Purification and characterization of a novel substrate for plasma kallikrein (PK-120) in human plasma.

A 120 kDa plasma protein, which is susceptible to plasma kallikrein, was purified from human plasma by polyethylene glycol fractionation followed by ion exchange chromatography using Q-Sepharose, S-Sepharose, and hydroxyapatite and gel filtration on Sephacryl S-200. The 120 kDa protein, termed PK-120 in this paper, was a single polypeptide chain containing about 20% sugar by weight and its concentration in plasma was estimated to be 80 micrograms/ml by ELISA. At least three fragments, 100, 70, and 35 kDa, were produced from PK-120 by plasma kallikrein. The N-terminal sequence and Western blot demonstrated that PK-120 was first cleaved to yield the 100 and 35 kDa fragments, then the 100 kDa fragment was cleaved into the 70 kDa fragment. N-Terminal sequence analyses of PK-120 and its fragments demonstrated that it is a novel plasma protein, distinct from high molecular weight kininogen, a natural substrate for plasma kallikrein.