BINDING OF PLASMINOGEN AND TISSUE-TYPE PLASMINOGEN-ACTIVATOR TO DIMERIC ALPHA(S2)-CASEIN ACCELERATES PLASMIN GENERATION

The effect of individual bovine caseins on the rate of plasminogen act ivation by tissue-type plasminogen activator (t-PA) was examined. Usin g a coupled peptidyl anilide plasminogen activator assay, we found tha t dimeric alpha(s2)-casein induced a concentration-dependent enhanceme nt of the t-PA-catalyzed plasminogen activation. On a weight basis, th e stimulating effect of dimeric alpha(s2)-casein was comparable to tha t of cyanogen bromide cleaved fibrinogen. In a direct catalytic assay with a t-PA sensitive peptidyl anilide substrate, dimeric alpha(s2)-ca sein had no effect on t-PA activity. Ligand blotting experiments showe d binding of both plasminogen and t-PA to the dimeric alpha(s2)-casein . The combined results from the binding experiments and the enzymatic assays suggest that the observed enhancement of plasminogen activation is mediated via the formation of a ternary complex between t-PA, plas minogen, and dimeric alpha(s2)-casein. We further investigated the bin ding specificity of t-PA towards dimeric alpha(s2)-casein using t-PA d eletion mutants and monoclonal antibodies. The results were compatible with the binding being mediated by kringle 2 and the finger domain of t-PA.