Studies on the structure and properties of the lectins from Abrus precatorius and Ricinus communis

Protein biosynthetic capacity in the endosperm tissue of ripening castor bean seeds

Endosperm tissue was excised from Ricinus communis plants at different stages during seed maturation. The various stages were characterized on the basis of total RNA, protein and lipid content. Polyadenylated RNA was recovered from the total RNA by affinity chromatography on oligo(dT)cellulose. With the exception of that isolated from dry seeds, this poly(A(+)) RNA actively programmed protein synthesis in cell-free systems containing either wheat germ S30 extracts or nuclease treated rabbit reticulocyte lysates at each developmental stage examined. Translational products were separated electrophoretically and were visualized by fluorography. The capacity to synthesize protein was also estimated during 'in vivo' labelling studies. Developmental changes in the capacity of maturing endosperm tissue to synthesize a characteristic protein, R. communis agglutinin, were followed by immunoprecipitating this protein from the total 'in vitro' products synthesized at various stages. Endoplasmic reticulum membranes were isolated from maturing endosperm tissue by sucrose density gradient centrifugation. The role of the endoplasmic reticulum (ER) in protein glycosylation was indicated by (a) localizing the enzymes catalysing the incorporation of N-acetylglucosamine and mannose into mono- and oligosaccharide lipid and into glycoprotein, (b) localizing particulate (3)H-labelled glycoprotein amongst cellular fractions prepared from endosperm tissue which had been incubated with [(3)H]N-acetylglucosamine.

Internalization of ricin in Chinese hamster ovary cells

Internalization of ricin into Chinese hamster ovary cells has been investigated. Combined treatment with galactose and pronase at 0 degrees C resulted in a complete release of surface-bound [125I]ricin into the media. Galactose-pronase-resistant cell-bound [125I]ricin represents internalized ricin molecules inside the cells. The internalization process is time, temperature, and concentration dependent. The pH optimum of internalization of ricin is about pH 7. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis has revealed that intact ricin molecules are internalized. Neither reduction nor proteolytic processing of ricin is required for the entry of ricin into Chinese hamster ovary cells.

Enhanced internalization of ricin in nigericin-pretreated Chinese hamster ovary cells

Biochemical and electron microscopic autoradiographic studies with [125I] ricin have revealed that nigericin-pretreated Chinese hamster ovary cells are more efficient than untreated cells in the internalization of the toxin into the cells. These results suggest that the enhanced rate of internalization of ricin in nigericin-pretreated cells may account for the enhancement of cytotoxicity of ricin in Chinese hamster ovary cells by nigericin.

Enhancement of cytotoxicities of ricin and Pseudomonas toxin in Chinese hamster ovary cells by nigericin

Nigericin and monensin, ionophores for Na+ and K+, have been found to enhance the cytotoxicities of abrin, ricin, and Pseudomonas aeruginosa exotoxin A in Chinese hamster ovary (CHO) cells. They do not affect the cytotoxicity of diphtheria toxin in the same cell line. Maximal sensitization of the CHO cells toward ricin and Pseudomonas toxin requires preculture of CHO cells in the presence of nigericin. Inhibition of protein synthesis in CHO cells by ricin or Pseudomonas toxin is also enhanced by preculture of CHO cells in the presence of nigericin. These results suggest a common step in the intoxication process of ricin and Pseudomonas toxin, the rate of which is facilitated by pretreatment with nigericin. This step is, however, not shared by the intoxication of CHO cells with diphtheria toxin.

Epidermal growth factor-toxin A chain conjugates: EGF-ricin A is a potent toxin while EGF-diphtheria fragment A is nontoxic

We have prepared a 2-pyridyl-dithiopropionate derivative of epidermal growth factor (EGF) and conjugated the derivative by disulfide interchange to the A chain of ricin (RTA) or to fragment A of diphtheria toxin (DTA). The EGF-RTA conjugate was toxic to 3T3 cells at concentrations (10(-9)--10(-11) M) similar to those at which EGF exerts its biological activity and within an order of magnitude of the toxicity of ricin. Ricin A chain alone only exerted toxic effects at concentrations (10(-6)--10(-7) M) three to four orders of magnitude higher than required for the activity of the EGF-RTA conjugate or ricin. An unconjugated mixture of RTA and EGF had no greater effect than RTA alone. Toxicity of the EGF-RTA conjugate on 3T3 cells was competed by EGF and was blocked by antibodies to RTA, but not by lactose or antibodies to the ricin B chain (RTB). In contrast to the EGF-RTA conjugate, the EGF-DTA conjugate proved virtually nontoxic at concentrations as high as 3 X 10(-8) M. Control experiments showed that the EGF-DTA conjugate retained EGF receptor binding activity; the DTA moiety of the hybrid retained ADP-ribosyltransferase activity; and the disulfide bridge linking DTA to EGF was readily reducible.

