Mutational and structural analysis of the lectin activity in binding domain 2 of ricin B chain

The study of the lectin binding sites of ricin B chain and of other homologous members of the small gene family that make up ricin-like molecules has revealed a number of key contact residues involved in sugar binding. In particular, on the basis of data generated by the X-ray crystallographic structure of ricin, comparisons of sequence homologies to other ricin-like molecules and substrate binding studies with these molecules, it has been proposed that His248 of Ricinus communis agglutinin (RCA) B chain may interfere with galactose binding in the second binding domain of that lectin. To test that hypothesis, single binding domain 2 (SBD2) of ricin B chain was expressed as a gene 3 fusion protein on the surface of fd phage to measure directly the effect of mutational changes on this binding site. Replacement of tyrosine with histidine at amino acid position 248 of SBD2 of ricin B chain was shown to reduce lectin activity. The sequences of RCA and ricin B chains were aligned and compared with the tertiary structure of ricin B chain to select various mutations that were introduced as controls in the study. One of these controls, Leu247 to Val247, displayed increased affinity for galactosides. The role of sequence changes is discussed in relation to the structural and functional divergence in these molecules.

Humanization of murine monoclonal antibodies through variable domain resurfacing

Two murine monoclonal antibodies, N901 (anti-CD56) and anti-B4 (anti-CD19), were humanized by a process we call "resurfacing." A systematic analysis of known antibody structures has been used to determine the relative solvent accessibility distributions of amino acid residues in murine and human antibody variable (Fv) regions and has shown that the sequence alignment positions of surface amino acids for human and murine variable region heavy (VH) and light (VL) chains are conserved with 98% fidelity across species. While the amino acid usage at these surface positions creates surface residue patterns that are conserved within species, there are no identical patterns across species. However, surprisingly few amino acid changes need to be made to convert a murine Fv surface pattern to that characteristic of a human surface. Resurfacing was used to change the patterns of surface accessible residues in the Fv regions of the N901 and anti-B4 antibodies to resemble those found on the Fv regions of human antibody sequences. Two different procedures for selecting a human sequence were compared. For anti-B4, a data base of clonally derived human VL-VH sequence pairs was used, while for N901, sequences for VL and VH were independently selected from the Kabat et al. data base [Kabat, E. A., Wu, T. T., Reid-Miller, M., Perry, H. M. & Gottesman, K. S. (1991) Sequences of Proteins of Immunological Interest (DHHS, Washington, DC), 5th Ed.]. Resurfaced N901 and anti-B4 antibodies had apparent affinities for their cell surface ligands that were identical to those of their respective parent murine antibodies. These data provide evidence that, despite the differences in the surfaces of mouse and human Fv regions, it is possible to substitute one for the other while retaining full antigen binding affinity.

Anti-CD6-blocked ricin: an anti-pan T-cell immunotoxin

We report the development of a potent anti-pan T-cell immunotoxin capable of killing cells in an antigen dependent manner. The immunotoxin is composed of a high affinity anti-CD6 antibody (IgG2a, Kd = 1.3 x 10(-11) M) conjugated to blocked ricin that is a chemically modified ricin molecule wherein the lectin binding sites of the B-chain have been blocked by covalent attachment of affinity ligands. Conjugation of blocked ricin to the antibody has minimal effect on the apparent affinity of the antibody and no effect on the ribosome-inactivating activity of the ricin A-chain moiety. Anti-CD6-blocked ricin is a specific and highly toxic immunoconjugate killing the antigen-positive Molt-4 cell line with an IC37 of 4 x 10(-12) M after a 24 h exposure of cells to the immunotoxin. Nonspecific cytotoxicity of anti-CD6-blocked ricin for the antigen-negative Namalwa cell line was more than 750-fold lower with an IC37 > 3 x 10(-9) M. The cytotoxicity of anti-CD6-blocked ricin is dependent on the length of the incubation of cells with the conjugate ranging from an IC37 of 1.5 x 10(-11) M leaving a surviving fraction of Molt-4 cells of 0.03 after a 2.5 h exposure to an IC37 of 5 x 10(-13) M and leaving a surviving fraction of 3 x 10(-6) after a continuous (3 weeks) exposure. Anti-CD6-blocked ricin is also capable of killing CD6 positive cells in human peripheral blood lymphocyte populations. Systemic toxicity of anti-CD6-blocked ricin in mice is similar to the toxicity of other immunotoxins containing blocked ricin that were found to be tolerated well by patients. An application of this immunoconjugate for the prevention and treatment of graft versus host disease or tissue graft rejection is suggested.

