Kinetic mechanisms of two NAD:arginine ADP-ribosyltransferases: the soluble, salt-stimulated transferase from turkey erythrocytes and choleragen, a toxin from Vibrio cholerae

A subunit of choleragen and an erythrocyte ADP-ribosyltransferase catalyze the transfer of ADP-ribose from NAD to proteins and low molecular weight guanidino compounds such as arginine. These enzymes also catalyze the hydrolysis of NAD to nicotinamide and ADP-ribose. The kinetic mechanism for both transferases was investigated in the presence and absence of the product inhibitor nicotinamide by using agmatine as the acceptor molecule. To obtain accurate estimates of kinetic parameters, the transferase and glycohydrolase reactions were monitored simultaneously by using [adenine-2,8-3H]NAD and [carbonyl-14C]NAD as tracer compounds. Under optimal conditions for the transferase assay, NAD hydrolysis occurred at less than 5% of the Vmax for ADP-ribosylation; at subsaturating agmatine concentrations, the ratio of NAD hydrolysis to ADP-ribosylation was significantly higher. Binding of either NAD or agmatine resulted in a greater than 70% decrease in affinity for the second substrate. All data were consistent with a rapid equilibrium random sequential mechanism for both enzymes.     

NAD-dependent ADP-ribosylation of arginine and proteins by Escherichia coli heat-labile enterotoxin.

Escherichia coli heat-labile enterotoxin (labile toxin, LT) catalyzed the hydrolysis of NAD to ADP-ribose and nicotinamide and the ADP-ribosylation of arginine (Moss, J., and Richardson, S.H. (1978) J. Clin. Invest. 62, 281-285). Analysis of the product of the ADP-ribosylation of arginine by nuclear magnetic resonance spectroscopy indicated that the reaction was stereospecific and resulted in the formation of alpha-ADP-ribosyl-L-arginine. This reaction product rapidly anomerized to yield a mixture of the alpha and beta forms. In the presence of [adenine-U-14C]NAD, E. coli enterotoxin catalyzed the transfer of the radiolabel to proteins; the ADP-ribosylation of proteins was inhibited by arginine methyl ester, an alternative substrate. Digestion of the 14C-protein with snake venom phosphodiesterase released predominantly 5'-AMP. No product was obtained with a mobility similar to that of 2'-(5'-phosphoribosyl)-5'-AMP. This result is consistent with the covalent attachment by the enterotoxin of ADP-ribose rather than poly(ADP-ribose) to protein. Thus, LT is catalytically equivalent to choleragen, an enterotoxin of Vibrio cholerae, and activates adenylate cyclase through a similar stereospecific ADP-ribosylation reaction.     

Mechanism of action of choleragen andE. coli heat-labile enterotoxin: activation of adenylate cyclase by ADP-ribosylation

