High molecular mass kininogen inhibits cathepsin G-induced platelet activation by forming a complex with cathepsin G

Preliminary studies have shown that high molecular mass kininogen (HK) inhibits cathepsin G-induced platelet activation. However, the potential mechanism underlying this inhibitory effect remains to be elucidated.

Suspensions of washed and gel-filtered platelets were used in radioligand binding and aggregation studies. The amidolytic activity of cathepsin G was measured using specific chromogenic substrate. Western blot technique was utilised to explore the potential complex formation between cathepsin G and HK. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was used to analyse the cleavage products of HK.

At a concentration of 1 microM, HK completely blocked cathepsin G-induced platelet shape change and secretion of ATP. HK inhibited cathepsin G-induced platelet aggregation in a concentration-dependent manner with an IC(50) of 0.48 microM. Moreover, HK was found to inhibit binding of (125)I-cathepsin G to gel-filtered platelets. (125)I-cathepsin G forms a complex with HK. The complex formation did not affect the amidolytic activity of cathepsin G. HK was proteolysed upon interaction with cathepsin G.

Our results show that high molecular mass kininogen down-regulates cathepsin G-induced platelet activation by forming a complex with cathepsin G and thus prevents binding of cathepsin G to platelets. These kininogen-cathepsin G interactions may be potential targets for pharmacological intervention.