Enzymic activity of cholera toxin. I. New method of assay and the mechanism of ADP-ribosyl transfer

Anhydrous Monoalkylguanidines in Aprotic and Nonpolar Solvents: Models for Deprotonated Arginine Side Chains in Membrane Environments

In this study, the synthesis of crystalline dodecylguanidine free base and its spectroscopic characterization in nonpolar environments are described. IR as well as ¹H and ¹⁵N NMR spectra of the free base dissolved in aprotic solvents are substantially different from the previously reported spectra of arginine, or other monoalkylguanidinium compounds, at high hydroxide concentrations. The current results provide improved modeling for the spectroscopic signals that would be expected from a deprotonated arginine in a nonpolar environment. On the basis of our spectra of the authentic dodecylguanidine free base, addition of large amounts of aqueous hydroxide to arginine or other monoalklyguanidinium salts does not deprotonate them. Instead, hydroxide addition leads to the formation of a guanidinium hydroxide complex, with a dissociation constant near ∼500 mM that accounts for the established arginine pK value of ∼13.7. We also report a method for synthesizing a compound containing both phenol and free-base guanidine groups, linked by a dodecyl chain that should be generalizable to other hydrocarbon linkers. Such alkyl-guanidine and phenolyl-alkyl-guanidine compounds can serve as small-molecule models for the conserved arginine-tyrosine groupings that have been observed in crystallographic structures of both microbial rhodopsins and G-protein-coupled receptors.

Characterization of pertussis-like toxin from Salmonella spp. that catalyzes ADP-ribosylation of G proteins

Salmonella Typhimurium definitive phage type (DT) 104 produces a pertussis-like toxin (ArtAB-DT104), which catalyzes ADP-ribosylation of pertussis toxin sensitive G proteins. However, the prevalence of ArtAB and its toxicity have not been established. We report here that, in addition to DT104, S. Worthington, and S. bongori, produce ArtAB homologs, designated ArtAB-SW and ArtAB-Sb, respectively. We purified and characterized these ArtAB toxins, which comprise a 27-kDa A subunit (ArtA) and 13.8-kDa pentameric B subunits (ArtB). While the sequence of the A subunit, which is ADP-ribosyltransferase, is similar to the A subunit sequences of other ArtABs, the B subunit of ArtAB-Sb is divergent compared to the B subunit sequences of other ArtABs. Intraperitoneal injection of purified ArtABs was fatal in mice; the 50% lethal doses of ArtAB-DT104 and ArtAB-SW were lower than that of ArtAB-Sb, suggesting that ArtB plays an influential role in the toxicity of ArtABs. ArtABs catalyzed ADP-ribosylation of G proteins in RAW 264.7 murine macrophage-like cells, and increased intracellular cyclic AMP levels. ArtAB-DT104 and ArtAB-SW, but not ArtAB-Sb, stimulated insulin secretion in mice; however, unlike Ptx, ArtABs did not induce leukocytosis. This disparity in biological activity may be explained by differences in ADP-ribosylation of target G proteins.

The NarE protein of Neisseria gonorrhoeae catalyzes ADP-ribosylation of several ADP-ribose acceptors despite an N-terminal deletion

The ADP-ribosylating enzymes are encoded in many pathogenic bacteria in order to affect essential functions of the host. In this study, we show that Neisseria gonorrhoeae possess a locus that corresponds to the ADP-ribosyltransferase NarE, a previously characterized enzyme in N. meningitidis The 291 bp coding sequence of gonococcal narE shares 100% identity with part of the coding sequence of the meningococcal narE gene due to a frameshift previously described, thus leading to a 49-amino-acid deletion at the N-terminus of gonococcal NarE protein. However, we found a promoter region and a GTG start codon, which allowed expression of the protein as demonstrated by RT-PCR and western blot analyses. Using a gonococcal NarE-6xHis fusion protein, we demonstrated that the gonococcal enzyme underwent auto-ADP-ribosylation but to a lower extent than meningococcal NarE. We also observed that gonoccocal NarE exhibited ADP-ribosyltransferase activity using agmatine and cell-free host proteins as ADP-ribose acceptors, but its activity was inhibited by human β-defensins. Taken together, our results showed that NarE of Neisseria gonorrhoeae is a functional enzyme that possesses key features of bacterial ADP-ribosylating enzymes.

Endogenous protein mono-ADP-ribosylation in Arabidopsis thaliana

Protein mono-ADP-ribosylation post-translationally transfers the ADP-ribose moiety from the β-NAD+ donor to various protein acceptors. This type of modification has been widely characterized and shown to regulate protein activities in animals, yeast and prokaryotes, but has never been reported in plants. In this study, using [³²P]NAD+ as the substrate, ADP-ribosylated proteins in Arabidopsis were investigated. One protein substrate of 32 kDa in adult rosette leaves was found to be radiolabeled. Heat treatment, protease sensitivity and nucleotide derivative competition assays suggested a covalent reaction of NAD+ with the 32 kDa protein. [carbonyl-¹⁴C]NAD+ could not label the 32 kDa protein, confirming that the modification was ADP-ribosylation. Poly (ADP-ribose) polymerase inhibitor failed to suppress the reaction, but chemicals that destroy mono-ADP-ribosylation on specific amino acid residues could break up the linkage, suggesting that the reaction was not a poly-ADP-ribosylation but rather a mono-ADP-ribosylation. This modification mainly existed in leaves and was enhanced by oxidative stresses. In young seedlings, two more protein substrates with the size of 45 kDa and over 130 kDa, respectively, were observed in addition to the 32 kDa protein, indicating that different proteins were modified at different developmental stages. Although the substrate proteins remain to be identified, this is the first report on the characterization of endogenously mono-ADP-ribosylated proteins in plants.

