Minireview: enzymatic properties of ribosome-inactivating proteins (RIPs) and related toxins

Mono-PEGylation of Alpha-MMC and MAP30 from Momordica charantia L.: Production, Identification and Anti-Tumor Activity

PEGylation is a well-established and effective strategy to decrease immunogenicity, which can increase the stability and in vivo half-life time. However, the generation of multi-site modified products is inevitable due to the lysine chemistry, which will bring difficulties in subsequent research, such as purification and quantification. Site-specific modification by mPEG-succinimidyl carbonate (mPEG-SC) is a widely used method for N-terminal conjugation. In this study, we used it for site-directed modification on two ribosome-inactivating proteins (RIPs), alpha-momorcharin (α-MMC) and momordica anti-HIV protein (MAP30), from Momordica charantia L. According to the optimization of previous modification conditions, we compared Macro-Cap SP with SP-Sepharose FF chromatography for separating the final mPEGylated RIPs. Two kinds of methods both can obtain homogenous mPEGylated RIPs which were identified by sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE), isoelectric focusing electrophoresis (IEF), and matrix-assisted laser desorption ionization-time of flight/time of flight (MALDI-TOF/TOF) analysis. We also used iodine staining method to detect the amount of unmodified PEG. Furthermore, the inhibition activity of both mPEGylated and non-PEGylated RIPs against human lung adenocarcinoma epithelial A549 cells was detected. All of the results suggested that the mPEGylated α-MMC/MAP30 might be potentially developed as new anti-tumor drugs.

Alpha-momorcharin possessing high immunogenicity, immunotoxicity and hepatotoxicity in SD rats

Momordica charantia L., a genus of Momordica Linn. of the family Cucurbitaceae, commonly known as bitter melon, has been widely planted in China, Southeast Asia, Turkey and other areas, and has been used as a medicine for a long time. Alpha-momorcharin (α-MMC) extracted and purified from bitter melon seeds has significant anti-tumor and anti-virus effects, and has potential toxicity as well, especially when taken overdose. However, up to date studies on its safety evaluation are still insufficient.

AIMS OF THE STUDY

The immunogenicity, immunotoxicity and general toxicity of α-MMC were investigated in rats and guinea-pigs, and the potential toxic effects of the agent on the body were also examined.

MATERIALS AND METHODS

The major ribosome-inactivating protein was isolated by column chromatographies from the protein extracted from bitter melon seeds, and was verified as α-MMC. After rats were immunized by α-MMC, titers of specific antibody to α-MMC in immunized rats serum were detected by indirect ELISA. Guinea-pigs and rats immunized with α-MMC were used to evaluate the active systemic anaphylaxis and passive cutaneous anaphylaxis induced by α-MMC relatively. α-MMC of 6.25 mg/kg, 2.08 mg/kg and 0.70 mg/kg was administered to rats every 2 days. Five weeks later, animals were sacrificed, and then, biochemical examination, analysis of bone marrow and peripheral blood cells, and histopathologic examination were performed.

RESULTS

The ribosome-inactivating protein isolated and purified from bitter melon seeds was identified as α-MMC. It induced high titer (1:46.4) of specific IgG and high positive results of the active systemic anaphylaxis and passive cutaneous anaphylaxis tests in animals. With the time of the α-MMC administration increasing, the body weights of the animals administered with α-MMC of 6.25 mg/kg decreased significantly, and point necrosis was also observed in liver cells, along with abnormal findings in serum chemistry, hematology and bone marrow histopathology test. The toxic effect lessened with the decrease of the dose of α-MMC and further reduced after the convalescence stage.

CONCLUSIONS

The results of the study show that α-MMC has high immunogenicity and immunotoxicity, and can cause obvious organic liver lesion.

Mannosidase 2, alpha 1 deficiency is associated with ricin resistance in embryonic stem (ES) cells

Host gene products required for mediating the action of toxins are potential targets for reversing or controlling their pathogenic impact following exposure. To identify such targets libraries of insertional gene-trap mutations generated with a PiggyBac transposon in Blm-deficient embryonic stem cells were exposed to the plant toxin, ricin. Resistant clones were isolated and genetically characterised and one was found to be a homozygous mutant of the mannosidase 2, alpha 1 (Man2α1) locus with a matching defect in the homologous allele. The causality of the molecular lesion was confirmed by removal of the transposon following expression of PB-transposase. Comparative glycomic and lectin binding analysis of the Man2α1 (-/-) ricin resistant cells revealed an increase in the levels of hybrid glycan structures and a reduction in terminal β-galactose moieties, potential target receptors for ricin. Furthermore, naïve ES cells treated with inhibitors of the N-linked glycosylation pathway at the mannosidase 2, alpha 1 step exhibited either full or partial resistance to ricin. Therefore, we conclusively identified mannosidase 2, alpha 1 deficiency to be associated with ricin resistance.

