Ricin subunit association. Thermodynamics and the role of the disulfide bond in toxicity.

The values of the thermodynamic parameters characterizing the association of the subunits of reduced ricin have been determined from equilibrium studies in the analytical ultracentrifuge. van't Hoff analysis indicates that the Gibbs free energy change for subunit association is predominantly of entropic origin. The positive values for the entropy and enthalpy changes suggest that hydrophobic forces may play a dominant role in the association. The association is characterized by values of Ka of 1.72 X 10(6) M-1 at 22 degrees C and 5.66 X 10(6) M-1 at 37 degrees C. The association was not affected by the presence of 20 mM lactose. Toxicity studies demonstrated that reduced ricin at a concentration where it was 52% associated had a toxicity equal to that of native ricin at that same concentration. At higher concentrations, reduced ricin was even more toxic than native ricin. Diethyl maleate, which reduces intracellular glutathione levels, blocked the toxicity of ricin but not the toxicity of reduced ricin. The disulfide bond linking the A and B subunits appears to play no role in toxicity other than to hold the two subunits together at low concentrations.