| Abstract: | In this paper we report the effect of sulfatides on the rate constants of factor XII activation by kallikrein and its isolated light chain (the domain of kallikrein that contains the active site of the enzyme). In the absence of sulfatides, kallikrein and the light chain were equally effective in factor XII activation (k1 = 1.57 X 10(3) M-1 s-1 at pH 7.0). The pH optima were the same (pH 7.0) and the reaction was not affected by variation of the ionic strength. Sulfatides strongly increased the rate constants of factor XIIa formation. In the presence of sulfatides kallikrein was, however, much more active than its light chain. At 330 microM sulfatides, pH 7.0 and 100 mM NaCl the rate constants of factor XII activation were 5.34 X 10(6) M-1 s-1 and 4.17 X 10(4) M-1 s-1 for kallikrein and its light chain, respectively. The pH optimum of factor XII activation by kallikrein in the presence of sulfatides was shifted to pH 6.3, and the reaction became highly ionic-strength-dependent. The rate constant increased considerably at decreasing NaCl concentrations. The optimum pH for light-chain-dependent factor XII activation in the presence of sulfatides remained unaltered and the reaction was not affected by the ionic strength. Binding studies revealed that both kallikrein and factor XII bind to the sulfatide surface, whereas no binding of the light chain of kallikrein was detectable. The isolated heavy chain of kallikrein had the same binding properties as kallikrein, which indicates that the heavy-chain domain contains the functional information for kallikrein binding to sulfatides. Since the effects of pH and ionic strength on the rate constants of kallikrein-dependent factor XII activation in the presence of sulfatides correlated with effects on the binding of kallikrein, it is concluded that under these conditions surface-bound factor XII is activated by surface-bound kallikrein. Our data suggest that sulfatides stimulate kallikrein-dependent factor XII activation by two distinct mechanisms: by making factor XII more susceptible to peptide bond cleavage by kallikrein and by promoting the formation of the enzyme-substrate complex through surface binding of kallikrein and factor XII. |