Summary Choleragen exerts its effects on cells through the activation of adenylate cyclase. The initial event appears to be the binding of the B subunit of the toxin to ganglioside G_M1 on the cell surface, following which there is a delay prior to activation of adenylate cyclase. Patching and capping of the toxin on the cell surface, perhaps involved in the internalization of the enzymatically active subunit, may be occuring during this time. The activation of adenylate cyclase, which is catalyzed by the A₁ peptide of choleragen, does not require the B subunit or ganglioside G_M1. The A₁ peptide catalyzes the transfer of ADP-ribose from NAD to an amino acid, probably arginine, in a 42 000 dalton membrane protein. This protein appears to be the GTP-binding component (or G/F factor) of the adenylate cyclase system and is cruical to the regulation of cyclase activity by hormones such as epinephrine. ADP-ribosylation of the G/F factor is enhanced by GTP and, in some systems, by a cytosolic factor. GTP is also required for stabilization and optimal catalytic function of the choleragen-activated cyclase. Calmodulin, a calcium-binding protein, is necessary for expression of catalytic activity of the toxin-activated adenylate cyclase in brain and other tissues. The ADP-ribosyltransferase activity required for activation of the cyclase is an intrinsic property of the A₁ peptide of choleragen which is expressed only after the peptide is released from the holotoxin by reduction of a single disulfide bond. In the absence of cellular components, choleragen catalyzes the ADP-ribosylation of small guanidino compounds such as arginine as well as peptides and proteins that contain arginine. It is assumed, therefore, that the site of ADP-ribosylation in the natural acceptor protein is an arginine or similar amino acid. When guanidino compounds are not present as ADP-ribose acceptors, choleragen hydrolyzes NAD to ADP-ribose and nicotinamide at a considerably slower rate. E. coli heat-labile enterotoxin (LT) is very similar to choleragen in structure and function. It consists of two types of subunits, A and B, with sizes comparable to those of the A and B subunits of choleragen. Binding of LT to the cell surface is enhanced by prior incorporation of G_M1 but not other gangliosides; the oligosaccharide of G_M1 specifically interacts with LT and its B subunit. The A subunit of LT exhibits ADP-ribosyltransferase activity following activation by thiol to release the A₁ peptide. The A subunit of LT can be isolated in an ‘unnicked’ form and thus requires, in addition to reduction by a thiol, proteolytic cleavage to generate the active A₁ peptide. Like choleragen, LT uses guanidino compounds as model ADP-ribose acceptors and catalyzes the ADP-ribosylation of a 42 000 dalton protein in cell membrane prepatations. ADP-ribosyltransferases that use arginine as ADP-ribose acceptors are not restricted to bacterial systems; such an enzyme has been purified to apparent homogeneity (>500 000-fold) from turkey erythrocytes. Based on a subunit molecular weight of 28 000, its turnover number with arginine as the ADP-ribose acceptor is considerably higher than that of either toxin. Although with low molecular weight guanidino derivatives the substrate specificity of the enzyme is similar to that of choleragen, with protein substrates it clearly differs. The physiological role of the turkey erythrocyte transferase remains to be established.     

Amino acid-specific ADP-ribosylation. Sensitivity to hydroxylamine of [cysteine(ADP-ribose)]protein and [arginine(ADP-ribose)]protein linkages

Hydroxylamine stability has been used to classify (ADP-ribose)protein bonds into sensitive and resistant linkages, with the former representing (ADP-ribose)glutamate, and the latter, (ADP-ribose)arginine. Recently, it was shown that cysteine also serves as an ADP-ribose acceptor. The hydroxylamine stability of [cysteine([32P]ADP-ribose)]protein and [arginine([32P] ADP-ribose)]protein bonds was compared. In transducin, pertussis toxin catalyzes the ADP-ribosylation of a cysteine residue, whereas choleragen (cholera toxin) modifies an arginine moiety. The (ADP-ribose)cysteine bond formed by pertussis toxin was more stable to hydroxylamine than was the (ADP-ribose)arginine bond formed by choleragen. The (ADP-ribose)cysteine bond apparently represents a third class of ADP-ribose bonds. Pertussis toxin ADP-ribosylates the inhibitory guanyl nucleotide-binding regulatory protein (Gi) of adenylate cyclase, whereas choleragen modifies the stimulatory guanyl nucleotide-binding regulatory protein (Gs). These (ADP-ribose)protein linkages are identical in stability to those formed in transducin by the two toxins, consistent with the probability that cysteine and arginine are modified in Gi and Gs, respectively. Bonds exhibiting differences in hydroxylamine-stability were found in membranes from various non-intoxicated mammalian cells following incubation with [32P]NAD, which may reflect the presence of endogenous NAD:protein-ADP-ribosyl-transferases.     

