| Abstract: | Previous studies from our laboratories (Sugo et al. (1980) Biochemistry 19, 3215-3220) have shown that bovine high-molecular-weight (HMW) kininogen remarkably accelerates the kaolin-mediated activation of Factor XII in the presence of prekallikrein, and that both fragment 1.2 and the light chain regions located in the COOH terminal half of the kininogen molecule are essential for the activation. In the present study, we demonstrate that the accelerating effect of HMW kininogen is mediated through its adsorption on the kaolin surface through the fragment 1.2 region and its complex formation with prekallikrein through the light chain region. The evidence is as follows: 1. HMW kininogen radio-labeled with 125I was adsorbed on kaolin and the adsorption was inhibited by the prior treatment of kaolin with fragment 1.2, fragment 1.2-light chain, kinin-free protein or HMW kininogen, but not with kinin- and fragment 1.2-free protein, light chain or low molecular-weight (LMW) kininogen. 2. The complex formation of HMW kininogen with prekallikrein in bovine plasma or in the purified system was examined by gel-filtration on a column of Sephacryl S-200 In bovine plasma, prekallikrein was eluted in the same fraction as HMW kininogen, showing an apparent molecular weight of 250,000, whereas purified prekallikrein was eluted in the fraction corresponding to an apparent molecular weight of 100,000. When purified prekallikrein was mixed with purified HMW kininogen in a mol ratio of 1 to 2, all prekallikrein was found to be associated with HMW kininogen. Furthermore, purified prekallikrein mixed with kininogen derivatives, such as kinin- and fragment 1.2-free protein, fragment 1.2-light chain or light chain, was eluted in the higher molecular weight fraction. HMW kininogen did not form a complex with prekallikrein. Using the same technique, it was shown that kinin- and fragment 1.2-free protein forms a complex not only with prekallikrein but also with kallikrein. |
