TI  - Association of terminal complement proteins in solution and modulation by
      suramin.
PG  - 6807-16
AB  - The association of terminal complement proteins was investigated by
      analytical ultracentrifugation and multi-angle laser light scattering.
      Native  <prot>C8</prot>  and  <prot>C9</prot>  formed a heterodimer in solution of physiological ionic
      strength with a free-energy change DeltaG degrees of -8.3 kcal/mol and a
      dissociation constant Kd of 0.6 &amp;mgr;M (at 20 degrees C) that was ionic
      strength- and temperature-dependent. A van't Hoff plot of the change in Kd
      was linear between 10 and 37 degrees C and yielded values of DeltaH
      degrees = -12.9 kcal/mol and DeltaS degrees = -15.9 cal mol-1 deg-1,
      suggesting that electrostatic forces play a prominent role in the
      interaction of <prot>C8</prot> with <prot>C9</prot>. Native  <prot>C8</prot>  also formed a heterodimer with  <prot>C5</prot> 
      and low concentrations of polyionic ligands such as protamine and suramin
      inhibited the interaction. Suramin induced high-affinity trimerization of
         <prot>C8</prot>    (Kd = 0.10 microM at 20 degrees C) and dimerization of   <prot>C9</prot>   (Kd = 0.86
      microM at 20 degrees C). Suramin-induced <prot>C8</prot> oligomerization may be the
      primary reason for the drug's ability to prevent complement-mediated
      hemolysis. Analysis of sedimentation equilibria and also of the
      fluorescence enhancement of suramin when bound to protein provided
      evidence for two suramin-binding sites on each <prot>C9</prot> and three on each <prot>C8</prot> in
      the oligomers. Oligomerization could be reversed by high suramin
      concentrations, but 8-aminonaphthalene-1,3,6- trisulfonate (ANTS2- ),
      which mimics half a suramin molecule, could not compete with suramin
      binding and oligomerization suggesting that the drug also binds
      nonionically to the proteins.
AD  - Division of Cell Biology and Biophysics, School of Biological Sciences,
