Cleavage of C2 in pathological serum and plasma studied by crossed immunoelectrophoresis

A cleavage product of C2, C2a was demonstrated by crossed immunoelectrophoresis in serum and EDTA plasma from patients with active systemic lupus erythematosus (SLE), patients in the early phase of acute poststreptococcal glomerulonephritis (AGN), and from two males with congenital deficiency of C1(-) inactivator, one of whom had symptoms of hereditary angioedema. C2a was not found in normal serum and plasma. C2 was more stable in plasma than in serum with regard to the effects of storage in room temperature and of repeated freezing and thawing. C2a concentrations were higher in serum than in plasma samples from SLE and AGN patients. Expressed in per cent of the total C2 protein, C2a was inversely correlated (r = -0.91) with the C2 hemolytic activity in the samples, which explains discrepancies between the results of immunochemical and functional assays, when used to measure C2 concentrations in disease.