Preliminary investigations in this laboratory have been directed to the isolation, purification and storage of human complement components applicable to large-scale preparation. Depletion of specific components by immunoprecipitation as a means of preparing simpler specific reagents for hemolytic assays will be studied, as will the application of radioimmunoassay to the quantitation of selected complement components. The components prepared will serve as a source of protein for investigations of the chemical and structural basis of their biological functions, initial emphasis being placed on C4, C3 and C5. An effort will be directed to an elucidation of the chemical nature of the interactions involved in the formation of the membrane receptor-component complex of C4 and C3. The complexes will be solubilized by detergent and isolated by precipitation with insolubilized anti-component sera. These precipitates will be characterized by electrophoresis on polyacrylamide gels in SDS. Models of covalent vs. noncovalent interactions will be tested. The subunit structures of C4, C3 and C5 has been shown to comprise a heavy and a light polypeptide chain. These chains will be isolated and characterized by amino acid and carbohydrate compositions, N- and C-terminal sequences, and peptide compositions. The subunits of components rendered hemolytically inactive by enzymes and chemical reagents will be studied in the same manner to establish the structural alterations involved in the process of activation. Sequence analysis will be undertaken where appropriate to establish homologies in primary structure within the subunits and between the subunits of C4 and C3 as a probe of the molecular evolution of the proteins. Finally, C3 and C4 of defined phenotype will be isolated from selected donors. Their subunits will again be isolated and characterized as above in an effort to delineate the nature of the chemical alterations involved in the observed polymorphism, presumably genetic in basis.