Characterization of the binding of purified human C1q to heart mitochondrial membranes

The binding of purified, radioiodinated human Clq to baboon heart mitochondrial membranes was investigated. The interaction of Clq with heart mitochondrial membranes was shown to be readily saturable, specific for Clq, and reversible upon addition of unlabeled Clq or increasing salt concentrations Scatchard plots of the binding data were biphasic and yielded association constants on the order of 1 x 10¹⁰ and 1 x 10⁹ M^-1 and binding capacities of approximately 0.16 and 0.24 nmol of Clq/mg of mitochondrial protein. The binding of Clq to isolated cardiac-derived mitochondrial membranes is implicated in the antibody-independent activation of the classical complement pathway by cellular membranes.