Role of TYR70 in the N-glycosidase activity of neo-trichosanthin

Trichosanthin (TCS) is a type I ribosome-inactivating protein (RIP) which possesses rRNA N-glycosidase activity. TCS has long been used as an abortifacient in China. In recent years, its immunomodulatory, anti-tumor and anti-HIV properties have attracted more and more attention. An isoform of trichosanthin, neo-trichosanthin (n-TCS), has been cloned and expressed as recombinant protein. The biochemical studies revealed that n-TCS has virtually the same rRNA N-glycosidase activity as TCS. The crystal structure of n-TCS is similar to TCS. The crystal of Y70A n-TCS, the mutant of recombinant n-TCS, was soaked in sodium citrate buffer (pH 5.5) containing 25% KCl and AMP (10 mg/ml) prior to data collection. After structure determination and refinement, no electron density corresponding to adenine can be detected around the active pocket. Furthermore, the reaction products of Y70A n-TCS and AMP incubated at various reaction times were analyzed using HPLC. No adenine can be detected. These results suggest that Tyr70 is crucial to n-TCS for its substrate recognition, binding and perhaps N-glycosidase activity.

Ribosome-inactivating proteins (RIPs) of wild Oregon cucumber (Marah oreganus)

Two type 1 RIPs, designated as MOR-I and MOR-II, have been isolated from Marah oreganus (manroot) seed extract. They are similar but not identical to trichosanthin, a type 1 RIP in the same family. MOR-I and MOR-II are monomeric proteins with molecular weights of 27989.0 and 27632.8 respectively and have pI values greater than 8.8. MOR-I and MOR-II inhibit cell-free protein synthesis with IC50s of 0.063 and 0.071 nM, respectively, and are relatively stable with respect to temperature and pH variations. They share a conserved N-terminal amino acid sequence (D-SF-LS) and cross-react with goat anti-trichosanthin polyclonal serum.

Demonstration of ribonuclease activity in the plant ribosome-inactivating proteins alpha- and beta-momorcharins

Alpha- and beta-momorcharins, ribosome-inactivating proteins from Momordica charantia seeds, were utilized in this investigation. Ribonucleolytic cleavage was observed after naked rRNA was incubated with either momorcharin. Beta-momorcharin, and to a lesser extent alpha-momorcharin, also acted on tRNA to release acid-soluble UV-absorbing products. Such activity was optimal at pH around 5.5. Using polyhomoribonucleotides as substrate, it was found that the momorcharins preferentially acted on polyU, but exerted negligible effects on polyA, polyC and polyG. Chromatographic analysis of the reaction product indicated that mono and/or oligo-ribonucleotides, but not free base, were generated from polyU, suggesting that the enzymatic action involved ribonucleolytic cleavage. Similar to the results obtained with tRNA as substrate, beta-momorcharin was about 15-fold more active than alpha-momorcharin on polyU.

Modeling of the three-dimensional structure of luffin-alpha and its simulated reaction with the substrate oligoribonucleotide GAGA

A fundamental problem in biochemistry and molecular biology is understanding the spatial structure of macromolecules and then analyzing their functions. In this study, the three-dimensional structure of a ribosome-inactivating protein luffin-alpha was predicted using a neural network method and molecular dynamics simulation. A feedforward neural network with the backpropagation learning algorithm were trained on model class of homologous proteins including trichosanthin and alpha-momorcharin. The distance constraints for the C alpha atoms in the protein backbone were utilized to generate a folded crude conformation of luffin-alpha by model building and the steepest descent minimization approach. The crude conformation was refined by molecular dynamics techniques and a simulated annealing procedure. The interaction between luffin-alpha and its analogous substrate GAGA was also simulated to understand its action mechanism.

