title: The variants of the protein toxins abrin and ricin. A useful guide to understanding the processing events in the toxin transport.
abstract: Kinetic data on inhibition of protein synthesis in thymocyte by three abrins and ricin have been obtained. The intrinsic efficiencies of A chains of four toxins to inactivate ribosomes, as analyzed by ki-versus-concentration plots were abrin II, III > ricin > abrin I. The lag times were 90, 66, 75 and 105 min at a 0.0744 nM concentration of each of abrin I, II, III and ricin, respectively. To account for the observed differences in the dose-dependent lag time, functional and structural variables of toxins such as binding efficiency of B chains to receptors and low-pH-induced structural alterations have been analyzed. The association constants obtained by stopped flow studies showed that abrin-I (4.13 x 10(5) M-1 s-1) association with putative receptor (4-methylumbelliferyl-alpha-D-galactoside) is nearly two times more often than abrin III (2.6 x 10(5) M-1 s-1) at 20 degrees C. Equilibrium binding 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 alterations as studied by a parallel enhancement in 8-anilino-L-naphthalene sulfonate fluorescence revealed a high degree of qualitative similarity. These results taken with a nearly identical concentration-independent lag time (minimum lag of 41-42 min) indicated that the binding efficiencies and internalization efficiencies of these toxins are the same and that the observed difference in the dose-dependent lag time is causally related to the proposed processing event. The rates of reduction of inter-subunit disulfide bond, an obligatory step in the intoxication process, have been measured and compared under a variety of conditions. Intersubunit disulfide reduction of abrin I is fourfold faster than that of abrin II at pH 7.2. The rate of disulfide reduction in abrin I could be decreased 11-fold by adding lactose, compared to that without lactose. The observed differences in the efficiencies of A chains, 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 toxin transport in detail.
authors.affiliation: Department of Biochemistry, Indian Institute of Science, Bangalore.
authors.forename: R
authors.initials: R
authors.lastname: Hegde
authors.forename: A A
authors.initials: AA
authors.lastname: Karande
authors.forename: S K
authors.initials: SK
authors.lastname: Podder
mesh_terms: D000036:Abrin / Q000378:metabolism* / Q000633:toxicity; D000818:Animals; D001692:Biological Transport; D002846:Chromatography, Affinity; D004220:Disulfides / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010084:Oxidation-Reduction; D014176:Protein Biosynthesis; D051381:Rats; D011975:Receptors, Mitogen / Q000378:metabolism; D012276:Ricin / Q000378:metabolism* / Q000633:toxicity; D013050:Spectrometry, Fluorescence; D013601:T-Lymphocytes / Q000378:metabolism*
chemical_list: D004220:Disulfides; D011975:Receptors, Mitogen; C035488:abrin receptor; D000036:Abrin; D012276:Ricin