Preparation and characterization of interleukin-2-gelonin conjugates made using different cross-linking reagents

Conjugates of IL-2 with the ribosome-inactivating protein gelonin were prepared using heterobifunctional reagents to link the proteins via disulfide, acid-labile, and noncleavable linkers. In each case, one protein was modified using 2-iminothiolane. The sulfhydryl groups so introduced were then reacted either with 2-nitro-5-dithiobenzoate groups or with iodoacetamido groups which had been introduced into the second protein. In the case of the acid-labile linkage, a reagent which forms a labile bond upon reaction with amino groups, 4-(iodoacetamido)-1-cyclohexene-1,2-dicarboxylic acid anhydride (its synthesis is described in this paper) was used to modify the toxin. The conjugates were separated from nonconjugated proteins by gel filtration on Sephadex G100 (SF). Each was analyzed with respect to its ribosome-inactivating activity, its ability to bind to the IL-2 receptor, and its in vitro cytotoxicity. The ribosome-inactivating activity of gelonin was unaffected by modification with 2-iminothiolane and was retained in conjugates prepared using this reagent. Modification of the toxin with 4-(iodoacetamido)-1-cyclohexene-1,2-dicarboxylic acid anhydride to form the acid-labile link drastically reduced the activity of the toxin. However, the activity of the toxin was recovered following acid treatment to release the native protein. Conjugates containing each type of linkage exhibited both specific binding and selective cytotoxicity toward cells expressing the IL-2 receptor. The most potent of these toxins, that containing the disulfide linkage, exhibited a cytotoxicity which was 2 orders of magnitude greater than that of unconjugated gelonin.

Evaluation of the systemic toxicity and pharmacokinetics of the immunoconjugate anti-B4-blocked ricin in non-human primates. Delivered by multiple bolus injections and by continuous infusion

The systemic toxicity of an immunoconjugate of blocked ricin and the anti-CD19 monoclonal antibody, anti-B4, was studied in cynomolgus monkeys to evaluate its safety for use in humans. Anti-B4-blocked Ricin (Anti-B4-bR) is a highly cytotoxic immunoconjugate which can kill up to 5 logs of antigen positive target cells at concentrations easily achievable in blood. Subacute toxicity studies with Anti-B4-bR were performed in 20 cynomolgus monkeys and 4 rhesus monkeys, which, unlike humans, do not express the CD19 epitope recognized by the anti-B4 antibody on their B-lymphocytes. Anti-B4-bR was administered to cynomolgus monkeys by 5 daily intravenous bolus injections of 10 or 100 micrograms/kg/day, and non-conjugated blocked ricin was administered by 5 daily intravenous bolus injections of 30 micrograms/kg/day. Total doses of the conjugate of 200, 500, 1000 or 1500 micrograms/kg were also delivered to rhesus monkeys by continuous intravenous infusion over seven days. The clinical signs of toxicity, clinical pathology parameters, and gross and microscopic tissue changes associated with Anti-B4-bR were minimal to moderate where present, and primarily hepatic. In monkeys treated with 5 x 10 micrograms/kg of Anti-B4-bR, lesions were noticeable on day 7 after the start of the treatment but were less severe or absent on day 14, suggesting that the toxic effects were reversible. Clearance of the conjugate from the serum after bolus injections of Anti-B4-bR was evaluated by ELISA and demonstrated an initial t 1/2(alpha) of 1.4-2.0 h and a secondary t 1/2(beta) of about 14 h. Serum concentrations of Anti-B4-bR were about 10-20-fold lower at 24 h as compared to 1 h after each of the 5 bolus injections in monkeys. Continuous infusion of Anti-B4-bR in primates achieved plateau levels of the immunotoxin in blood for almost the entire duration of the infusion. The therapeutic utility of the Anti-B4-bR is currently being evaluated in patients with B-cell malignancies.

