Binding of plasminogen and tissue-type plasminogen activator to dimeric α s2-casein accelerates plasmin generation

Abstract

Summary The effect of individual bovine caseins on the rate of plasminogen activation by tissue-type plasminogen activator (t-PA) was examined. Using a coupled peptidyl anilide plasminogen activator assay, we found that dimeric α en -casein induced a concentration-dependent enhancement of the t-PA-catalyzed plasminogen activation. On a weight basis, the stimulating effect of dimeric α en -casein was comparable to that of cyanogen bromide cleaved fibrinogen. In a direct catalytic assay with a t-PA sensitive peptidyl anilide substrate, dimeric α en -casein had no effect on t-PA activity. Ligand blotting experiments showed binding of both plasminogen and t-PA to the dimeric α en -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, plasminogen, and dimeric α en -casein. We further investigated the binding specificity of t-PA towards dimeric α en -casein using t-PA deletion mutants and monoclonal antibodies. The results were compatible with the binding being mediated by kringle 2 and the finger domain of t-PA.