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  • Title: Ribosome inactivation by the toxic lectins abrin and ricin. Kinetics of the enzymic activity of the toxin A-chains.
    Author: Olsnes S, Fernandez-Puentes C, Carrasco L, Vazquez D.
    Journal: Eur J Biochem; 1975 Dec 01; 60(1):281-8. PubMed ID: 1204642.
    Abstract:
    A sensitive test system for toxin-treated ribosomes was worked out by treating rabbit reticulocyte ribosomes with abrin A-chain, ricin A-chain or ricinus agglutinin A-chain, adding neutralizing amounts of specific antitoxins and testing for polyphenylalanine-synthesizing activity in a system where the concentration of elongation factors and ribosomes were varied. The strongest inhibition was obtained in the presence of low concentrations of elongation factor (EF-2). The activity of the ribosomes decreased with time of incubation with the toxin A-chains. Addition of anti-toxins stopped further inactivation. In systems containing untreated and toxin-treated ribosomes the ability to polymerize phenylalanine was proportional to the concentration of untreated ribosomes. There was a linear relationship between toxin A-chain concentration and the number of ribosomes inactivated per minute. The inactivation rate increased with temperature, and the estimated activation energy was 10.6 kcal (44.3 kJ). Linewaver-Burk plots of the data obtained by incubating various ribosome concentrations with toxins indicated a molecular activity of about 1500 ribosomes/minute for abrin and ricin A-chains and 100 ribosomes/minute for ricinus agglutinin A-chain. The apparent Michaelis constant was 0.1-0.2 muM for all three A-chains. The activity of the A-chains in the intact cell is discussed.
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