Adjuvant immunotoxin therapy with anti-B4-blocked ricin after autologous bone marrow transplantation for patients with B-cell non-Hodgkin's lymphoma

Anti-B-blocked ricin (anti-B4-bR) combines the specificity of the anti-B4 (CD19) monoclonal antibody with the protein toxin "blocked ricin." In blocked ricin, affinity ligands are attached to the ricin B-chain to attenuate its lectin binding capacity. In a phase I trial, Anti-B4-bR was administered by 7-day continuous infusion to 12 patients in complete remission after autologous bone marrow transplantation (ABMT) for relapsed B-cell non-Hodgkin's lymphoma (NHL). Patients were treated at 20, 40, and 50 micrograms/kg/d for 7 days. Potentially therapeutic serum levels could be sustained for 3 to 4 days. The maximum tolerated dose was 40 micrograms/kg/d for 7 days (total 280 micrograms/kg). The dose-limiting toxicities were reversible grade IV thrombocytopenia and elevation of hepatic transaminases. Mild capillary leak syndrome was manifested by hypoalbuminemia, peripheral edema (4 patients), and dyspnea (1 patient). Anti-immunotoxin antibodies developed in 7 patients. Eleven patients remain in complete remission between 13 and 26 months post-ABMT (median 17 months). These results show that Anti-B4-bR can be administered with tolerable, reversible toxicities to patients with B-cell NHL in complete remission following ABMT.

Anti-B4-blocked ricin: a phase I trial of 7-day continuous infusion in patients with B-cell neoplasms

PURPOSE

This phase I trial was undertaken to determine the maximum-tolerated dose (MTD) and dose-limiting toxicities (DLTs) of the B-cell-restricted immunotoxin anti-B4-blocked ricin (anti-B4-bR) when it is administered by 7-day continuous infusion.

PATIENTS AND METHODS

Thirty-four patients with relapsed and refractory B-cell neoplasms (26 non-Hodgkin's lymphoma [NHL], four chronic lymphocytic leukemia [CLL], four acute lymphoblastic leukemia [ALL]) received 7-day continuous infusion anti-B4-bR. Successive cohorts of at least three patients were treated at doses of 10 to 70 micrograms/kg/d for 7 days with the dose increased by 10 micrograms/kg/d for each cohort. The initial three cohorts of patients (10, 20, and 30 micrograms/kg/d x 7 days) also received a bolus infusion of 20 micrograms/kg before beginning the continuous infusion.

RESULTS

The MTD was reached at 50 micrograms/kg/d x 7 days. The DLTs were National Cancer Institute Common Toxicity Criteria (NCI CTC) grade IV reversible increases in AST and ALT, and grade IV decreases in platelet counts. Adverse reactions included fevers, nausea, headaches, myalgias, hypoalbuminemia, dyspnea, edema, and capillary leak syndrome. Potentially therapeutic serum levels of anti-B4-bR could be sustained for 4 days in patients treated at the MTD. Two complete responses (CRs), three partial responses (PRs), and 11 transient responses (TRs) were observed.

CONCLUSION

Anti-B4-bR can be administered safely by 7-day continuous infusion with tolerable, reversible toxicities to patients with relapsed B-cell neoplasms. Although occasional responses were seen, future trials will use anti-B4-bR in patients with lower tumor burdens to circumvent the obstacle of immunotoxin delivery to bulk disease.

Correlation between in vivo toxicity and preclinical in vitro parameters for the immunotoxin anti-B4-blocked ricin