A highly efficient procedure for purifying the ribosome-inactivating proteins alpha- and beta-momorcharins from Momordica charantia seeds, N-terminal sequence comparison and establishment of their N-glycosidase activity

A new purification scheme, involving two successive ion exchange chromatographic steps on DEAE-cellulose and Mono-S FPLC, was developed for the isolation of the ribosome-inactivating proteins, alpha- and beta-momorcharins, from the Chinese herb Kuquazi (seeds of Momordica charantia). This simple and rapid procedure yielded 3.1 and 1.7 mg of alpha- and beta-momorcharins, respectively, from 2.5 g of decorticated seeds in only two days. The N-terminal amino acid sequence of beta-momorcharin was found to be DVNFDLSTATAKTYTKFIED. It differed from that of alpha-momorcharin (DVSFRLSGADPRSYGMFIKD) in 10 out of the 20 positions investigated. Like other ribosome-inactivating proteins, the purified momorcharins showed specific N-glycosidase activity at nanomolar concentrations, when rRNA from rabbit reticulocyte lysate was used as substrate. The N-glycosidase activity of both momorcharins was optimal at pH7, not inhibited by K+ and not appreciably affected by NH4+. The activity of alpha-momorcharin was not drastically altered by Mn2+ but (1-10mM) Mn2+ inhibited the activity of beta-momorcharin by about 40%.

A new approach in the synthesis of immunotoxins: ribosome inactivating protein noncovalently bound to monoclonal antibody

This study describes the synthesis of a new generation of immunotoxins made by a noncovalent interaction between a monoclonal antibody derivatized with a dichlorotriazinic dye and the ribosomal inhibitor protein gelonin. The scheme of preparation has several advantages with respect to the traditional methods, which used heterobifunctional cross-linkers, such as a higher overall yield of production and the homogeneity of the obtained conjugate. Moreover, because no chemical derivatization of the gelonin was required, the unconjugated ribosome inactivating protein was recovered unaltered and therefore can be reused in other synthetic processes. This immunoconjugate was stable when tested in mouse serum and showed an interesting slow elimination rate when administered intravenously in mice. Although a high dye derivatization degree induced a modification of the specificity of the monoclonal antibody, the native specificity was restored after conjugation with gelonin. Furthermore the noncovalent linkage did not affect the gelonin inhibitory activity; in fact, the specific cytotoxic activity seemed to be similar to that of other disulfide-linked immunotoxins previously prepared in our laboratories.

The N-glycosidase mechanism of ribosome-inactivating proteins implied by crystal structures of alpha-momorcharin

BACKGROUND

alpha-Momorcharin (alpha MMC) is a type I ribosome-inactivating protein. It inhibits protein synthesis by hydrolytically removing a specific adenine residue from a highly conserved, single-stranded loop of rRNA.

RESULTS

Here we describe the determination and refinement of the crystal structures of alpha MMC in the native state and in complexes with the product, adenine, and a substrate analogue, formycin 5'-monophosphate (FMP) at high resolution. Both adenine and the base of FMP are tightly bound; the ribose of bound FMP adopts a strained, high-energy conformation, which may mimic the structure of the transition state.

CONCLUSIONS

These structures indicate that residues Tyr70, Glu160 and Arg163 of alpha MMC are the most critical for catalysis. We propose that the strained conformation of the ribose in the target adenosine weakens the glycoside bond. Partial protonation mediated by Arg163 then facilitates N-glycoside bond cleavage, leading to the formation of an oxycarbonium ion intermediate which is stabilized by the negatively-charged Glu160. Tyr70 adopts subtly different conformations in the three structures implying that it may be important in substrate recognition and perhaps catalysis.

Cusativin, a new cytidine-specific ribonuclease accumulated in seeds of Cucumis sativus L

Dry seeds of Cucumis sativus L. were found to contain a heat-sensitive endoribonuclease of a novel type which we have named cusativin. It was purified to apparent electrophoretic homogeneity by chromatography through S-Sepharose Fast Flow, Sephadex G-75, CM-Sepharose, Superdex 75-FPLC (fast protein liquid chromatography) and Mono S-FPLC. It is a single unglycosylated polypeptide chain with an apparent molecular mass (M(r)) of 22900. Polyclonal anti-cusativin antibodies raised in rabbits only reacted with melonin, the translation inhibitor from Cucumis melo L. Functional, Western blot and enzyme-linked immunosorbent assay (ELISA) analyses indicated that cusativin is present in the coat and cotyledons of dry seeds, but not in embryonic axes. Cusativin is accumulated in maturing seeds. By contrast, after seed germination there is degradation of the cusativin present in cotyledons but not that present in the seed coat. The preference of cusativin for polynucleotide cleavage was poly(C) >> poly(A) acids, poly(U) and poly(G) being unaffected by cusativin. Under the denaturing conditions used for RNA sequencing, cusativin acted only on poly(C). Cusativin proved to be useful for RNA sequencing, in particular, complementing the data obtained with RNase CL3. Cusativin represents a new class of plant RNase and, as far as we are aware, is the first plant enzyme that shows cleavage specificity for cytidine under the denaturing conditions of RNA sequencing.

