When serum from a patient with membrano-proliferative glomerulonephritis and normal serum are mixed at 37°C, C3 is rapidly broken down to two more rapidly migrating components. In the mixture, a heat-labile pseudoglobulin, designated as the C3 nephritic factor or C3NeF, reacts with a pseudogolbulin in the normal serum, designated as cofactor, to form a C3 inactivator. By analogy with the cobra venom factor, the C3 inactivator is most likely a complex of the nephritic factor and cofactor. The complex has been designated as the C3 lytic nephritic factor or C3LyNeF. The reaction which results in the Formation of C3LyNeF requires the presence of Mg++, is highly temperature sensitive but occurs very rapidly at 37°C. In 20 min at 37°C, C3LyNeF can break down over 80% of the C3 in a mixture of normal and nephritic serum.
The two-step reaction which leads to C3 breakdown has an optimum pH ranging from 6.0 to 9.0. Experiments employing serum depleted of C4 and C2, as well as certain characteristics of the C3NeF system provide evidence that C3 breakdown with nephritic serum is not dependent on complement-inactivating immune complexes or on the action of convertase (C4, 2). Data relating rate of C3 breakdown to the concentrations of C3NeF, C3, and C3LyNeF in the reaction mixture are similar to those for the reaction of enzyme with substrate. The biological significance of C3LyNeF in the production of glomerular inflammation has not been established.