A New Method for Extraction and Analysis of Ricin Samples through MALDI-TOF-MS/MS

Theoretical Investigation of Repurposed Drugs Potentially Capable of Binding to the Catalytic Site and the Secondary Binding Pocket of Subunit A of Ricin

Recently, we reported a library of 82 compounds, selected from different databanks through virtual screening and docking studies, and pointed to 6 among them as potential repurposed dual binders to both the catalytic site and the secondary binding pockets of subunit A of ricin (RTA). Here, we report additional molecular modeling studies of an extended list of compounds from the original library. Rounds of flexible docking followed by molecular dynamics simulations and further rounds of MM-PBSA calculations using a more robust protocol, enabled a better investigation of the interactions of these compounds inside RTA, the elucidation of their dynamical behaviors, and updating the list of the most important residues for the ligand binding. Four compounds were pointed as potential repurposed ricin inhibitors that are worth being experimentally investigated.

Highly sensitive MALDI-MS measurement of active ricin: insight from more potential deoxynucleobase-hybrid oligonucleotide substrates

Herein, we report an improved MALDI-MS method for active ricin to contribute toward countermeasures against its real threat to the public. Compared with commonly used DNA or RNA substrates, the deoxynucleobase-hybrid oligonucleotide (RNA_dA, Rd) substrate containing functional Gd[combining low line]A[combining low line]GA loop was revealed as a substrate with more potential and used for the first time in ricin measurement via MALDI-MS. The Rd sequence greatly prompted ricin to exhibit its catalytic activity as rRNA N-glycosylase in ex vitro condition, which was supported by molecular docking simulation and enzymatic parameters depicted in MALDI-MS. Furthermore, we discovered that a highly pure matrix was the most crucial parameter for enhancing the sensitivity, which addressed the major obstacle encountered in the oligo(deoxy)nucleotide measurement, i.e., the interfering alkali metal ion-adducted signals in MALDI-MS. After the optimization of pH and enzymatic reaction buffer composition in this ex vitro condition, this method can provide a wide linearity of up to three orders of magnitude, i.e., 1-5000 ng mL-1, and a high sensitivity of 1 ng mL-1 without any enrichment. Denatured and active ricin could be distinctly differentiated, and the application to practical samples from one international exercise and a soft drink proved the feasibility of this new method. We believe this MALDI-MS method can contribute to the first response to ricin occurrence events in public safety and security, as well as pave a new way for a deep understanding of ricin and other type II ribosome inactivating proteins involved toxicology.

[Highly toxic type Ⅱ ribosome-inactivating proteins ricin and abrin and their detection methods: a review]

Type Ⅱ ribosome-inactivating proteins (RIPs) are an important class of protein toxins that consist of A and B chains linked by an interchain disulfide bond. The B-chain with lectin-like activity is responsible for binding to the galactose-containing receptors on eukaryotic cell surfaces, which is essential for A-chain internalization by endocytosis. The A-chain has N-glycosidase activity that irreversibly depurinates a specific adenine from 28S ribosomal RNA (28S rRNA) and terminates protein synthesis. The synergistic effect of the A-B chain inactivates the ribosome, inhibits protein synthesis, and exhibits high cytotoxicity. Ricin and abrin that are expressed by the plants Ricinus communis and Abrus precatorius, respectively, are typical type Ⅱ RIPs. The toxicity of ricin and abrin are 385 times and 2885 times, respectively, more that of the nerve agent VX. Owing to their ease of preparation, wide availability, and potential use as a bioterrorism agent, type Ⅱ RIPs have garnered increasing attention in recent years. Ricin is listed as a prohibited substance under schedule 1A of the Chemical Weapons Convention (CWC). The occurrence of ricin-related bioterrorism incidents in recent years has promoted the development of accurate, sensitive, and rapid detection and identification technology for type Ⅱ RIPs. Significant progress has been made in the study of toxicity mechanisms and detection methods of type Ⅱ RIPs, which primarily involve qualitative and quantitative analysis methods including immunological assays, mass spectrometry analysis methods, and toxin activity detection methods based on depurination and cytotoxicity. Immunoassays generally involve the specific recognition of antigens and antibodies, which is based on oligonucleotide molecular recognition elements called aptamers. These methods are fast and highly sensitive, but for highly homologous proteins in complex samples, they provide false positive results. With the rapid development of biological mass spectrometry detection technology, techniques such as electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are widely used in the identification of proteins. These methods not only provide accurate information on molecular weight and structure of proteins, but also demonstrate accurate quantification. Enzyme digestion combined with mass spectrometry is the predominantly used detection method. Accurate identification of protein toxins can be achieved by fingerprint analysis of enzymatically digested peptides. For analysis of protein toxins in complex samples, abundant peptide markers are obtained using a multi-enzyme digestion strategy. Targeted mass spectrometry analysis of peptide markers is used to obtain accurate qualitative and quantitative information, which effectively improves the accuracy and sensitivity of the identification of type Ⅱ RIP toxins. Although immunoassay and mass spectrometry detection methods can provide accurate identification of type Ⅱ RIPs, they cannot determine whether the toxins will retain potency. The widely used detection methods for activity analysis of type Ⅱ RIPs include depurination assay based on N-glycosidase activity and cytotoxicity assay. Both the methods provide simple, rapid, and sensitive analysis of type Ⅱ RIP toxicity, and complement other detection methods. Owing to the importance of type Ⅱ RIP toxins, the Organization for the Prohibition of Chemical Weapons (OPCW) has proposed clear technical requirements for the identification and analysis of relevant samples. We herein reviewed the structural characteristics, mechanism of action, and the development and application of type Ⅱ RIP detection methods; nearly 70 studies on type Ⅱ RIP toxins and their detection methods have been cited. In addition to the technical requirements of OPCW for the unambiguous identification of biotoxins, the trend of future development of type Ⅱ RIP-based detection technology has been explored.

