Carboxypeptidase U, a plasma carboxypeptidase with high affinity for plasminogen

A novel basic carboxypeptidase clearly different from carboxypeptidase N has been isolated from human plasma. It circulates as an enzymatically inactive precursor enzyme bound to plasminogen. During fibrinolysis, it can be converted to its active form, carboxypeptidase U, through the action of plasmin. The active enzyme has an apparent molecular weight of 53,000 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It hydrolyzes the synthetic peptides hippuryl-L-arginine and hippuryl-L-lysine but, in contrast to other human basic carboxypeptidases, has only a limited esterase activity. After its activation, carboxypeptidase U tends to be very unstable.