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

The binding of purified, radioiodinated human C1q to baboon heart mitochondrial membranes was investigated. The interaction of C1q with heart mitochondrial membranes was shown to be readily saturable, specific for C1q, and reversible upon addition of unlabeled C1q or increasing salt concentrations. Scatchard plots of the binding data were biphasic and yielded association constants on the order of 1 X 10(10) and 1 X 10(9) M-1 and binding capacities of approximately 0.16 and 0.24 nmol of C1q/mg of mitochondrial protein. The binding of C1q to isolated cardiac-derived mitochondrial membranes is implicated in the antibody-independent activation of the classical complement pathway by cellular membranes.