Isolation and characterization of cyanogen bromide fragments of the A and B chains of the antitumor toxin ricin D

Both the A and B chains of ricin D consist of four CNBr fragments. These fragments have been isolated from the separated chains of S-carboxymethylated ricin D obtained from the small bean variety of Ricinus communis. The peptides have been characterized by molecular weight, amino acid composition and amino terminal sequence. A unique order of the peptides is evident for each chain. These sequence data are compared with those obtained for ricin D isolated from the large bean variety.

Physical studies on three lectins from the seeds of Abrus precatorius

The physical properties of three lectins from the seeds of the Abrus precatorius plant, abrin C, abrin A and the Abrus agglutinin, were studied. All three exhibited similar circular dichroic (CD) spectra in the near-ultraviolet having negative maxima at 286 and 293 nm. In addition, D-galactose induced similar conformational alterations in the three proteins as observed through changes in the near-ultraviolet CD from 280 to 295 nm. The near-ultraviolet CD spectrum of the toxic subunit of abrin C was very different from that of the parent molecule. The fluorescence emission spectra of the three proteins were also studied. All exhibited fluorescence near 335 nm which is quenched 9% by galactose. Iodide quenching of fluorescence using the Stern-Volmer analysis indicated different tryptophan accessibilities in the presence and absence of D-galactose for the Abrus agglutinin. The results suggest that there is a saccharide-induced conformational change which buries several partially exposed tryptophan residues. A comparable analysis of the closely related Ricinus agglutinin revealed that its tryptophan residues are more buried than those of the Abrus agglutinin and, unlike the Abrus agglutinin, there was no saccharide-induced change in tryptophan accessibility.

Effect of sulfhydryl reagents and protease inhibitors on sodium dodecyl sulfate-heat induced dissociation of Ricinus communis agglutinin

Ricinus communis agglutinin dissociated to lower molecular weight forms when heated in sodium dodecyl sulfate in the absence of reducing agents, while ricin was little affected by such treatment. The data suggest that strong noncovalent bonds hold together two A-B heterodimers in the Ricinus communis agglutinin tetramer. Protease inhibitors such as diisopropylfluorophosphate, phenylmethansefulonyl fluoride, and EDTA, did not prevent the sodium dodecyl sulfate-heat induced dissociation; however, sulfhydryl specific reagents (N-ethylmaleimide, 5,5'-dithiobis (2-nitrobenzoic acid) and p-chloromercuribenzoate) were effective. Titration of the lectins in sodium dodecyl sulfate indicated that ricin contains one sulfhydryl and Ricinus communis agglutinin four sulfhydryl groups, none of which react in the presence of 8 M urea. The sulfhydryl groups that could be titrated in the intact proteins in sodium dodecyl sulfate were on the A chains.

[Lectins, sugar-binding plant proteins]

Lectins are proteins which can bind to the surfaces of animal and human cells. They are widespread in the plant and animal kingdom. Though many lectins have been isolated and characterized, and though some are extensively used as cell surface "tools", their biological role is only subject to hypotheses.

Binding of ricin A chain to rat liver ribosomes: relationship to ribosome inactivation

Ricin A chain was radioactively labeled using reductive alkylation, lactoperoxidase catalyzed iodination, and reaction with iodoacetamide or N-ethylmaleimide (NEM). The inhibition of cell-free rat liver protein synthesis by the modified A chains and the ribosome binding characteristics of each of the labeled derivatives was examined. [3H] NEW was found to quantitatively react with the A chain sulfhydryl group normally involved in a disulfide bond with the B chain in intact ricin. Labeling the protein with [3H] NEM had no effect on the in vitro inhibition of protein synthesis by the A chain. [3H] NEM-labeled A chain binds to rat liver ribosomes in a manner which is dependent on the concentrations of NaCl and Mg2+. At optimal Mg2+ concentration (5.5 mM), A chain binding to ribosomes is saturable and fully reversible either by dilution of the reaction mixture or by addition of unlabeled A chain. At 5.5 mM Mg2+, A chain was found to bind to a single site on rat liver ribosomes with a dissociation constant of 6.2 x 10(-8) M. [3H] NEM-labeled A chain did not bind to isolated 40S ribosomal subunits and bound to 60S ribosomal subunits with a 1 : 1 molar stoichiometry and a dissociation constant of 2.2 x 10(-7) M. The relationship between ribosome binding and A chain inhibition of eucaryotic protein synthesis is discussed.

