogenic factor.

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