Isoelectric Focusing of Components of the Complement System and Certain Related Proteins in Human Serum (1)

Abstract

The isoelectric points for nine components and the three component inactivators of human complement are defined. The results indicate that C1, C8 and the C3 inactivator exist in serum in at least two forms, each variant of which appears to have similar biological activity. The two variants of C3 inactivator have slightly different pi values depending on the methods used for purification. C4 has been found mainly at pH 6.0 and 6.4, but minor variants have been detected at six other pH values. A hemolytically inactive form of C4 has been found by either precipitin or hemagglutination inhibition tests at pH 4.2 to 4.5. C3, possessing hemolytic and immune adherence reactivity, has a pi of 5.75 and appears to be a different protein from β1c which has a pi of 6.05. Hemolytic C5, with a pi of 4.1, reacts in hemagglutination inhibition assays with antisera purported to be monospecific for β1c/β10.