Mechanism of action of choleragen

Choleragen exerts its effect on cells through activation of adenylate cyclase. Choleragen initially interacts with cells through binding of the B subunit of the toxin to the ganglioside G_M1 on the cell surface. Subsequent events are less clear. Patching or capping of toxin on the cell surface may be an obligatory step in choleragen action. Studies in cell-free systems have demonstrated that activation of adenylate cyclase by choleragen requires NAD. In addition to NAD, requirements have been observed for ATP, GTP, and calcium-dependent regulatory protein. GTP also is required for the expression of choleragen-activated adenylate cyclase. In preparations from turkey erythrocytes, choleragen appears to inhibit an isoproterenol-stimulated GTPase. It has been postulated that by decreasing the activity of a specific GTPase, choleragen would stabilize a GTP-adenylate cyclase complex and maintain the cyclase in an activated state. Although the holotoxin is most effective in intact cells, with the A subunit having 1/20th of its activity and the B subunit (choleragenoid) being inactive, in cell-free systems the A subunit, specifically the A₁ fragment, is required for adenylate cyclase activation. The B protomer is inactive. Choleragen, the A subunit, or A₁ fragment under suitable conditions hydrolyzes NAD to ADP-ribose and nicotinamide (NAD glycohydrolase activity) and catalyzes the transfer of the ADP-ribose moiety of NAD to the guandino group of arginine (ADP-ribosyltransferase activity). The NAD glycohydrolase activity is similar to that exhibited by other NAD-dependent bacterial toxins (diphtheria toxin, Pseudomonas exotoxin A), which act by catalyzing the ADP-ribosylation of a specific acceptor protein. If the ADP-ribosylation of arginine is a model for the reaction catalyzed by choleragen in vivo, then arginine is presumably an analog of the amino acid which is ADP-ribosylated in the acceptor protein. It is postulated that choleragen exerts its effects on cells through the NAD-dependent ADP-ribosylation of an arginine or similar amino acid in either the cyclase itself or a regulatory protein of the cyclase system.     

Inhibition of the transport of citrate during ADP-ribosylation of inner membrane proteins of the mitochondria

The ability of rat liver submitochondrial particles to catalyze NAD+ hydrolysis with a transfer of ADP-ribose residues to protein membranes has been demonstrated ADP-ribosylation is directly dependent on NAD+ concentration upon saturation with 1 mM NAD+ and is inhibited by physiological compounds (e.g., ATP, 10 mM; nicotinamide, 10 mM); besides, it is an artificial acceptor of ADP-ribose, arginine methyl ester. It was found that ADP-ribose is accepted by inner mitochondrial membrane protein, whose molecular masses amount to 25-30 kDa. The fact that 5'-AMP is a product of ADP-ribose degradation by snake venom phosphodiesterase suggests that the inner membrane vesiculate proteins are modified by mono(ADP-ribose). Covalent modification of membrane proteins by ADP-ribose leads to citrate transport inhibition in inner membrane vesicles the [14C]citrate uptake is significantly decreased thereby. The ability of ADP-ribosylation inhibitors to restore the citrate transport rate is suggestive of a direct regulatory effect of NAD+-dependent ADP-ribosylation on the activity of citrate-translocating system of inner mitochondrial membranes.     

Dependence of ADP-ribosylation of histones of chicken liver nuclei on the intranuclear content of NAD

The dependence of ADP-ribosylation of chicken liver nuclear histones on NAD concentration in the nuclei was studied under conditions of stimulation of coenzyme synthesis by the nicotinamide and nicotinic acid as well as upon addition of various concentrations of the [Ado-U-14C]NAD nuclei to the incubation mixture. In the first case, the rate of [Ado-U-14C]NAD incorporation into the histones was decreased due to the dilution of the label by the de novo synthesized NAD. The amount of the latter formed under effects of nicotinic acid and nicotinamide increased, correspondingly, from 2,2 X 10(-5) mmol up to 4.1 X 10(-5) and 7.0 X 10(-5) mmol per mg of nuclear protein. The incorporation of [Ado-U-14C]NAD into the histones decreased from 12.0 X 10(-8) mmol after incubation of liver slides with nicotinic acid and nicotinamide down to 8.0 X 10(-8) and 7.0 X 10(-8) mmol, respectively. With a rise in the concentration of exogenous [Ado-U-14C]NAD, the level of ADP-ribosylation of nuclear histones increased, the plot [14C]NAD incorporation at the labeled coenzyme concentration of 25 X 10(-7) mM/mg of histone had a plateau. Changes in the labeled substrate concentration brought about corresponding changes in the average length of the histone-linked poly-(ADP-ribose) chain.     