Salmonella enterica serotype Typhimurium DT104 ArtA-dependent modification of pertussis toxin-sensitive G proteins in the presence of [32P]NAD

Salmonella enterica serotype Typhimurium (S. Typhimurium) definitive phage type (DT) 104 has become a widespread cause of human and other animal infections worldwide. The severity of clinical illness in S. Typhimurium DT104 outbreaks suggests that this strain possesses enhanced virulence. ArtA and ArtB - encoded by a prophage in S. Typhimurium DT104 - are homologues of components of pertussis toxin (PTX), including its ADP-ribosyltransferase subunit. Here, we show that exposing DT104 to mitomycin C, a DNA-damaging agent, induced production of prophage-encoded ArtA/ArtB. Pertussis-sensitive G proteins were labelled in the presence of [(32)P]NAD and ArtA, and the label was released by HgCl(2), which is known to cleave cysteine-ADP-ribose bonds. ADP-dependent modification of G proteins was markedly reduced in in vitro-synthesized ArtA(6Arg-Ala) and ArtA(115Glu-Ala), in which alanine was substituted for the conserved arginine at position 6 (necessary for NAD binding) and the predicted catalytic glutamate at position 115, respectively. A cellular ADP-ribosylation assay and two-dimensional electrophoresis showed that ArtA- and PTX-induced ADP-ribosylation in Chinese hamster ovary (CHO) cells occur with the same type of G proteins. Furthermore, exposing CHO cells to the ArtA/ArtB-containing culture supernatant of DT104 resulted in a clustered growth pattern, as is observed in PTX-exposed CHO cells. Hydrogen peroxide, an oxidative stressor, also induced ArtA/ArtB production, suggesting that these agents induce in vivo synthesis of ArtA/ArtB. These results, taken together, suggest that ArtA/ArtB is an active toxin similar to PTX.

From bench to bedside: stealth of enteroinvasive pathogens

Bacterial enteric infections are often associated with diarrhoea or vomiting, which are clinical presentations commonly referred to as gastroenteritis. However, some enteric pathogens, including typhoidal Salmonella serotypes, Brucella species and enteropathogenic Yersinia species are associated with a clinical syndrome that is characterized by abdominal pain and/or fever and is distinct from acute gastroenteritis. Recent insights into molecular mechanisms of the host-pathogen interaction show that these enteric pathogens share important characteristics that explain why the initial host responses associated with these agents more closely resemble host responses to viral or parasitic infections. Host responses contribute to the clinical presentation of disease and improved understanding of these responses in the laboratory is beginning to bridge the gap between bench and bedside.

Design, synthesis, and evaluation of bisubstrate analog inhibitors of cholera toxin

Bisubstrate analog inhibitors in which a nicotinamide mimic is attached to a series of structurally diversified guanidines (arginine mimics) were synthesized and evaluated for inhibition of cholera toxin. The mechanism-based bisubstrate inhibitors were up to 1400-fold more potent than the natural substrate NAD+ and 400-fold more potent than the artificial substrate diethylamino (benzylidine-amino)guanidine (DEABAG) in an assay toward an intrinsically active mutant of wild-type cholera toxin.

Toxins which activate adenylate cyclase

Cholera toxin and other heat-labile enterotoxins have the same subunit structure (A5B) and all catalyse the mono ADP-ribosylation of Ns, a regulator of adenylate cyclase, probably at an arginine residue. They also ADP-ribosylate a variety of other membrane and soluble proteins at much slower rates. The rates differ from protein to protein but it may be that every arginine residue in every protein is ADP-ribosylated at some slow rate. A guanine nucleotide triphosphate is required for the ADP-ribosylation of the major (Ns) and minor substrates alike. It used to be thought that all the substrates were GTP-binding proteins but this cannot be so. Rather, the GTP is required because it has to bind to some additional site on the membrane, termed 'S', in a cooperative event that involves a soluble protein called cytosolic factor (CF). If we expose erythrocyte membranes to CF and the GTP analogue Gpp(NH)p we can later extract in detergent a factor or complex that confers upon naive erythrocyte membranes the ability to be ADP-ribosylated. Pertussis toxin also has an A5B structure and acts on an intracellular substrate for ADP-ribosylation, namely the negative regulator of adenylate cyclase, called Ni. ADP-ribosylation prevents the reduction of cyclase activity by inhibitory hormones. The ADP-ribosylation of Ns or Ni does not affect the rate of ADP-ribosylation of the other protein.