Anti-B4-blocked ricin (Anti-B4-bR) is an immunotoxin comprised of the anti-B4 monoclonal antibody and the protein toxin, "blocked ricin." In blocked ricin, the galactose-binding sites of the ricin B-chain which mediate nonspecific binding to cells are blocked by covalently linked affinity ligands prepared from N-linked oligosaccharides of fetuin. Blocked ricin consists of two species, one with two covalently attached ligands and one with three covalently attached ligands. In a Phase I dose escalation clinical trial, Anti-B4-bR was administered to patients with relapsed and refractory B-cell neoplasms by 7-day continuous infusion. Although several different lots of Anti-B4-bR had similar IC37 values as determined by in vitro cytotoxicity testing on cultured human cell lines, these lots differed in their in vivo toxicity when administered to patients. Thus, IC37 values alone were not sufficient to predict in vivo toxicity. We report that the degree of cell kill at concentrations of drug that saturate the B4 antigen and murine 50% lethal dose values provide additional parameters that may be predictive of in vivo cytotoxicity. Furthermore, we performed detailed cytotoxicity studies of the ricin species containing two and three covalently attached ligands, respectively. In vitro cytotoxicity testing using these samples revealed that Anti-B4-bR made with blocked ricin containing two covalently attached ligands is capable of depleting five logs of target cells in an in vitro cytotoxicity assay, while Anti-B4-bR comprised of blocked ricin with three ligands can deplete only one log of cells. Log cell kill at antigen saturating concentration, murine 50% lethal dose and biochemical analysis of the composition of blocked ricin are therefore important considerations for establishing the potential efficacy and safety of Anti-B4-bR.

Effects of anti-CD33 blocked ricin immunotoxin on the capacity of CD34+ human marrow cells to establish in vitro hematopoiesis in long-term marrow cultures

Human marrow cells that express the CD34 antigen but lack CD33 are able to initiate sustained, multilineage in vitro hematopoiesis in long-term Dexter cultures and are believed to include the primitive stem cells responsible for effecting long-term hematopoietic reconstitution in vivo following marrow transplantation. In studies described in this report we investigated the effects of a novel anti-CD33 immunotoxin on the clonogenic potential of normal human CD34+ marrow cells and on the ability of these cells to initiate hematopoiesis in two-stage Dexter cultures (long-term marrow cultures, LTMC). This immunotoxin (anti-CD33-bR), shown previously to kill both clonogenic myelogenous leukemia cells and normal mature myeloid progenitor cells (granulocyte-macrophage colony-forming units, CFU-GM), consists of an anti-CD33 monoclonal antibody conjugated to purified ricin that has been modified by blocking the carbohydrate binding domains of the ricin B-chain to eliminate nonspecific binding. For our studies, normal CD34+ human marrow cells were isolated from the light-density (less than 1.070 g/ml) cells of aspirated marrow by positive selection with immunomagnetic beads linked to the monoclonal antibody K6.1. These cell isolates were highly enriched with both multipotential and lineage-restricted clonogenic, hematopoietic progenitors (mixed lineage colony-forming units, CFU-Mix; CFU-GM; and erythroid burst-forming units, BFU-E) which constituted greater than or equal to 20% of the cells. Recovery of clonogenic progenitors from these CD34+ cell preparations, following treatment with anti-CD33-bR (10 nM), was reduced by greater than or equal to 85% for CFU-GM and 20%-40% for CFU-Mix and BFU-E. However, the capacity of these cells to initiate hematopoietic LTMC was preserved. Indeed, the production of high proliferative potential (HPP) CFU-GM, BFU-E, and CFU-Mix in cultures seeded with 10(5) anti-CD33-bR-treated CD34+ marrow cells was substantially greater than that observed in LTMC seeded with equivalent numbers of untreated CD34+ cells. Moreover, concentrations of long-term culture initiating cells in CD34+ cell isolates, quantified by a limiting dilution technique, were found to be increased following anti-CD33-bR treatment. These findings support the potential usefulness of anti-CD33-bR for in vitro marrow purging or in vivo treatment to eliminate CD33+ leukemic clones, while sparing normal CD34+/CD33- stem cells that support normal hematopoiesis and hematopoietic reconstitution in vivo.