Stimulation of choleragen enzymatic activities by GTP and two soluble proteins purified from bovine brain

Choleragen (cholera toxin) activates adenylate cyclase by catalyzing ADP-ribosylation of Gs alpha, the stimulatory guanine nucleotide-binding protein. It was recently found (Tsai, S.-C., Noda, M., Adamik, R., Moss, J., and Vaughan, M. (1987) Proc. Natl. Acad. Sci. U. S. A. 84, 5139-5142) that a bovine brain membrane protein known as ADP-ribosylation factor or ARF, which enhances ADP-ribosylation of Gs alpha, also increases the GTP-dependent NAD:arginine and NAD:protein ADP-ribosyltransferase, NAD glycohydrolase, and auto-ADP-ribosylation activities of choleragen. We report here the purification and characterization of two soluble proteins from bovine brain that similarly enhance the Gs alpha-dependent and independent ADP-ribose transfer reactions catalyzed by toxin. Like membrane ARF, both soluble factors are 19-kDA proteins dependent on GTP or GTP analogues for activity. Maximal ARF effects were observed at a molar ratio of less than 2:1, ARF/toxin A subunit. Dimyristoyl phosphatidylcholine was necessary for optimal ADP-ribosylation of Gs alpha but inhibited auto-ADP-ribosylation of the choleragen A1 subunit and NAD:agmatine ADP-ribosyltransferase activity. It appears that the soluble factors directly activate choleragen in a GTP-dependent fashion. The relationships of the ARF proteins to the ras oncogene products and to the family of guanine nucleotide-binding regulatory proteins that includes Gs alpha remains to be determined.     

Role of the dinitrogenase reductase arginine 101 residue in dinitrogenase reductase ADP-ribosyltransferase binding, NAD binding, and cleavage

Dinitrogenase reductase is posttranslationally regulated by dinitrogenase reductase ADP-ribosyltransferase (DRAT) via ADP-ribosylation of the arginine 101 residue in some bacteria. Rhodospirillum rubrum strains in which the arginine 101 of dinitrogenase reductase was replaced by tyrosine, phenylalanine, or leucine were constructed by site-directed mutagenesis of the nifH gene. The strain containing the R101F form of dinitrogenase reductase retains 91%, the strain containing the R101Y form retains 72%, and the strain containing the R101L form retains only 28% of in vivo nitrogenase activity of the strain containing the dinitrogenase reductase with arginine at position 101. In vivo acetylene reduction assays, immunoblotting with anti-dinitrogenase reductase antibody, and [adenylate-(32)P]NAD labeling experiments showed that no switch-off of nitrogenase activity occurred in any of the three mutants and no ADP-ribosylation of altered dinitrogenase reductases occurred either in vivo or in vitro. Altered dinitrogenase reductases from strains UR629 (R101Y) and UR630 (R101F) were purified to homogeneity. The R101F and R101Y forms of dinitrogenase reductase were able to form a complex with DRAT that could be chemically cross-linked by 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide. The R101F form of dinitrogenase reductase and DRAT together were not able to cleave NAD. This suggests that arginine 101 is not critical for the binding of DRAT to dinitrogenase reductase but that the availability of arginine 101 is important for NAD cleavage. Both DRAT and dinitrogenase reductase can be labeled by [carbonyl-(14)C]NAD individually upon UV irradiation, but most (14)C label is incorporated into DRAT when both proteins are present. The ability of R101F dinitrogenase reductase to be labeled by [carbonyl-(14)C]NAD suggested that Arg 101 is not absolutely required for NAD binding.     

Evidence for a rabbit luteal ADP-ribosyltransferase activity which appears to be capable of activating adenylyl cyclase.