Regulation of virulence in Vibrio cholerae: the ToxR regulon

Vibrio cholerae is a gram-negative bacterium that is the causative agent of cholera. This disease consists of enormous fluid loss through stools, which can be fatal. Cholera epidemics appear in explosive outbreaks that have occurred repeatedly throughout history. The virulence factors toxin coregulated pilus (TCP) and cholera toxin (CT) are essential for colonization of the host and enterotoxicity, respectively. These virulence factors are under the control of ToxT, an AraC/XylS family protein that activates transcription of the genes encoding TCP and CT. ToxT is under the control of a virulence regulatory cascade known as the ToxR regulon, which responds to environmental stimuli to ensure maximal virulence-factor induction within the human intestine. An understanding of this intricate signaling pathway is essential for the development of methods to treat and prevent this devastating disease.

Cholera toxin, LT-I, LT-IIa and LT-IIb: the critical role of ganglioside binding in immunomodulation by type I and type II heat-labile enterotoxins

The heat-labile enterotoxins expressed by Vibrio cholerae (cholera toxin) and Escherichia coli (LT-I, LT-IIa and LT-IIb) are potent systemic and mucosal adjuvants. Coadministration of the enterotoxins with a foreign antigen produces an augmented immune response to that antigen. Although each enterotoxin has potent adjuvant properties, the means by which the enterotoxins induce various immune responses are distinctive for each adjuvant. Various mutants have been engineered to dissect the functions of the enterotoxins required for their adjuvanticity. The capacity to strongly bind to one or more specific ganglioside receptors appears to drive the distinctive immunomodulatory properties associated with each enterotoxin. Mutant enterotoxins with ablated or altered ganglioside-binding affinities have been employed to investigate the role of gangliosides in enterotoxin-dependent immunomodulation.

Human alpha-defensins neutralize toxins of the mono-ADP-ribosyltransferase family

Various bacterial pathogens secrete toxins, which are not only responsible for fatal pathogenesis of disease, but also facilitate evasion of host defences. One of the best-known bacterial toxin groups is the mono-ADP-ribosyltransferase family. In the present study, we demonstrate that human neutrophil alpha-defensins are potent inhibitors of the bacterial enzymes, particularly against DT (diphtheria toxin) and ETA (Pseudomonas exotoxin A). HNP1 (human neutrophil protein 1) inhibited DT- or ETA-mediated ADP-ribosylation of eEF2 (eukaryotic elongation factor 2) and protected HeLa cells against DT- or ETA-induced cell death. Kinetic analysis revealed that inhibition of DT and ETA by HNP1 was competitive with respect to eEF2 and uncompetitive against NAD+ substrates. Our results reveal that toxin neutralization represents a novel biological function of HNPs in host defence.

Bile acids stimulate biofilm formation in Vibrio cholerae

Vibrio cholerae is a Gram-negative bacterium that causes the acute diarrhoeal disease cholera. After the bacterium is ingested, it passes through the digestive tract, encountering various environmental stresses including the acidic milieu of the stomach and the toxic effects of bile in the duodenum. While these stresses serve as part of a host defence system, V. cholerae has evolved resistance mechanisms that allow it to evade these defences and establish infection. We examined the expression profiles of V. cholerae in response to bile or bile acids and found an induction of biofilm genes. We found that V. cholerae shows significantly enhanced biofilm formation in response to bile acids, and that bacteria within the biofilm are more resistant to the toxicity of bile acids compared with planktonic cells. Bile acid induction of biofilms was found to be dependent on the vps genes (Vibrio polysaccharide synthesis) and their transcriptional activator VpsR, but VpsT is not required. These results contribute to the developing picture of a complex relationship between V. cholerae and its environment within the host during infection.

NarE: a novel ADP-ribosyltransferase from Neisseria meningitidis

Mono ADP-ribosyltransferases (ADPRTs) are a class of functionally conserved enzymes present in prokaryotic and eukaryotic organisms. In bacteria, these enzymes often act as potent toxins and play an important role in pathogenesis. Here we report a profile-based computational approach that, assisted by secondary structure predictions, has allowed the identification of a previously undiscovered ADP-ribosyltransferase in Neisseria meningitidis (NarE). NarE shows structural homologies with E. coli heat-labile enterotoxin (LT) and cholera toxin (CT) and possesses ADP-ribosylating and NAD-glycohydrolase activities. As in the case of LT and CT, NarE catalyses the transfer of the ADP-ribose moiety to arginine residues. Despite the absence of a signal peptide, the protein is efficiently exported into the periplasm of Neisseria. The narE gene is present in 25 out of 43 strains analysed, is always present in ET-5 and Lineage 3 but absent in ET-37 and Cluster A4 hypervirulent lineages. When present, the gene is 100% conserved in sequence and is inserted upstream of and co-transcribed with the lipoamide dehydrogenase E3 gene. Possible roles in the pathogenesis of N. meningitidis are discussed.

