A comparative study of ricin and diphtheria toxin-antibody-conjugate kinetics on protein synthesis inactivation

Kinetic data on toxin and antibody-toxin-conjugate inactivation of protein synthesis have been used to assess the variables which affect the transport of these toxins into the cytosol compartment. First-order inactivation rate constants of protein synthesis (ki) are compared under conditions of known receptor occupancy. The effect of inclusion of toxin B chains, both homologous and heterologous, in antibody-toxin conjugates is observed, and factors which affect toxin lag periods are studied. The results show that the inclusion of B chains in conjugates increases ki values 3-10-fold, but only if the B chain is homologous with the A chain. In spite of the augmentation of antibody-toxin-conjugate ki values by homologous toxin B chain, these ki values are only 1/20 those observed with unmodified toxins on sensitive cells. A further difference noted between toxins and antibody-toxin conjugates is the presence of a dose-dependent lag when toxins, but not antibody-toxin conjugates, effect sensitive cell types. This lag period for ricin can be shortened by alkalinizing the cell medium. The kinetic data can be fit by assuming a processing step interposed between the binding of ricin to surface receptors and the interaction of the A chain with ribosomes which is first-order in toxin concentration and pH-dependent. The time constant of this event is reflected in the dose-dependent lag period. It is proposed that antibody-toxin conjugates do not participate in this processing event and therefore fail to achieve the high entry levels exhibited by unmodified toxins.