1. An ADP-ribosyltransferase activity which appears to be capable of activating adenylyl cyclase was identified in a plasma membrane fraction from rabbit corpora lutea and partially characterized by comparing the properties of the luteal transferase with those of cholera toxin. 2. Incubation of luteal membranes in the presence of GTP and varying concentrations of NAD resulted in concentration-dependent increases in adenylyl cyclase activity. 3. Stimulation of adenylyl cyclase by NAD and cholera toxin plus NAD was observed in the presence of GTP but not in the presence of guanosine-5'-O-(2-thiodiphosphate) or guanyl-5'-yl imidodiphosphate. 4. NAD or cholera toxin plus NAD reduced the Kact values for luteinizing hormone to activate adenylyl cyclase 3- to 3.5-fold. 5. NAD or cholera toxin plus NAD increased the extent to which cholate extracts from luteal membranes were able to reconstitute adenylyl cyclase activity in S49 cyc- mouse lymphoma membranes. 6. It was necessary to add ADP-ribose and arginine to the incubation mixture in order to demonstrate cholera toxin-specific ADP-ribosylation of a protein corresponding to the alpha subunit of the stimulatory guanine nucleotide-binding regulatory component (alpha Gs). 7. Treatment of luteal membranes with NAD prior to incubation in the presence of [32P]NAD plus cholera toxin resulted in reduced labeling of alpha Gs. 8. Endogenous ADP-ribosylation of alpha Gs was enhanced by Mg but was not altered by guanine nucleotide, NaF or luteinizing hormone and was inhibited by cAMP. 9. Incubation of luteal membranes in the presence of [32P]ADP-ribose in the absence and presence of cholera toxin did not result in the labeling of any membrane proteins.     

Inhibition of the GTPase activity of transducin by an NAD+:arginine ADP-ribosyltransferase from turkey erythrocytes.

The bacterial toxins, choleragen and pertussis toxin, inhibit the light-stimulated GTPase activity of bovine retinal rod outer segments by catalysing the ADP-ribosylation of the alpha-subunit (T alpha) of transducin [Abood, Hurley, Pappone, Bourne & Stryer (1982) J. Biol. Chem. 257, 10540-10543; Van Dop, Yamanaka, Steinberg, Sekura, Manclark, Stryer & Bourne (1984) J. Biol. Chem. 259, 23-26]. Incubation of retinal rod outer segments with NAD+ and a purified NAD+:arginine ADP-ribosyltransferase from turkey erythrocytes resulted in approx. 60% inhibition of GTPase activity. Inhibition was dependent on both enzyme and NAD+, and was potentiated by the non-hydrolysable GTP analogues guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG) and guanosine 5'-[beta gamma-methylene]triphosphate (p[CH2]ppG). The transferase ADP-ribosylated both the T alpha and T beta subunits of purified transducin. T alpha (39 kDa), after ADP-ribosylation, migrated as two distinct peptides with molecular masses of 42 kDa and 46 kDa on SDS/polyacrylamide-gel electrophoresis. T beta (36 kDa), after ADP-ribosylation, migrated as a 38 kDa peptide. With purified transducin subunits, it was observed that the GTPase activity of ADP-ribosylated T alpha, reconstituted with unmodified T beta gamma and photolysed rhodopsin, was decreased by 80%; conversely, reconstitution of T alpha with ADP-ribosyl-T beta gamma resulted in only a 19% inhibition of GTPase. Thus ADP-ribosylation of T alpha, the transducin subunit that contains the guanine nucleotide-binding site, has more dramatic effects on GTPase activity than does modification of the critical 'helper subunits' T beta gamma. To elucidate the mechanism of GTPase inhibition by transferase, we studied the effect of ADP-ribosylation on p[NH]pp[3H]G binding to transducin. It was shown previously that modification of transducin by choleragen, which like transferase ADP-ribosylates arginine residues, did not affect guanine nucleotide binding. ADP-ribosylation by the transferase, however, decreased p[NH]pp[3H]G binding, consistent with the hypothesis that choleragen and transferase inhibit GTPase by different mechanisms.     