Mono(ADP-ribosyl)ation of 2'-deoxyguanosine residue in DNA by an apoptosis-inducing protein, pierisin-1, from cabbage butterfly

Pierisin-1 is a potent apoptosis-inducing protein derived from the cabbage butterfly, Pieris rapae. It has been shown that pierisin-1 has an A small middle dotB structure-function organization like cholera or diphtheria toxin, where the "A" domain (N-terminal) exhibits ADP-ribosyltransferase activity. The present studies were designed to identify the target molecule for ADP-ribosylation by pierisin-1 in the presence of beta-[adenylate-(32)P]NAD, and we found DNA as the acceptor, but not protein as is the case with other bacteria-derived ADP-ribosylating toxins. ADP-ribosylation of tRNAs from yeast was also catalyzed by pierisin-1, but the efficiency was around 110 of that for calf thymus DNA. Pierisin-1 efficiently catalyzed the ADP-ribosylation of double-stranded DNA containing dG small middle dotdC, but not dA small middle dotdT pairs. The ADP-ribose moiety of NAD was transferred to the amino group at N(2) of 2'-deoxyguanosine to yield N(2)-(alpha-ADP-ribos-1-yl)-2'-deoxyguanosine and its beta form, which were determined by several spectral analyses including (1)H- and (13)C-NMR and mass spectrometry. The chemical structures were also ascertained by the independent synthesis of N(2)-(D-ribos-1-yl)-2'-deoxyguanosine, which is the characteristic moiety of ADP-ribosylated dG. Using the (32)P-postlabeling method, ADP-ribosylated dG could be detected in DNA from pierisin-1-treated HeLa cells, in which apoptosis was easily induced. Thus, the targets for ADP-ribosylation by pierisin-1 were concluded to be 2'-deoxyguanosine residues in DNA. This finding may open a new field regarding the biological significance of ADP-ribosylation.

Biological and biochemical characterization of variant A subunits of cholera toxin constructed by site-directed mutagenesis

Cholera toxin (CT) is the prototype for the Vibrio cholerae-Escherichia coli family of heat-labile enterotoxins having an AB5 structure. By substituting amino acids in the enzymatic A subunit that are highly conserved in all members of this family, we constructed 23 variants of CT that exhibited decreased or undetectable toxicity and we characterized their biological and biochemical properties. Many variants exhibited previously undescribed temperature-sensitive assembly of holotoxin and/or increased sensitivity to proteolysis, which in all cases correlated with exposure of epitopes of CT-A that are normally hidden in native CT holotoxin. Substitutions within and deletion of the entire active-site-occluding loop demonstrated a prominent role for His-44 and this loop in the structure and activity of CT. Several novel variants with wild-type assembly and stability showed significantly decreased toxicity and enzymatic activity (e.g., variants at positions R11, I16, R25, E29, and S68+V72). In most variants the reduction in toxicity was proportional to the decrease in enzymatic activity. For substitutions or insertions at E29 and Y30 the decrease in toxicity was 10- and 5-fold more than the reduction in enzymatic activity, but for variants with R25G, E110D, or E112D substitutions the decrease in enzymatic activity was 12- to 50-fold more than the reduction in toxicity. These variants may be useful as tools for additional studies on the cell biology of toxin action and/or as attenuated toxins for adjuvant or vaccine use.

pepA, a gene mediating pH regulation of virulence genes in Vibrio cholerae

ToxT, a member of the AraC family of transcriptional regulators, controls the expression of several virulence factors in Vibrio cholerae. In the classical biotype of V. cholerae, expression of toxT is regulated by the same environmental conditions that control expression of the virulence determinants cholera toxin and the toxin coregulated pilus. Several genes that activate toxT expression have been identified. To identify genes that repress toxT expression in nonpermissive environmental conditions, a genetic screen was used to isolate mutations which alter the expression of a toxT-gusA transcriptional fusion. Several mutants were isolated, and the mutants could be divided into two classes. One class of mutants exhibited higher expression levels of toxT-gusA at both the nonpermissive pH and temperature, while the second class showed elevated toxT-gusA expression only at the nonpermissive pH. One mutant from the second class was chosen for further characterization. This mutant was found to carry a TnphoA insertion in a homolog of the Escherichia coli pepA gene. Disruption of pepA in V. cholerae resulted in elevated levels of expression of cholera toxin, tcpA, toxT, and tcpP at the noninducing pH but not at the noninducing temperature. Elevated levels of expression of toxT and tcpP at the nonpermissive pH in the pepA mutant were abolished in tcpP toxR mutant and aphB mutant backgrounds, respectively. A putative binding site for PepA was identified in the tcpPH-tcpI intergenic region, suggesting that PepA may act at the level of tcpPH transcription. Disruption of pepA caused only partial deregulation at the noninducing pH, suggesting the involvement of additional factors in the pH regulation of virulence genes in V. cholerae.

