THE VARIANTS OF THE PROTEIN TOXINS ABRIN AND RICIN - A USEFUL GUIDE TO UNDERSTANDING THE PROCESSING EVENTS IN THE TOXIN TRANSPORT

Kinetic data on inhibition of protein synthesis in thymocyte by three abrins and ricin have been obtained. The intrinsic efficiencies of A c hains of four toxins to inactivate ribosomes, as analyzed by k(i)-vers us-concentration plots were abrin II, III > ricin > abrin I. The lag t imes were 90, 66, 75 and 105 min at a 0.0744 nM concentration of each of abrin I, II, III and ficin, respectively. To account for the observ ed differences in the dose-dependent lag time, functional and structur al variables of toxins such as binding efficiency of B chains to recep tors and low-pH-induced structural alterations have been analyzed. The association constants obtained by stopped flow studies showed that ab rin-I (4.13 X 10(5) M-1 s-1) association with putative receptor (4-met hylumbelliferyl-alpha-D-galactoside) is nearly two times more often th an abrin III (2.6 X 10(5) M-1 s-1) at 20-degrees-C. Equilibrium bindin g constants of abrin I and II to thymocyte at 37-degrees-C were 2.26 X 10(7) M-1 and 2.8 X 107 M-1 respectively. pH-induced structural alter ations as studied by a parallel enhancement in 8-anilino-L-naphthalene sulfonate fluorescence revealed a high degree of qualitative similari ty. These results taken with a nearly identical concentration-independ ent lag time (minimum lag of 41-42 min) indicated that the binding eff iciencies and internalization efficiencies of these toxins are the sam e and that the observed difference in the dose-dependent lag time is c ausally related to the proposed processing event. The rates of reducti on of inter-subunit disulfide bond, an obligatory step in the intoxica tion process, have been measured and compared under a variety of condi tions. Intersubunit disulfide reduction of abrin I is fourfold faster than that of abrin II at pH 7.2. The rate of disulfide reduction in ab rin I could be decreased 11-fold by adding lactose, compared to that w ithout lactose. The observed differences in the efficiencies of A chai ns, the dose-dependent lag period, the modulating effect of lactose on the rates of disulfide reduction and similarity in binding properties make the variants a valuable tool to probe the processing events in t oxin transport in detail.