The specific cytotoxicity of immunoconjugates containing blocked ricin is dependent on the residual binding capacity of blocked ricin: evidence that the membrane binding and A-chain translocation activities of ricin cannot be separated

Recently we have developed blocked ricin, a derivative of native ricin in which the galactose-binding sites of the B-chain are blocked by covalent modification with affinity ligands. This modification impedes the binding function of the B-chain, while sparing its ability to facilitate the entry of the toxic subunit of ricin, the A-chain, into the cytoplasm. Immunotoxins prepared with blocked ricin approach the cytotoxic potency of native ricin with antibody-dependent specificity. Here we report that the high cytotoxic potency of these immunoconjugates, which is attributed to the preserved translocation function of the ricin B-chain, is dependent on the minimal residual lectin activity of blocked ricin. Our findings support the notion that two functions of ricin, membrane binding and translocation, cannot be separated.

Photoactivation of toxin conjugates

A novel photocleavable protein cross-linking reagent has been used for conjugation of the ribosome-inactivating protein pokeweed antiviral protein from seeds of Phytolacca americana (PAP-S), with either the monoclonal antibody 5E9 directed against the human transferrin-receptor or the B-chain of ricin that binds to cell-surface oligosaccharides bearing terminal D-galactose residues. When irradiated with near-UV light (350 nm), the linker of these conjugates undergoes photolytic degradation, resulting in the release of native toxin that is fully functional. The cytotoxicities of these 5E9-PAP-S and ricin B-chain-PAP-S conjugates for HeLa cells could be enhanced by irradiating the cells with light after they had internalized the conjugates.

Serotherapy of B-cell neoplasms with anti-B4-blocked ricin: a phase I trial of daily bolus infusion

Anti-B4-blocked Ricin (Anti-B4-bR) is an immunotoxin comprised of the anti-B4 monoclonal antibody (MoAb) and the protein toxin "blocked ricin." The anti-B4 MoAb is directed against the B-lineage-restricted CD19 antigen expressed on more than 95% of normal and neoplastic B cells. Blocked ricin is an altered ricin derivative that has its nonspecific binding eliminated by chemically blocking the galactose binding domains of the B chain. In vitro cytotoxicity studies demonstrate that the IC37 of Anti-B4-bR is 2 x 10(-11) mol/L compared with 4 x 10(-12) mol/L for native ricin. A phase I dose escalation clinical trial was conducted in 25 patients with refractory B-cell malignancies. Anti-B4-bR was administered by daily 1-hour bolus infusion for 5 consecutive days at doses ranging from 1 microgram/kg/d to 60 micrograms/kg/d. Serum levels above 1 nmol/L were achieved transiently in the majority of patients treated at the maximum tolerated dose of 50 micrograms/kg/d for 5 days for a total dose of 250 micrograms/kg. The dose-limiting toxicity was defined by transient, reversible grade 3 elevations in hepatic transaminases, without impaired hepatic synthetic function. Minor toxicities included transient hypoalbuminemia, thrombocytopenia, and fevers. Human antimouse antibody and human anti-ricin antibody were detected in nine patients. One complete response, two partial responses, and eight mixed or transient responses were observed. These results show the in vitro and in vivo cytotoxicity of Anti-B4-bR and indicate that this immunotoxin can be administered as a daily bolus infusion for 5 days with tolerable, reversible toxicity.

Blocked ricin-conjugated T cell immunotoxins: effect of anti-CD6-blocked ricin on normal T cell function

The biological properties of an immunotoxin composed of an anti-CD6 monoclonal antibody conjugated to whole ricin, which had been modified so that the galactose-binding sites of the B chain were blocked ("blocked ricin"), were examined. Treatment of peripheral blood lymphocytes with anti-CD6-blocked ricin for a 24-h period prevented T cell proliferation induced by phytohemagglutinin in a dose-dependent manner with concentrations causing 50% inhibition (IC50) ranging from 5 pM to 30 pM. In contrast, treatment with either blocked ricin alone or with a control immunotoxin prepared with a B-cell-lineage-restricted monoclonal antibody gave IC50 values of approximately 2 nM. Although shortening the duration of the anti-CD6-blocked ricin treatment to as little as 3 h had little significant effect on the observed inhibition, T cell viability experiments demonstrated that the magnitude of immunotoxin-induced killing after a given time period is significantly higher when the target cells become activated. Thus, from the initial concentration of cells treated with anti-CD6-blocked ricin placed in culture, 40%-45% viable cells remained after 2 days yet only 3%-9% remained if phorbol ester and Ca2+ ionophore were added; activation of T cells after mock treatment using blocked ricin plus nonconjugated anti-CD6 demonstrated that this effect was not the result of activation alone. The toxicity of anti-CD6-blocked ricin was also measured by inhibition of PHA-induced clonogenic growth of normal T cells. Continuous treatment of the cells using anti-CD6-blocked ricin at 0.1 nM resulted in a surviving fraction of about 3.5 x 10(-3); when immunotoxin treatment was for 24 h or less, the surviving fraction was only about 10(-1). As an indication of the unique specificity of anti-CD6-blocked ricin, immunotoxin pretreatment of potential responder cells prevented the generation of allogeneic cytolytic T lymphocytes in mixed lymphocyte cultures yet had little effect on the generation of interleukin-2-induced lymphokine-activated killer cell activity. We conclude that anti-CD6-blocked ricin demonstrates a cellular specificity and potency that make it a highly promising anti-T cell reagent.