The Arg7Lys mutant of heat-labile enterotoxin exhibits great flexibility of active site loop 47-56 of the A subunit

The heat-labile enterotoxin from Escherichia coli (LT) is a member of the cholera toxin family. These and other members of the larger class of AB5 bacterial toxins act through catalyzing the ADP-ribosylation of various intracellular targets including Gs alpha. The A subunit is responsible for this covalent modification, while the B pentamer is involved in receptor recognition. We report here the crystal structure of an inactive single-site mutant of LT in which arginine 7 of the A subunit has been replaced by a lysine residue. The final model contains 103 residues for each of the five B subunits, 175 residues for the A1 subunit, and 41 residues for the A2 subunit. In this Arg7Lys structure the active site cleft within the A subunit is wider by approximately 1 A than is seen in the wild-type LT. Furthermore, a loop near the active site consisting of residues 47-56 is disordered in the Arg7Lys structure, even though the new lysine residue at position 7 assumes a position which virtually coincides with that of Arg7 in the wild-type structure. The displacement of residues 47-56 as seen in the mutant structure is proposed to be necessary for allowing NAD access to the active site of the wild-type LT. On the basis of the differences observed between the wild-type and Arg7Lys structures, we propose a model for a coordinated sequence of conformational changes required for full activation of LT upon reduction of disulfide bridge 187-199 and cleavage of the peptide loop between the two cysteines in the A subunit.(ABSTRACT TRUNCATED AT 250 WORDS)

Mutational analysis of the ganglioside-binding activity of the type II Escherichia coli heat-labile enterotoxin LT-IIb

The Escherichia coli type II heat-labile enterotoxin LT-IIb IIb consists of a single A polypeptide and five B polypeptides. The A polypeptide is responsible for the toxic activity, and the B polypeptides function to bind the toxin to gangliosides on the surface of the plasma membrane. Previous studies on the related type II enterotoxin LT-IIa demonstrated the importance of threonine (Thr) residues at positions 13, 14, and 34 in the mature B polypeptide for ganglioside GD1bp-binding activity. In this study, we used sitespecific mutagenesis to investigate ganglioside GD1a-binding activity of the B polypeptide of LT-IIb. We determined that Thr-13 and Thr-14 were involved in binding of ganglioside GD1a by the B polypeptides of LT-IIb but that Thr-34 was not essential. Substitution of serine, but not other amino acids, at position 13 or 14 in the B polypeptide of LT-IIb resulted in retention of ganglioside-binding activity equivalent to that of the wild-type enterotoxin, providing strong evidence that the hydroxyl groups of threonine or serine at positions 13 and 14 are important for the ganglioside-binding activity of LT-IIb. Chimeric genes that expressed hybrids of the B polypeptides of LT-IIb and LT-IIa were also constructed, and analysis of the hybrids showed that the specificity of their ganglioside-binding activity was determined by the N-terminal half of the molecule.

ADP-ribosylation factors: a family of approximately 20-kDa guanine nucleotide-binding proteins that activate cholera toxin

ADP-ribosylation factors (ARFs) comprise a family of approximately 20 kDa guanine nucleotide-binding proteins that were discovered as one of several cofactors required in cholera toxin-catalyzed ADP-ribosylation of Gs alpha, the guanine nucleotide-binding protein responsible for stimulation of adenylyl cyclase, and was subsequently found to enhance all cholera toxin-catalyzed reactions and to directly interact with, and activate the toxin. ARF is dependent on GTP or its analogues for activity, binds GTP with high affinity in the presence of dimyristoylphosphatidylcholine/cholate and contains consensus sequences for GTP-binding and hydrolysis. Six mammalian family members have been identified which have been classified into three groups (Class I, II, and III) based on size, deduced amino acid sequence identity, phylogenetic analysis and gene structure. ARFs are ubiquitous among eukaryotes, with a deduced amino acid sequence that is highly conserved across diverse species. They have recently been shown to associate with phospholipid and Golgi membranes in a GTP-dependent manner and are involved in regulating vesicular transport.

Role of ADP-ribosylation in endothelial signal transduction and prostacyclin production

ADP-ribosylation of proteins by the enzymatic transfer of ADP-ribose from NAD has been implicated in a number of biological processes. We report that inhibitors of ADP-ribosylation, most notably the novel inhibitor of arginine specific cellular mono(ADP-ribosyl) transferase, meta-iodobenzylguanidine (MIBG) as well as nicotinamide, L-arginine methyl ester (LAME) and guanyltyramine, inhibit histamine-induced endothelial production of inositol phosphates, release of arachidonic acid and production of prostacyclin (PGI2). Those same responses were unaffected by MIBG when triggered by thrombin or leukotriene C4. These findings suggest that ADP-ribosylation serves a role in histamine-induced production of prostacyclin and imply differences in transduction pathways employed by the different agonists.

Role of G proteins in stimulation of Na-H exchange by cell shrinkage

Many cells respond to shrinkage by stimulating specific ion transport processes (e.g., Na-H exchange). However, it is not known how the cell senses this volume change, nor how this signal is transduced to an ion transporter. We have studied the activation of Na-H exchange in internally dialyzed barnacle muscle fibers, measuring intracellular pH (pHi) with glass microelectrodes. When cells are dialyzed to a pHi of approximately 7.2, Na-H exchange is active only in shrunken cells. We found that the shrinkage-induced stimulation of Na-H exchange, elicited by increasing medium osmolality from 975 to 1,600 mosmol/kgH2O, is inhibited approximately 72% by including in the dialysis fluid 1 mM guanosine 5'-O-(2-thiodiphosphate). The latter is an antagonist of G protein activation. Even in unshrunken cells, Na-H exchange is activated by dialyzing the cell with 1 mM guanosine 5'-O-(3-thiotriphosphate), which causes the prolonged activation of G proteins. Activation of Na-H exchange is also elicited in unshrunken cells by injecting cholera toxin, which activates certain G proteins. Neither exposing cells to 100 nM phorbol 12-myristate 13-acetate nor dialyzing them with a solution containing 20 microM adenosine 3',5'-cyclic monophosphate (cAMP) (or 50 microM dibutyryl cAMP) plus 0.5 mM 3-isobutyl-1-methylxanthine substantially stimulates the exchanger. Thus our data suggest that a G protein plays a key role in the transduction of the shrinkage signal to the Na-H exchanger via a pathway that involves neither protein kinase C nor cAMP.