Distribution and properties of major ribosome-inactivating proteins (28 S rRNA N-glycosidases) of the plant Saponaria officinalis L. (Caryophyllaceae)

We have studied the distribution of the protein synthesis inhibitory activity in the tissues of Saponaria officinalis L. (Caryophyllaceae). Seven major saporins, ribosome-inactivating proteins, were purified to apparent homogeneity from leaves, roots and seeds using a new procedure of RIPs isolation including ion-exchange and hydrophobic chromatography. They all catalysed the depurination of rat liver ribosomes, which generate the Endo's diagnostic rRNA fragment upon treatment with acid aniline, thus indicating that A4324 from the 28S rRNA has been released (Endo et al. (1987) J. Biol. Chem. 262, 5908-5912). The molecular mass of saporins by SDS-PAGE ranged between 30.2 and 31.6 kDa and by gel-filtration between 27.5 and 30.1 kDa. Amino acid composition and amino-terminal amino acid sequence indicate that all saporins may be considered isoforms. Only two saporins present in roots were glycosylated (SO-R1 and SO-R3). All saporins are very active on cell-free translation systems derived from rabbit reticulocyte lysates, rat liver, Triticum aestivum L., Cucumis sativus L. and Vicia sativa L. However, they are poor inhibitors of an Escherichia coli translation system. They inhibit protein synthesis in HeLa, BeWo and NB 100 cells, HeLa cells being the most resistant. The enzymatic activity of at least one saporin isoform was dependent on magnesium concentration in the standard rat liver cell-free system.

Mutations dissociating the inhibitory activity of the pokeweed antiviral protein on eukaryote translation and Escherichia coli growth

The pokeweed antiviral protein is a ribosome inactivating protein acting on eukaryotic as well as on prokaryotic ribosomes thus is toxic for both cell types. Using the PCR technique to clone the PAP open reading frame, we characterized two cDNAs coding for proteins inhibiting eukaryotic translation process and which are not toxic for Escherichia coli, unlike the wild type protein. The sequence of the two cDNAs showed that the proteins contain only one and two point mutations. This result suggest that the wild type amino acids in the mutated positions participate in the prokaryotic ribosome recognition. These mutants might be useful for the construction of immunotoxins containing the pokeweed antiviral protein as toxin.

Rationale for the clinical use of immunotoxins: monoclonal antibodies conjugated to ribosome-inactivating proteins

The use of chemotherapeutic drugs in combination with bone marrow transplantation to treat cancer patients has markedly improved the disease-free survival and cure rate. Part of the tumor cells, however, can escape from therapy due to resistance. Tumor-specific delivery of toxins that do not interfere with conventional drugs and are not cell cycle dependent seems to be a reasonable approach to overcome this problem. Natural ribosome-inhibiting-proteins (RIPs) from plants, bacteria and fungi which are extremely toxic inhibitors of protein synthesis are isolated and coupled to monoclonal antibodies (MoAbs) and receptor-specific ligands, immunotoxins (ITs), to fulfil this purpose. ITs are very suitable to eliminate malignant cells in vitro and in vivo. RIPs contain two or three active sites: a binding site which can be absent in a part of the RIPs and can be replaced by the MoAb; a translocation site that facilitates transport into the cytosol after internalization, and a cytotoxic site that enzymatically inhibits protein synthesis. Binding site containing toxins induce strong nonspecific cytotoxicity when coupled to MoAbs. Recent developments in recombinant DNA techniques enable genetic elimination of the binding site to reduce nonspecific cytotoxicity of these toxins. In this review the structures and mechanisms of action of RIPs as well as factors that influence cytotoxicity of immunotoxins are discussed. Moreover the problems dealing with in vivo application of ITs such as blood clearance by instability of the IT and hepatic entrapment, and production of antibodies directed against MoAb and toxin are reviewed.

Deoxyribonucleolytic activity of alpha- and beta-momorcharins

alpha- and beta-Momorcharins were purified by an improved procedure using the affinity Affi-gel Blue gel and the ion exchange Mono-S FPLC column. Both purified alpha- and beta-momorcharins possessed deoxyribonucleolytic activity. Under normal digestion conditions, they cleaved the supercoiled, double-stranded SV-40 DNA to produce nicked circular and linear DNAs. Prolonged incubation did not have any further effects. On the other hand, the linear DNAs, lambda, Ad-2 and T7 were not digested by alpha- nor beta-momorcharin. Thus, it appears that the conformation of the DNA may be the determining factor for the deoxyribonucleolytic activity of these momorcharins.