Development and Evaluation of an Immuno-MALDI-TOF Mass Spectrometry Approach for Quantification of the Abrin Toxin in Complex Food Matrices

The toxin abrin found in the seeds of Abrus precatorius has attracted much attention regarding criminal and terroristic misuse over the past decade. Progress in analytical methods for a rapid and unambiguous identification of low abrin concentrations in complex matrices is essential. Here, we report on the development and evaluation of a MALDI-TOF mass spectrometry approach for the fast, sensitive and robust abrin isolectin identification, differentiation and quantification in complex food matrices. The method combines immunoaffinity-enrichment with specific abrin antibodies, accelerated trypsin digestion and the subsequent MALDI-TOF analysis of abrin peptides using labeled peptides for quantification purposes. Following the optimization of the workflow, common and isoform-specific peptides were detected resulting in a ~38% sequence coverage of abrin when testing ng-amounts of the toxin. The lower limit of detection was established at 40 ng/mL in milk and apple juice. Isotope-labeled versions of abundant peptides with high ionization efficiency were added. The quantitative evaluation demonstrated an assay variability at or below 22% with a linear range up to 800 ng/mL. MALDI-TOF mass spectrometry allows for a simple and fast (<5 min) analysis of abrin peptides, without a time-consuming peptide chromatographic separation, thus constituting a relevant alternative to liquid chromatography-tandem mass spectrometry.

Identification of novel potential ricin inhibitors by virtual screening, molecular docking, molecular dynamics and MM-PBSA calculations: a drug repurposing approach

Ricin is a potent cytotoxin with no available antidote. Its catalytic subunit, RTA, damages the ribosomal RNA (rRNA) of eukaryotic cells, preventing protein synthesis and eventually leading to cell death. The combination between easiness of obtention and high toxicity turns ricin into a potential weapon for terrorist attacks, urging the need of discovering effective antidotes. On this context, we used computational techniques, in order to identify potential ricin inhibitors among approved drugs. Two libraries were screened by two different docking algorithms, followed by molecular dynamics simulations and MM-PBSA calculations in order to corroborate the docking results. Three drugs were identified as potential ricin inhibitors: deferoxamine, leucovorin and plazomicin. Our calculations showed that these compounds were able to, simultaneously, form hydrogen bonds with residues of the catalytic site and the secondary binding site of RTA, qualifying as potential antidotes against intoxication by ricin.Communicated by Ramaswamy H. Sarma.

Quantitative detection of ricin in beverages using trypsin/Glu-C tandem digestion coupled with ultra-high-pressure liquid chromatography-tandem mass spectrometry

The toxic protein of ricin has drawn wide attention in recent years as a potential bioterrorism agent due to its high toxicity and wide availability. For the verification of the potential anti-terrorism activities, it is urgent for the quantification of ricin in food-related matrices. Here, a novel strategy of trypsin/Glu-C tandem digestion was introduced for quantitative detection of ricin marker peptides in several beverage matrices using isotope-labeled internal standard (IS)-mass spectrometry. The ricin in beverages was captured and enriched by biotinylated anti-ricin polyclonal antibodies conjugated to streptavidin magnetic beads. The purified ricin was cleaved using the developed trypsin/Glu-C tandem digestion method and then quantitatively detected by ultra-high-pressure liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) with isotope-labeled T7A and TG11B selected as IS. The use of trypsin/Glu-C digestion allows shorter peptides, which are more suitable for MS detection, to be obtained than the use of single trypsin digestion. Under the optimized tandem digestion condition, except for T7A in the A-chain, two resulting specific peptides of TG13A, TG28A from the A-chain and two of TG11B, TG33B from the B-chain were chosen as novel marker peptides with high MS response. The uniqueness of the selected marker peptides allows for unambiguous identification of ricin among its homologous proteins in a single run. The MS response of the four novel marker peptides is increased by more than 10 times compared with that of individual corresponding tryptic peptides. Both the marker peptides of A-chain T7A and B-chain TG11B were selected as quantitative peptides based on the highest MS response among the marker peptides from their individual chains. The limit of detection (LOD) of ricin is 0.1 ng/mL in PBS and 0.5 ng/mL in either milk or orange juice. The linear range of calibration curves for ricin were 0.5-300 ng/mL in PBS, 1.0-400 ng/mL in milk, and 1.0-250 ng/mL in orange juice. The method accuracy ranged between 82.6 and 101.8% for PBS, 88.9-105.2% for milk, and 95.3-118.7% for orange juice. The intra-day and inter-day precision had relative standard deviations (%RSD) of 0.3-9.4%, 0.7-8.9%, and 0.2-6.9% in the three matrices respectively. Furthermore, whether T7A or TG11B is used as a quantitative peptide, the quantitative results of ricin are consistent. This study provides not only a practical method for the absolute quantification of ricin in beverage matrices but also a new strategy for the investigation of illegal use of ricin in chemical weapon verification tasks such as OPCW biotoxin sample analysis exercises.

Introduction to the Toxins Special Issue "Ricin Toxins"

Ricin toxin isolated from the castor bean (Ricinus communis) is one of the most potent and lethal molecules known [...].