1. The cytokinase (tissue activator of plasminogen) content of several mammalian tissues was evaluated by a quantitative casein hydrolysis method. 2. An alkaline (pH10·5) extraction of cytokinase from rabbit kidney lysosome–microsome fraction, followed by chromatography on DEAE-cellulose at pH7·6 with stepwise or linear increase in concentration of phosphate buffer, gave an 86-fold purification of the enzyme. The purified material was non-proteolytic against casein and heated fibrin and was freeze-dried without significant loss of activity or solubility. 3. Cytokinase is a protein with E0·1%1cm.=0·87 at 280mμ, and does not possess sufficient hexose or sialic acid to be classified as a glycoprotein. It has S20,w 2·9–3·1s and molecular weight 50000 when measured on a calibrated Sephadex G-100 column. It has an isoelectric point between pH8 and pH9, and is maximally active and stable at pH8·5. It is inactivated by heat at 78°. 4. Cytokinase and human urokinase have the same Km value and are inhibited in a partially competitive manner by ∈-aminohexanoic acid and aminomethylcyclohexanecarboxylic acid. They are also inhibited by cysteine and arginine, but are unaffected by iodoacetamide and p-chloromercuribenzoate. 5. On the basis of this and other evidence it is suggested that rabbit kidney cytokinase and human urokinase are similar, if not identical, enzymes.