Intracellular inhibition of mono(ADP-ribosylation) by meta-iodobenzylguanidine: specificity, intracellular concentration and effects on glucocorticoid-mediated cell lysis

meta-Iodobenzylguanidine (MIBG) is a high-affinity substrate for mono(ADP-ribosyl)transferase of cholera toxin and turkey erythrocyte membranes (Loesberg, C., Van Rooij, H. and Smets, L.A.(1990) Biochim. Biophys. Acta 1037, 92-99). In the present study the drug was investigated as a potential inhibitor of intracellular ribosyltransferases by competition with endogenous acceptors. To this end, MIBG was compared with the conventional ADP-ribosylation inhibitors nicotinamide and 3-aminobenzamide in cell-free ribosylation systems and in intact L1210 leukemia cells. Poly(ADP-ribose)polymerase (poly-ADPRP) was assayed by the DNAse-I-induced incorporation of [14C]NAD in nuclei of permeabilized L1210 cells. Mono(ADP-ribosyl)transferase (mono-ADPRT) was assayed as NAD linkage to [125I]iodoguanyltyramine catalysed by turkey erythrocyte membranes or activated cholera toxin. Poly-ADPRP was inhibited by nicotinamide (IC50 = 0.03 mM) and by 3-aminobenzamide (IC50 less than or equal to 0.03 mM) but was insensitive to MIBG. Conversely, mono-ADPRT was inhibited by MIBG (IC50 = approx. 0.1 mM) but not by 3-aminobenzamide and only weakly so by nicotinamide in high concentration (10 mM). In L1210 cells, intracellular levels of nicotinamide equilibrated at 60-70% of the extracellular drug concentrations assayed at 1 and 10 mM. In contrast, MIBG was concentrated 15-fold by nonspecific uptake. The preferential interference of the drugs with endogenous mono- or poly-ADP ribosylations, predicted from inhibitory capacity in vitro and intracellular concentrations, was confirmed by their effect on dexamethasone-induced lysis of L1210 cell lines. Inhibition of endogenous mono-ADPRT with 0.03 mM MIBG or 10 mM nicotinamide induced sensitivity to glucocorticoids in refractory L1210-wt cells. In contrast, inhibition of poly-ADPRP by 3-aminobenzamide or nicotinamide (1 mM each) did not confer susceptibility to refractory cells but enhanced the lytic process in the sensitive subline L1210-H7 or in L1210-wt cells sensitized by MIBG. These results indicate that MIBG is the first substrate for guanidino-specific mono-ADPRT which accumulates in intact mammalian cells and effectively competes with intracellular acceptors for endogenous enzymes.

Meta-iodobenzylguanidine (MIBG), a novel high-affinity substrate for cholera toxin that interferes with cellular mono(ADP-ribosylation)

Meta-iodobenzylguanidine (MIBG) is a guanidine analogue of the neurotransmitter norepinephrine. Radioiodinated [131I]MIBG is clinically used as a tumor-targeted radiopharmaceutical in the diagnosis and treatment of adrenergic tumors. Moreover, non-radiolabelled MIBG exerts several cell-biological effects, tentatively ascribed to interference with cellular mono(ADP-ribosyl) transferases (Smets, L.A., Bout, B. and Wisse, J. (1988) Cancer Chemother. Pharmacol. 21, 9-13; Smets, L.A., Metwally, E.A.G., Knol, E. and Martens, M. (1988) Leukemia Res. 12, 737-743). In the present study it was investigated whether MIBG could serve as an acceptor for the ribosyl transferase activity of cholera toxin and of erythrocyte membranes. MIBG appeared a substrate for the cholera toxin-catalyzed transfer of the ADP-ribose moiety of NAD to arginine-like residues with the highest affinity for this enzyme reported as yet (Km = 6.5 microM). MIBG was also ADP-ribosylated by the mono(ADP-ribosyl)transferase(s) of turkey erythrocyte membranes. Moreover, the drug appeared a potent affector of the ADP-ribose linkage to membrane proteins by these enzymes. Interference by MIBG was stronger than by related guanyltyramine, the monoamine precursors of MIBG, meta-iodobenzylamine had no effect at all. In contrast, the drug failed to affect endogenous, O-linked poly(ADP-ribose) polymerase, induced in nuclei of S49-leukemia cells by deoxyribonuclease. Since MIBG is the first described drug that specifically interferes with the cellular N-linked mono(ADP-ribosyl) transferase reactions, it may be an important tool to elucidate the physiological role of this posttranscriptional protein modification.

