Ribosome inactivation by the toxic lectins abrin and ricin. Kinetics of the enzymic activity of the toxin A-chains.
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.
D000036:Abrin / Q000494:pharmacology*; D000818:Animals; D007700:Kinetics; D037102:Lectins / Q000494:pharmacology; D010441:Peptide Chain Elongation, Translational / Q000187:drug effects; D010445:Peptide Elongation Factors; D037121:Plant Lectins; D010940:Plant Proteins / Q000494:pharmacology*; D010947:Plants, Toxic; D014176:Protein Biosynthesis / Q000187:drug effects; D011817:Rabbits; D012156:Reticulocytes / Q000378:metabolism; D012270:Ribosomes / Q000187:drug effects / Q000378:metabolism*; D012276:Ricin / Q000494:pharmacology*; D012278:Ricinus
D037102:Lectins; D010445:Peptide Elongation Factors; D037121:Plant Lectins; D010940:Plant Proteins; D000036:Abrin; D012276:Ricin