[Comparison of consumption kinetics of the C4 and C2 components of complement by immune complexes "in vitro"...

We study the consumption kinetic of the fourth and second component of complement by the C1 esterase in different systems with EA or EAC1 and fresh NHS, or functionally purified C4 and C2 preparations. The results show that the two kinetics are similar with a comparable decrease in both C4 and C2 activities till 90 seconds (the mean residual values are from 21 to 34% at this time). The results show moreover that the consumption of the C2 component requires necessarily the presence of the C4 component. The C4 reaction kinetic compared with the C2 "in vitro" one refutes the hypothesis of a greater sensitivity of the C4 component compared with the C2, for the enzymatic action of the C1 esterase. So, a discrepancy in the reaction speed cannot be evoked in order to explain the obvious C4 depression in the consumption hypocomplementemia, and different hypothesis have to be investigated, such as, for instance, a lower synthesis rate of the C4 component compared with the C2 one.