Activation of the zymogen of hepatocyte growth factor activator by thrombin.

Hepatocyte growth factor activator (HGF activator) is a serine protease which converts single-chain HGF to the active two-chain form. HGF activator purified from human serum has a molecular mass of 34 kDa and consists of two chains held together by a disulfide bond. The nucleotide sequence of HGF activator cDNA shows that HGF activator is derived from the COOH-terminal region of a precursor of 655 amino acids by proteolytic cleavage of the bonds between Arg372 and Val373 and between Arg407 and Ile408 and that the precursor consists of multiple domains homologous to those observed in blood coagulation factor XII. In this study, we identified the precursor of HGF activator in human plasma using an enzyme-linked immunosorbent assay system. The precursor was purified from plasma by a five-step procedure. The purified precursor did not activate single-chain HGF. The precursor was efficiently cleaved in vitro by thrombin, at the bond between Arg407 and Ile408, in the presence of negatively charged substances. The cleaved precursor activated single-chain HGF. These findings led us to conclude that HGF activator is present in plasma as an inactive zymogen and that the zymogen is activated by the cleavage of the bond between Arg407 and Ile408 by thrombin. Characteristic structural domains in the NH2-terminal region of the zymogen may be involved in the binding of the zymogen to negatively charged substances, which stimulates the activation of the zymogen by thrombin.