THE MACROMOLECULAR NATURE OF THE FIRST COMPONENT OF HUMAN COMPLEMENT

Abstract

Kinetic and ultra-centrifugal experiments demonstrated that the previously described subcomponents of human C[image]l, designated C[image]lq, andC[image]lr, interacted with each other in liquid phase to form a macromolecule which was then capable of converting sensitized erythrocytes (EA) to the state EAC[image]l. The apparent sedimentation constants of C[image]lq, C[image]lr, and C[image]ls and of the macromolecular product of their interaction were approximately US, 7S, 4S, and 18S respectively. Association of C[image]l subcomponents was prevented and dissociation of macromolecular C[image]l was effected by NasHEDTA and Na2MgEDTA but not by Na2CaEDTA. The rate of formation of macromolecular C[image]l was a function of concentration of subcomponents and temperature of interaction, with an apparent energy of activation of 21,000 calories per mol. Ultra-centrifugal studies further indicate the macromolecular nature of C[image]l in normal human serum. In the absence of EDTA, C[image]l sedimented with the serum macroglobulins and C[image]l subcomponents were not detected. Conversely, in the presence of EDTA, macromolecular C [image]1 was not demonstrable and individual C[image] 1 subcomponents could be measured in lighter fractions. The significance of these observations in relation to previous studies on C[image]l subcomponents, the role of Ca++ in C[image]l function, and the subunit structure of enzymes is discussed.