Amino acid specific ADP-ribosylation: substrate specificity of an ADP-ribosylarginine hydrolase from turkey erythrocytes

An ADP-ribosylarginine hydrolase, which catalyzes the degradation of ADP-ribosyl[14C]arginine to ADP-ribose plus arginine, was separated by ion exchange, hydrophobic, and gel permation chromatography from NAD:arginine ADP-ribosyltransferases, which are responsible for the stereospecific formation of alpha-ADP-ribosylarginine. As determined by NMR, the specific substrate for the hydrolase was alpha-ADP-ribosylarginine, the product of the transferase reaction. The ADP-ribose moiety was critical for substrate recognition; (phosphoribosyl) [14C]arginine and ribosyl[14C]arginine were poor substrates and did not significantly inhibit ADP-ribosyl[14C]arginine degradation. In contrast, ADP-ribose was a potent inhibitor of the hydrolase and significantly more active than ADP greater than AMP greater than adenosine. In addition to ADP-ribosyl[14C]arginine, both ADP-ribosyl[14C]guanidine and (2'-phospho-ADP-ribosyl)[14C]arginine were also substrates; at pH greater than 7, ADP-ribosyl[14C]guanidine was degraded more readily than the [14C]arginine derivative. Neither arginine, guanidine, nor agmatine, an arginine analogue, was an effective hydrolase inhibitor. Thus, it appears that the ADP-ribosyl moiety but not the arginine group is critical for substrate recognition. Although the hydrolase requires thiol for activity, dithiothreitol accelerated loss of activity during incubation at 37 degrees C. Stability was enhanced by Mg2+, which is also necessary for optimal enzymatic activity. The findings in this paper are consistent with the conclusion that different enzymes catalyze ADP-ribosylarginine synthesis and degradation. Furthermore, since the hydrolase and transferases possess a compatible stereospecificity and substrate specificity, it would appear that the two enzymatic activities may serve as opposing arms in an ADP-ribosylation cycle.     

Inactivation of glutamine synthetases by an NAD:arginine ADP-ribosyltransferase

Glutamine synthetase from ovine brain has a critical arginine residue at the catalytic site (Powers, S. G., and Riordan, J.F. (1975) Proc. Natl. Acad. Sci. U.S. A. 72, 2616-2620). This enzyme is now shown to be a substrate for a purified NAD:arginine ADP-ribosyltransferase from turkey erythrocyte cytosol that catalyzes the transfer of ADP-ribose from NAD to arginine and purified proteins. The transferase catalyzed the inactivation of the synthetase in an NAD-dependent reaction; ADP-ribose and nicotinamide did not substitute for NAD. Agmatine, an alternate ADP-ribose acceptor in the transferase-catalyzed reaction, prevented inactivation of glutamine synthetase. MgATP, a substrate for the synthetase which was previously shown to protect that enzyme from chemical inactivation, also decreased the rate of inactivation in the presence of NAD and ADP-ribosyltransferase. Using [32P]NAD, it was observed that approximately 90% inactivation occurred following the transfer of 0.89 mol of [32P]ADP-ribose/mol of synthetase. The erythrocyte transferase also catalyzed the NAD-dependent inactivation of glutamine synthetase purified from chicken heart; 0.60 mol of ADP-ribose was transferred per mol of enzyme, resulting in a 95% inactivation. As noted with the ovine brain enzyme, agmatine and MgATP protected the chicken synthetase from inactivation and decreased the extent of [32P]ADP-ribosylation of the synthetase. These observations are consistent with the conclusion that the NAD:arginine ADP-ribosyltransferase modifies specifically an arginine residue involved in the catalytic site of glutamine synthetase. Although the transferase can use numerous proteins as ADP-ribose acceptors, some characteristics of this particular arginine, perhaps the same characteristics that are involved in its function in the catalytic site, make it a favored ADP-ribose acceptor site for the transferase.     

Metabolic fate of 14C-labelled nicotinamide and adenine in germinating propagules of the mangrove Bruguiera gymnorrhiza

We studied the metabolic fate of [carbonyl-14C]nicotinamide and [8-(14)C]adenine in segments taken from young and developing leaves, stem, hypocotyls, and roots of a shoot-root type emerging propagule of the mangrove plant Bruguiera gymnorrhiza. Thin-layer chromatography was used together with a bioimaging analyser system. During 4 h of incubation, incorporation of radioactivity from [carbonyl-14C]nicotinamide into NAD and trigonelline was found in all parts of the propagules; the highest incorporation rates into NAD and trigonelline were found in newly emerged stem and young leaves, respectively. Radioactivity from [8-(14)C]adenine was distributed mainly in the salvage products (adenine nucleotides and RNA), and incorporation was less in catabolites (allantoin, allantoic acid, and CO2). Adenine salvage activity was higher in young leaves and stem than in hypocotyls and roots. Over a short time, the effect of 500 mM NaCl on nicotinamide and adenine metabolism indicated that NaCl inhibits both salvage and degradation activities in roots.     

