Physico-chemical and antigenic properties of human C3

Abstract

Information about structure and composition of the third component of human complement (C3) and the fragments C3b, C3c and C3d, obtained after trypsin conversion, was gathered by means of electron microscopy, polyacrylamide (PAA) gel electrophoresis with or without sodium dodecyl sulphate (SDS), analysis of the antigenic properties of reduced C3 and its polypeptide chains, circular dichroism measurements and amino acid analysis. The electron micrographs of C3 probably representing aggregated molecules showed spherical shapes. Aggregated C3b had a thread-like form, while the shape of the C3c could not be defined and C3d was invisible. The results of the mol. wt analysis of C3, C3b, C3c and C3d in PAA gels and of that of the component polypeptide chains in SDS-PAA gels were in good agreement with each other. C3, C3c and C3d consists of two polypeptide chains, whereas C3b has four chains. From the SDS-PAA experiments and the analysis of the antigenic properties of reduced C3 and its separated polypeptide chains it was concluded that C3a consists of a part of the high mol. wt chain of C3, whereas C3c and C3d consist of parts of the high as well as the low mol. wt chain of C3. Circular dichroism measurements demonstrated changes in secondary structure after conversion of C3 into C3b and after conversion of C3b into C3c and C3d. Summation of the amino acid composition of all the fragments is in agreement with the composition of C3. The findings suggest that changes in conformation rather than large scale destructive process occur during conversion of C3 into the fragments.