C5b-9 dimer: isolation from complement lysed cells and ultrastructural identification with complement-dependent membrane lesions.

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Abstract
The membrane attack complex (MAC) of complement [C] was extracted from the membranes of [rabbit erythrocyte] cells lysed by human C and its properties were compared with those of the fluid phase complex SC5b-9. Upon sodium dodecyl sulfate polyacrylamide gel electrophoresis and immunochemical analysis, the 2 isolated complexes had identical subunit compositions, except that the MAC lacked the S-protein. The sedimentation coefficient and MW of the extracted and isolated MAC were, respectively, 33.5 S and 1.7 .times. 106 daltons, compared to 23 S and 1.0 .times. 106 dalton for SC5b-9. Because the MW of the MAC was approximately 2 times greater than that of C5b-9 (800,000 daltons), the MAC was considered the dimer of C5b-9. Under specified conditions, the 33.5 S dimer could be converted to the 23 S monomer without dissociation of subunits. The MAC had the EM appearance and dimensions that are characteristic for the C produced ultrastructural membrane lesions. SC5b-9 had a different ultrastructure that is dissimilar to the morphology of the lesions. The isolated MAC could be reincorporated into phospholipid bilayers and assumed on the surface of the resultant lipid vesicles the orientation and appearance of typical complement lesions.