Stimulation by D-glucose of the direct conversion of arginine to citrulline in enterocytes isolated from pig jejunum

In enterocytes isolated from pig jejunum, L-arginine is metabolized to L-citrulline either directly or indirectly through the sequence of reactions catalysed by arginase and ornithine transcarbamylase. In the presence of 5 mM D-glucose, the direct conversion of 1mM L-[guanido-14C] arginine to L-citrulline was increased more than 4 times. Isolated enterocytes exhibit a high glycolytic capacity. Furthermore, the decarboxylation of 5mM D-[1-14C] glucose was 3.6 fold higher than the decarboxylation of 5 mM D-[6-14C] glucose which suggests the presence of a pentose phosphate pathway in enterocytes. Since the production of labelled L-citrulline from L-[guanido-14C] arginine in pig enterocyte homogenates was markedly increased in the presence of NADPH, it is proposed that the direct conversion of L-arginine to L-citrulline could be stimulated by the production of NADPH from D-glucose in the pentose phosphate pathway.     

Characterization of the endogenous ADP-ribosylation of wild-type and mutant elongation factor 2 in eukaryotic cells.

Anti-[ADP-ribosylated elongation factor 2 (EF-2)] antiserum has been used to immunoprecipitate the modified form of EF-2 from polyoma-virus-transformed baby hamster kidney (pyBHK) cells [Fendrick, J. L. & Iglewski, W. J. (1989) Proc. Natl Acad. Sci. USA 86, 554-557]. This antiserum also immunoprecipitates a 32P-labelled protein of similar size to EF-2 from a variety of primary and continuous cell lines derived from many species of animals. One of these cell lines, chinese hamster ovary CHO-K1 cells was further characterized. The time course of labelling of ADP-ribosylated EF-2 with [32P]orthophosphate was similar in pyBHK cells and in CHO-K1 cells. The kinetics of labelling were more rapid for cells cultured in 2% serum than 10% serum, with incorporation of 32P reaching a maximum at 6 h and 10 h, respectively. EF-2 mutants of pyBHK and CHO-K1 cells resistant to diphtheria-toxin-catalyzed ADP-ribosylation of EF-2 remain sensitive to cellular ADP-ribosylation of EF-2. The 32P-labelled moiety of ADP-ribosylated EF-2 was digested by snake venom phosphodiesterase and the product was identified as AMP. The same 32P-labelled tryptic peptide was modified by toxin in wild-type EF-2 and by the cellular transferase in mutant EF-2. When purified EF-2 from pyBHK cells was incubated with [carbonyl-14C]nicotinamide and diphtheria toxin fragment A, under conditions for reversal of the ADP-ribosylation reaction, [14C]NAD was generated. The results suggest that cellular ADP-ribosylated EF-2 exists in a variety of cell types, and the ribosylated product is identical to that produced by toxin ADP-ribosylation of EF-2, except in diphthamide mutant cells. Studies with the mutant cell lines indicate that the toxin and the cellular transferase, however, recognize different determinants at the ADP-ribose acceptor site in EF-2. The cellular transferase does not require the diphthamide modification of the histidine ring in the amino acid sequence of EF-2 for the transfer of ADP-ribose to the ring. Therefore, we would expect the cellular transferase active site to be similar to, but not identical to, the critical amino acids demonstrated in the active site of diphtheria toxin and Pseudomonas exotoxin A.     

