The C4A and C4B isotypic forms of @entity1 complement fragment @entity9199 have the same intrinsic affinity for complement receptor 1 ( XXXX / @entity3432 ).

@entity1 @entity3432 @entity9199 @entity1520 @entity27367 @entity10287
Several previous reports concluded that the @entity9199 fragment of @entity1 @entity10287 ( @entity10287 ) binds with higher affinity to @entity3432 than does C4Bb. Because the isotypic residues, (1101)PCPVLD and (1101)LSPVIH in @entity10287 and @entity9199 , respectively, are located within the C4d region, one may have expected a direct binding contribution of C4d to the interaction with @entity3432 . However, using surface plasmon resonance as our analytical tool, with soluble @entity27367 immobilized on the biosensor chip, we failed to detect significant binding of C4d of either isotype. By contrast, binding of C4c was readily detectable. @entity10287 and @entity9199 , purified from plasma lacking one of the isotypes, were Cs converted to @entity10287 and C4Bb. Spontaneously formed @entity1520 -linked dimers were separated from monomers and higher oligomers by sequential chromatographic steps. The binding sensorgrams of @entity10287 and C4Bb monomers as analytes reached steady state plateaus, and these equilibrium data yielded essentially superimposable saturation curves that were well fit by a one-site binding model. Although a two-site model was required to fit the equilibrium-binding data for the dimeric forms of @entity9199 , once again there was little difference in the K(D) values obtained for each isotype. Independent verification of our surface plasmon resonance studies came from ELISA-based inhibition experiments in which monomers of @entity10287 and C4Bb were equipotent in inhibiting the binding of soluble @entity3432 to plate-bound @entity9199 . Although divergent from previous reports, our results are consistent with recent @entity10287 structural data that raised serious doubts about there being a conformational basis for the previously reported isotypic differences in the @entity9199 - @entity3432 interaction.