An immunotoxin prepared with blocked ricin: a natural plant toxin adapted for therapeutic use

Ricin, the cytotoxic protein isolated from castor beans, is composed of two subunits, A-chain and B-chain. Ricin intoxicates cells by binding through its B-chain to galactose-terminated oligosaccharides found on the surface of all eukaryotic cells and then transferring its A-chain to the cytosol where it disrupts protein synthesis by inactivating ribosomes. In addition to binding, the B-chain plays an important, but not yet understood, role in the translocation of the A-chain through a cellular membrane to the cytosol. Blocking the two galactose-binding sites of native ricin by chemical modification with affinity ligands created an altered toxin, called blocked ricin, that has at least a 3500-fold lower binding affinity and is more than 1000-fold less cytotoxic than native ricin for Namalwa cells (a Burkitt's lymphoma line) but that has maintained the translocation function of the B-chain and the catalytic activity of the A-chain. Conjugation of blocked ricin to monoclonal antibodies that bind to cell surface antigens creates new cytotoxins that approach the potency of native ricin. These cytotoxins incorporate the three essential functions of natural toxins, i.e., binding to cells, transport through a membrane, and catalytic inactivation of an essential cellular process; but in addition they possess a defined cellular target specificity. Such potent immunotoxins may play an important therapeutic role in cancer treatment. Clinical trials with an anti-CD19-blocked ricin and an anti-CD33-blocked ricin conjugate against B-cell cancers and acute myeloblastic leukemia have begun.

Anti-MY9-blocked-ricin: an immunotoxin for selective targeting of acute myeloid leukemia cells

The use of immunotoxins (IT) to selectively destroy acute myeloid leukemia (AML) cells in vivo or in vitro is complicated by both the antigenic similarity of AML cells to normal progenitor cells and the difficulty of producing a sufficiently toxic conjugate. The monoclonal antibody (MoAb) anti-MY9 is potentially ideal for selective recognition of AML cells because it reacts with an antigen (CD33) found on clonogenic AML cells from greater than 80% of cases and does not react with normal pluripotent stem cells. In this study, we describe an immunotoxin that is selectively active against CD33+ AML cells: Anti-MY9-blocked-Ricin (Anti-MY9-bR), comprised of anti-MY9 conjugated to a modified whole ricin that has its nonspecific binding eliminated by chemical blockage of the galactose binding domains of the B-chain. A limiting dilution assay was used to measure elimination of HL-60 leukemic cells from a 20-fold excess of normal bone marrow cells. Depletion of CD33+ HL-60 cells was found to be dependent on the concentration of Anti-MY9-bR and on the duration of incubation with IT at 37 degrees C. More than 4 logs of these leukemic cells were specifically depleted following short exposure to high concentrations (10(-8) mol/L) of Anti-MY9-bR. Incubation with much lower concentrations of Anti-MY9-bR (10(-10) mol/L), as compatible with in vivo administration, resulted in 2 logs of depletion of HL-60 cells, but 48 to 72 hours of continuous exposure were required. Anti-MY9-bR was also shown to be toxic to primary AML cells, with depletion of greater than 2 logs of clonogenic cells following incubation with Anti-MY9-bR 10(-8) mol/L at 37 degrees C for 5 hours. Activity of Anti-MY9-bR could be blocked by unconjugated Anti-MY9 but not by galactose. As expected, Anti-MY9-bR was toxic to normal colony-forming unit granulocyte-monocyte (CFU-GM), which expresses CD33, in a concentration- and time-dependent manner, and also to burst-forming unit-erythroid and CFU-granulocyte, erythroid, monocyte, megakaryocyte, although to a lesser extent. When compared with anti-MY9 and complement (C'), Anti-MY9-bR could be used in conditions that provided more effective depletion of AML cells with substantially less depletion of normal CFU-GM. Therefore, Anti-MY9-bR may have clinical utility for in vitro purging of AML cells from autologous marrow when used at high IT concentrations for short incubation periods. Much lower concentrations of Anti-MY9-bR that can be maintained for longer periods may be useful for elimination of AML cells in vivo.