ADP-ribosyltransferase from hen liver nuclei. Purification and characterization

Chromatin-bound ADP-ribosyltransferase from adult hen liver nuclei was purified to a homogeneous state through salt extraction, gel filtration, hydroxyapatite, phenyl-Sepharose, Cm-cellulose, and DNA-Sepharose. The ADP-ribosyltransferase has a pH optimum at 9.0 and does not require DNA for reaction. The purified enzyme has a molecular weight of 27,500 +/- 500. Agmatine sulfate, arginine methyl ester, histones, and casein proved to be effective acceptors for the ADP-ribose molecule. Among histones, H3 was most active, followed by H2a, H4, and H2b, in that order, the lowest activity seen with H1. With all the acceptors tested, the rate of nicotinamide release was in excess of the ADP-ribosylation. However, changes in the ratio of nicotinamide release to ADP-ribosylation seemed to depend on concentrations of the acceptor used. ADP-ribose-whole histones X adducts formed by ADP-ribosyltransferase served as initiators for poly(ADP-ribose) synthesis when these adducts were incubated in the presence of NAD, DNA, Mg2+, and the purified poly(ADP-ribose) synthetase, in which poly(ADP-ribose) formation can occur.     

Identity of V factor in culture medium used for prior growth of two strains of Staphylococcus aureus.

Staphylococcus aureus ATCC 12598 and ATCC 25923 were starved of pyridine nucleotides and precursors and then grown in a semidefined medium containing [carbonyl-14C]nicotinamide. Samples of medium from late-exponential-phase and stationary-phase cultures were analyzed for 14C-metabolites. In all cases, V factor was present primarily as NAD.     

Effect of oxidative stress on in vivo ADP-ribosylation of eukaryotic elongation factor 2

Different lines of evidence indicate that eukaryotic elongation factor 2 (eEF2) can be ADP-ribosylated endogenously. The physiological significance of this reaction has, however, remained unclarified. In order to address this issue we investigated the in vivo ADP-ribosylation of eEF2 and the effect of oxidative stress thereon. The investigation revealed that the endogenous ADP-ribosylation of eEF2 is complex and can take place in K562 cell lysates either under the action of endogenous transferase from [adenosine-14C]NAD or by direct binding of free [14C]ADP-ribose. These two types of ADP-ribosylation were distinguished by use of different treatments based on the chemical stability of the respective bonds formed. Under standard culture conditions, in vivo labeling of eEF2 in the presence of [14C]adenosine was reversed to about 65% in the presence of diphtheria toxin and nicotinamide. This finding implied that the modification that took place under physiological circumstances was, mainly, of an enzymic nature. On the other hand, H2O2-promoted oxidative stress gave rise to a nearly two-fold increase in the extent of in vivo labeling of eEF2. This was accompanied by a loss of eEF2 activity in polypeptide chain elongation. Oxidative stress specifically inhibited the subsequent binding of free ADP-ribose to eEF2. The results thus provide evidence that endogenous ADP-ribosylation of eEF2 can also take place by the binding of free ADP-ribose. This nonenzymic reaction appears to account primarily for in vivo ADP-ribosylation of eEF2 under oxidative stress.     

Pyridine nucleotide cycling and poly(ADP-ribose) synthesis in resting human lymphocytes

NAD is a critical cofactor for the oxidation of fuel molecules. The exposure of human PBL to agents that cause DNA strand breaks to accumulate can deplete NAD pools by increasing NAD consumption for poly(ADP-ribose) formation. However, the pathways of NAD synthesis and degradation in viable PBL have not been carefully documented. The present experiments have used radioactive labeling techniques to trace the routes of NAD metabolism in resting PBL. The cells could generate NAD from either nicotinamide or nicotinic acid. PBL incubated with [14C]nicotinic acid excreted [14C]nicotinamide into the medium. Approximately 50% of a prelabeled [14C]NAD pool was metabolized during 6 to 8 hr in tissue culture. Basal NAD turnover was prolonged threefold to fourfold by 3-aminobenzamide (3-ABA), an inhibitor of poly(ADP-ribose) synthetase. Supplementation of the medium with 3-ABA also prevented the accelerated NAD degradation that ensued after exposure of PBL to deoxyadenosine plus deoxycoformycin at concentrations previously shown to cause DNA strand break accumulation. These results demonstrate that quiescent human PBL continually produce NAD and utilize the nucleotide for poly(ADP-ribose) synthesis.