Mechanisms of Action of Ribotoxins

Comparative analysis of depurination catalyzed by ricin A-chain on synthetic 32mer and 25mer oligoribonucleotides mimicking the sarcin/ricin domain of the rat 28S rRNA and E. coli 23S rRNA

Ricin A-chain can inactivate eukaryotic ribosomes, but exhibits no N-glycosidase activity on intact E. coli ribosomes. In the present research, in order to avoid using radiolabeled oligoribonucleotides, two kinds of synthetic 5'-FAM fluorescence-labeled oligoribonucleotide substrates were used to mimic the sarcin/ricin domains of rat 28S rRNA and E. coli 23S rRNA (32mer and 25mer, named as Rat FAM-SRD and E. coli FAM-SRD, respectively). Ricin A-chain was able to specifically release adenine from the first adenosine of the GAGA tetraloop and exhibited specific N-glycosidase activity under neutral and weak acidic conditions with both substrates. However, under more acidic conditions, ricin A-chain was able to release purines from other sites on eukaryotic substrates, but it retained specific depurination activity on prokaryotic substrates. At pH 5.0, the Michaelis constant (K(m)) for the reaction with Rat FAM-SRD (4.57+/-0.28microM) corresponded to that with E. coli FAM-SRD (4.64+/-0.26microM). However, the maximum velocity (V(max)) for ricin A-chain with Rat FAM-SRD was 0.5+/-0.024microM/min, which is higher than that with E. coli FAM-SRD (0.32+/-0.011microM/min).

High-performance liquid chromatography–mass selective detection assay for adenine released from a synthetic RNA substrate by ricin A chain

High-performance liquid chromatography (HPLC) and selected ion monitoring mass spectrometry (MS) were used to develop a quantitative assay for adenine released from a synthetic RNA substrate by ricin A chain, which contains the toxin's N-glycosidase activity. Because ricin and ricin A chain have potential applications as biotherapeutics and bioweapons, assays are needed to evaluate potency and potential inhibitors of activity. The detection limit for adenine was 0.02 microM (2.4 ng/ml), and the standard curve was linear up to 27.3 microM. The lower limit of quantitation was 0.27 microM and was reproducible throughout this range. Reaction characterization showed that most adenine was released by 5h and that the reaction could not be fully stopped with formic acid concentrations up to 0.75 mM (the maximum typically used for HPLC-MS). Injections were made at 2-min intervals, 10 injections could be performed before the column was backflushed, and no ricin A chain was observed in the column effluent. This assay would also be useful for ricin since ricin A chain did not pass through the HPLC column. With minor modifications to this system, the assay should provide rapid, sensitive, selective, and quantitative assessment of the activity of most ribosome-inactivating proteins. In addition, further chromatographic and mass spectrometric improvements could reduce sample requirements and analysis times.