The galactose-binding sites of the cytotoxic lectin ricin can be chemically blocked in high yield with reactive ligands prepared by chemical modification of glycopeptides containing triantennary N-linked oligosaccharides

A glycopeptide containing a triantennary N-linked oligosaccharide from fetuin was modified by a series of chemical and enzymic reactions to afford a reagent that contained a terminal residue of 6-(N-methylamino)-6-deoxy-D-galactose on one branch of the triantennary structure and terminal galactose residues on the other two branches. Binding assays and gel filtration experiments showed that this modified glycopeptide could bind to the sugar-binding sites of ricin. The ligand was activated at the 6-(N-methylamino)-6-deoxy-D-galactose residue by reaction with cyanuric chloride. The resulting dichlorotriazine derivative of the ligand reacts with ricin, forming a stable covalent linkage. The reaction was confined to the B-chain and was inhibited by lactose. Bovine serum albumin and ovalbumin were not modified by the activated ligand under similar conditions, and we conclude, therefore, that the reaction of the ligand with ricin B-chain was dependent upon specific binding to sugar-binding sites. Ricin that had its galactose-binding sites blocked by the covalent reaction with the activated ligand was purified by affinity chromatography. The major species in this fraction was found to contain 2 covalently linked ligands per ricin B-chain, while a minor species contained 3 ligands per B-chain. The cytotoxicity of blocked ricin was at least 1000-fold less than that of native ricin for cultured cells in vitro, even though the activity of the A-chain in a cell-free system was equal to that from native ricin. Modified ricin that contained only 1 covalently linked ligand was also purified. This fraction retained an ability to bind to galactose affinity columns, although with a lower affinity than ricin, and was only 5- to 20-fold less cytotoxic than native ricin.

Relationships among benzo(a)pyrene metabolism, benzo(a)pyrene-diol-epoxide:DNA adduct formation, and sister chromatid exchanges in human lymphocytes from smokers and nonsmokers

In the present study, benzo(a)pyrene (BP) metabolism, DNA adduct formation, ethoxyresorufin-O-deethylase activity, and sister chromatid exchange induction by BP were compared in human lymphocytes prepared from whole blood of smokers and nonsmokers following an in vitro incubation with BP. There was an approximate 7- to 10-fold variation in all parameters measured. To determine the source of this variation, participants were resampled, the assays were repeated, and all the data were analyzed to assess (a) smoking-related effects, (b) differences in multiple samples from the same individual, and (c) intraindividual, experimental, and interindividual variation. No smoking-related effects were observed except for baseline sister chromatid exchange frequency. The variation observed for BP-related DNA adducts and ethoxyresorufin-O-deethylase activity was primarily due to interindividual variation. For example, in vitro formation of DNA adducts did not change when samples were obtained at different times from the same individual and were not influenced significantly by culture conditions. No significant correlation existed between DNA adduct formation and BP metabolism [correlation coefficient (r) = 0.27] for either the total population or when segregated based on smoking status. Furthermore, no correlation was seen between DNA adducts and sister chromatid exchange induction by BP. Our studies have compared a number of commonly used lymphocyte markers and conclude that it is difficult to predict changes in one marker based on changes in another. However, in vitro formation, of PB-derived DNA adducts is consistent over time for individuals.