Autocatalytic Activation of C1r Subcomponent of the First Component of Human Complement

抄録

Autoactivation of the proenzyme form of a subunit of the first component (C1r) was performed in the presence and absence of diisopropyl fluorophosphate (DFP). The time-course of autoactivation of zymogen C1r followed a sigmoidal curve and was accelerated by addition of the enzyme C_??_r and by increasing the concentration of C1r, suggesting that autoactivation of C1r consists of two intermolecular reactions, i.e. zymogen(C1r)- and enzyme(C1r)-catalyzed reactions. In the presence of 10mM DFP, the enzyme-catalyzed autoactivation of C1r was completely inhibited, while the zymogen-catalyzed autoactivation still proceeded depending upon C1r concentration. These results suggested that the zymogen-catalyzed autoactivation of C1r is a DFP-insensitive second-order reaction and is mediated by an active site generated in a single chain C_??_r through a conformational change (Kasahara et al. (1982) FEBS lett. 141, 128-131).
Based on these results, a possible reaction process of autoactivation of C1r was proposed, as follows:
_??_
where C_??_r represents a conformational isomer which catalyzes the autoactivation of C1r, and the rate constants, k₂ and k₃, are of second-order. Utilizing a computer, we simulated the autoactivation of C1r and found the above scheme to be a reasonable model of C1r autoactivation.
Evidence which supports the formation of a conformational isomer of C1r, C_??_r, as an intermediate in its autoactivation was also obtained by a surface radio-labeling method.