Characterization of allotypic determinants of murine complement C3.

Abstract Three allotypes of murine complement C3(C3-1 A, -B, -C) have been distinguished on the basis of their isoelectric point (pI) and alloantigenicity. In order to explore the structural basis of murine C3 allotypes, we purified to homogeneity all 3 allotypes from plasma of NC(C3-1a/a), BALB/c(C3-1b/b), and ICR(C3-1c/c) and compared their physicochemical and immunochemical properties. Highly purified C3 preparations of these allotypes showed the same allotype characteristics, as judged by analytical isoelectric focusing and reactivity with alloantisera, as C3 in EDTA-plasma. Except for this allotype specificity, all allotypes of mouse C3 shared essentially the same physicochemical properties, including the composition and size of constituent polypeptide chains, distribution and apparent content of neutral sugars, and the sensitivity to tryptic digestion. This observation rendered it less likely that the allotype specificity of murine C3 arises from a gross molecular difference such as that observed between certain allotypes of the Ss(C4) protein. In comparison to human C3, mouse C3 commonly exhibited a few distinctive properties, namely, the size of β-chain and the distribution pattern of sugars between constituent polypeptide chains. Split products of extensive tryptic digestion of C3-1 A and C3-1 B allotype retained unreduced reactivity with the corresponding alloantiserum. Therefore, tryptic fragments (C3c and C3d) were isolated from C3-1 A and C3-1 B allotypes and tested for alloantigenicity by using alloantisera directed either to C3-1 A or to C3-1 B. Results of immunodiffusion tests showed that allotypic determinants reside in the C3c fragment in these allotypes of mouse C3.