[Biology of lectins and their application in clinical biochemistry (author's transl)]

Lectins are proteins or glycoproteins, originally isolated from plant seeds. Characteristics are their ability to bind glycoproteins or glycolipids depending on the carbohydrate residues. The present review describes the structure of the lectins, their binding specificity and their functions with respect to precipitation of glycoproteins, agglutination of cells, transformation of lymphocytes and toxic action. Recently, lectin-analogs have been described in rabbit liver, which are responsible for hepatic uptake of circulating glycoproteins. The regulation of this process is intimately linked to the terminal N-Acetylneuraminic acid (NA-NA). Moreover, its significance is shown during fetal development, oncogenic transformation, immunologic recognition as well as homostasis. Due to the different terminal carbohydrate residues, glycoproteins of adult, fetal or transformed cells can be separated using affinity chromatography. Besides the purification of glycoproteins, lectins are also used for the separation of intact cells. Therefore the use of lectins is recommended for preparative and analytical methods, for the measurements of glycoprotein-turnover and for clinical diagnostics.

Toxicity, distribution and elimination of the cancerostatic lectins abrin and ricin after parenteral injection into mice

The survival time of mice after i.v. injection of the cancerostatic lectins, abrin and ricin was recorded. The LD50 dose was found to be 10-13 ng and 55-65 ng per mouse for abrin and ricin, respectively. Increasing amounts of toxin reduced the survival time, reaching a minimum of about 10 h. Lactose injected with ricin, provided partial protection against ricin, as measured by the survival time. Abrin and ricin labelled with 125I, and shown to retain their full toxic activity, were injected into mice. Most of the radioactivity found in the organs was present in the form of intact toxins, at least up to 5 h after injection. After i.v. injection the highest concentration/g tissue was found in spleen, followed by kidneys, heart, liver and thymus. The relative concentration in liver was considerably higher for ricin than for abrin. Similar results were found after i.p. injection. When lactose was administered together with ricin, almost 80% of the ricin injected was found in the liver after 30 min, compared to 48% without lactose, and the amount in other organs was concurrently reduced. The elimination of total radioactivity was much faster for ricin than abrin. The radioactivity found in the urine was largely present in non-trichloroacetic acid precipitable form, indicating that the toxins were extensively degraded before excretion.

Isolation and characterization of a hemagglutinin from Amphitrite ornata, a polychaetous annelid

Extracts of the marine polychaetous annelid, Amphitrite ornata, agglutinate rat, rabbit, chicken and human erythrocytes and in other work have been shown to inhibit the growth of Ehrlich ascites tumors in mice. Fractionation of extracts on Sephadex G-100 gave three active fractions with molecular weights of 30 000, 54 000 and 100 000. The 30 000 dalton fraction (B) was purified 72-fold by ammonium sulfate precipitation, gel filtration and preparative disc gel electrophoresis. The purified hemagglutinin, amphitritin, was homogenous on analytical disc gel electrophoresis at four different pH values and gave a sharp boundary in sedimentation velocity ultracentrifugation. The three fractions showed paralled specificity toward rat and chicken erythrocytes, the former giving the higher titer. The purified agglutinin was active toward human blood groups A, B and O and exhibited 4-fold higher activity toward group A. The hemagglutinin titer against rat red blood cells was lowered only by N-acetylgalactosamine, the terminal sugar residue of the group A determinant. None of the saccharides tested inhibited agglutination of chicken erythrocytes. Hemagglutinin activity was insensitive to dialysis or treatment with EDTA. The activity was not affected by digestion with trypsin or pronase, but was destroyed by phenol extraction. Analytical disc gel electrophoresis showed one protein band with high anodal mobility at pH 8.5, which was not affected by proteolytic enzymes but was removed by phenol. Activity was unaffected by heating at 70 degrees C for 30 min but was destroyed by similar treatemtn at 85 degrees C. Activity was at a maximum at pH 7-9 and decreased reversibly down to pH 4 at which point it was irreversibly inactivated. The higher molecular weight agglutinin (A1) could be dissociated to give amphitritin by treatment with 6M urea of precipitation in 55% (NH4)2SO4. This dissociation was not reversed by dialysis. Amphitritin is a glycoprotein with a molecular weight determined by gel filtration of 30 000 and by approach to equilibrium sedimentation of 32 000. Amino acid analysis showed a preponderance of aspartic and glutamic acids and relatively large amounts of glycine, proline, alanine, valine and cysteine. The carbohydrate moeity which represented 12.8% of the molecule, contained mannose, galactose, glucosamine and sialic acid. Amphitritin is the first hemagglutinin to be isolated from a polychaetous annelid.