Localization of transfer RNA genes on the physical map of Vibrio eltor phage e4 genome

Transfer RNAs were isolated from uninfected and phage e4-infected Vibrio eltor Mak 757 cells. These tRNAs were then aminoacylated with 3H-labeled amino acids and hybridized to DNA isolated from phage e4. Significant hybridization was observed only with tRNA isolated from phage e4-infected cells. Restriction enzyme digestion of phage e4 DNA followed by Southern blot using [32P]tRNA from infected cells revealed that tRNA genes were contained in a 3.4-kb Kpnl fragment. The tRNA genes were located on the physical map of the phage genome 19 kb from one of the termini.

Potentiation of glucocorticoid-induced lysis in refractory and resistant leukemia cells by inhibitors of ADP-ribosylation

Meta-iodo-benzylguanidine (MIBG; 3 x 10(-5) M), a novel inhibitor of mono(ADP-ribosylation)-and the general ribosylation inhibitor nicotinamide (NA; 5-20 mM) both stimulated the glucocorticoid-mediated lysis of sensitive L1210 leukemia cells and even induced susceptibility in various human and murine lines refractory or resistant to dexamethasone (DEX). Potentiation and induction of DEX-sensitivity by ADP-ribosylation inhibitors was accompanied by an increase in saturable 3H-DEX binding sites and by a 2-3 fold increase in the affinity of intracellular receptors for hormone binding. Moreover, the ribosylation inhibitors converted the glucocorticoid antagonist RU-486 into a potent agonist for cytolysis of L1210 cells. We conclude that the cytolytic action of glucocorticoid hormones in leukemic cells is negatively controlled by (mono)ADP-ribosylation of receptor proteins.

Use of substituted (benzylidineamino)guanidines in the study of guanidino group specific ADP-ribosyltransferase

A number of substituted (benzylidineamino)guanidines with different substitutents in the benzene nucleus are synthesized by coupling substituted benzaldehydes with aminoguanidine, and these compounds are tested as substrates for cholera toxin catalyzed ADP-ribosylation. A spectrophotometric assay method for the measurement of ADP-ribosyltransferase activity is developed, making use of the absorption characteristics of some of these compounds and the difference in the ionic character of the free compounds and the ADP-ribosylated products. The kinetic parameters for the ADP-ribosylation of these compounds are evaluated. A correlation between log kcat or log (kcat/Km) and the Hammett substituent constant sigma is observed. This correlation suggests the importance of substrate electronic effects on the enzymatic reaction. The reactivity of these compounds as acceptors of ADP-ribosyl groups in the reaction catalyzed by cholera toxin increases with increasing electron-donating power of the substituents in the benzene function. The effect is primarily on the catalytic rate constant, kcat, not on the binding constant, Km. The results are consistent with an SN2 reaction mechanism in which the deprotonated guanidino group makes a nucleophilic attack on the C-1 carbon of the ribose moiety.

Cholera toxin A subunit: functional sites correlated with regions of secondary structure

The A subunit of cholera toxin contains the ADP-ribosyltransferase activity in its major constituent polypeptide A1 (Mr 23,000) which is responsible for the elevation of cAMP typically observed with most mammalian cell types after exposure to the toxin. The primary structure of the A subunit, recently established by sequence analyses, is presented and used as the basis for the secondary structure prediction according to the method of Chou and Fasman. The results indicated the presence of 27% alpha-helix, 25% beta-structure, 12% beta-turn, and 36% random coil. The majority of the beta-structure consisted of six strands located in the NH2-terminal portion of the molecule (residues 33-106) covering one-half of the region corresponding to the A1 polypeptide portion. The beta-sheet domain led immediately into the active site region characterized by the alternating structures of beta-pleated sheet and alpha-helix (residues 95-140) similar to that reported for other NAD+ binding proteins. The presence of this structural feature in the region was confirmed by the use of another predictive method (J. Garnier et al., J. Mol. Biol. 1978, 120, 97-120). In addition, two regions (residues 14-18 and 200-214), previously identified to contain binding sites for the B subunit as evidenced by chemical modification and monoclonal antibody studies, were found to be in alpha-helix configuration.

Location and amino acid sequence around the ADP-ribosylation site in the cholera toxin active subunit A1

Renatured, S-carboxymethylated subunit A1 of cholera toxin possess the ADP-ribose transferase activity (Lai, et.al., Biochem. Biophys. Res. Commun. 1981, 102, 1021). In the absence of acceptor self ADP-ribosylation of A1 subunit was observed. Stoicheometric incorporation of ADP-ribose moiety was achieved in 20 min at room temperature in a 0.1 - 0.2M PO4(Na) buffer, pH 6.6. On incubation of the complex with polyarginine, 75% of the enzyme-bound ADP-ribose moiety was transferred to the acceptor in 25 min. The ADP-ribosylated A1 was stable at low pH, and on cleavage with BrCN, the ADP-ribose moiety was found associated with peptide Cn I, the COOH-terminal fragment of A1 subunit. On further fragmentation with cathepsin D, a dodecapeptide containing ADP-ribose moiety was isolated whose structure was determined as: Asp-Glu-Glu-Leu-His-Arg-Gly-Tyr-Arg*-Asp-Arg-Tyr. The Arg* in the peptide was indicated to be the site of ADP-ribosylation.

Assay of mono ADP-ribosyltransferase activity by using guanylhydrazones

Guanylhydrazones of p-nitrobenzaldehyde and methylglyoxal serve as acceptors of ADP-ribosyl groups for the reactions catalyzed by cholera toxin. The absorption spectrum of the ADP-ribosylated p-nitrobenzylidine aminoguanidine is similar to that of a 1:1 mixture of ADP-ribose and p-nitrobenzylidine aminoguanidine. Results from fast atom bombardment mass spectrometry prove that the product is mono-ADP-ribosylated. ADP-ribosylation lowers the pKa of the p-nitrobenzylidine aminoguanidine by 0.7-0.8 pH unit. Assay methods are developed for measuring the ADP-ribosyltransferase reaction by following the rate of disappearance of p-nitrobenzylidine aminoguanidine by high-performance liquid chromatography or spectrophotometrically by monitoring the absorbance increase at 370 nm accompanying ADP-ribosylation of p-nitrobenzylidine aminoguanidine. The high-performance liquid chromatographic system can be utilized to measure ADP-ribosyltransferase activity in animal tissues. By using this procedure, the presence and quantitation of an ADP-ribosyltransferase in a homogenate of rabbit skeletal muscle is reported.

Presence of a dinucleotide fold in cholera toxin: possible approach to chemoprophylaxis?

The ADP-ribosyltransferase activity of the A1 subunit of cholera toxin is specifically inhibited by the dye cibacron blue 3GA. The presence of a 'dinucleotide fold' in the A1 subunit is thus established for the first time. This specific inhibition observed in vitro is successfully exploited in vivo for the inhibition of te diarrheal response brought out by the pure toxin in the rabbit ileal-loop test.

Conversion of a primary amine to a labeled secondary amine by the addition of phenolic group and radioiodination

To preserve the nucleophilicity of amino compounds during conjugative radioiodination, a new method for converting primary amines to phenolic secondary amines was developed. Amino acids were used as model compounds for establishing optimal conditions for the reductive amination. In the first step of the reaction, the aldehyde group of 4-hydroxybenzaldehyde (formylphenol) was reacted reversibly with an amino group to form an imine. The irreversible attachment of formylphenol to the amino group was accomplished by reduction of the imine with sodium cyanoborohydride. The pH optimum for the reaction was 5.0. Higher temperature has favorable effects on the rate and extent of the conjugation. Phenolic derivatives of amino compounds suitable for radioiodination are produced by the reactions described.

Repair of ultraviolet-light-induced DNA damage in vibrio cholerae

Repair of ultraviolet-light-induced DNA damage in a highly pathogenic Gram-negative bacterium, Vibrio cholerae, has been examined. All three strains of V. cholerae belonging to two serotypes, Inaba and Ogawa, are very sensitive to ultraviolet irradiation, having inactivation cross-sections ranging from 0.18 to 0.24 m2/J. Although these cells are proficient in repairing the DNA damage by a photoreactivation mechanism, they do not possess efficient dark repair systems. The mild toxinogenic strain 154 of classical Vibrios presumably lacks any excision repair mechanism and studies of irradiated cell DNA indicate that the ultraviolet-induced pyrimidine dimers may not be excised. Ultraviolet-irradiated cells after saturation of dark repair can be further photoreactivated.

Endogenous and cholera toxin-catalyzed ADP-ribosylation of a plasma membrane protein by RL-PR-C cloned rat hepatocytes

Cholera toxin catalyzed the ADP-ribosylation of a single plasma membrane protein (Mr 55 000) of both RL-PR-C rat hepatocytes and purified rat liver plasma membranes. Labeling of this protein from nicotinamide [2,8-3H]adenine dinucleotide was competitively inhibited by free arginine, but by no other amino acid tested, including lysine. The same protein was ADP-ribosylated from NAD+ endogenously, i.e., in the absence of toxin. This process was, however, not competitively inhibited by added arginine nor by any other amino acid tested lysine. Free ADP-ribose, even in 50-fold molar excess over the nicotinamide [2,8-3H]adenine dinucleotide substrate, did not reduce (by isotope dilution) the endogenous or cholera toxin-catalyzed labeling of the 55 000 dalton membrane protein. It is likely, therefore, that hepatocyte plasma membranes contain an ADP-ribosyltransferase, with a mechanism similar to that of the A subunit of cholera toxin, in that both transfer ADP-ribose to the same membrane protein and in that neither apparently produce free ADP-ribose as an intermediate. It is also clear that the acceptor residue in the 55 000 dalton protein is different for each process. Cholera toxin-catalyzed and endogenous transfer of ADP-ribose to the hepatocyte plasma membrane protein, in contrast to a pigeon erythrocyte system, required no cytosolic factors. The results indicate that ADP-ribosylation in cloned differentiated rat hepatocytes differs from that in pigeon erythrocytes in that the acceptor protein is larger (55 000 compared to 42 000 daltons), cytosolic factors are not required and transfer of ADP-ribose to the acceptor protein occurs endogenously.