Nicotinamide 2-fluoroadenine dinucleotide unmasks the NAD+ glycohydrolase activity of Aplysia californica adenosine 5'-diphosphate ribosyl cyclase

ADP-ribosyl cyclases catalyze the transformation of nicotinamide adenine dinucleotide (NAD+) into the calcium-mobilizing nucleotide second messenger cyclic adenosine diphosphoribose (cADP-ribose) by adenine N1-cyclization onto the C-1' ' position of NAD+. The invertebrate Aplysia californica ADP-ribosyl cyclase is unusual among this family of enzymes by acting exclusively as a cyclase, whereas the other members, such as CD38 and CD157, also act as NAD+ glycohydrolases, following a partitioning kinetic mechanism. To explore the intramolecular cyclization reaction, the novel nicotinamide 2-fluoroadenine dinucleotide (2-fluoro-NAD+) was designed as a sterically very close analogue to the natural substrate NAD+, with only an electronic perturbation at the critical N1 position of the adenine base designed to impede the cyclization reaction. 2-Fluoro-NAD+ was synthesized in high yield via Lewis acid catalyzed activation of the phosphoromorpholidate derivative of 2-fluoroadenosine 5'-monophosphate and coupling with nicotinamide 5'-monophosphate. With 2-fluoro-NAD+ as substrate, A. californica ADP-ribosyl cyclase exhibited exclusively a NAD+ glycohydrolase activity, catalyzing its hydrolytic transformation into 2-fluoro-ADP-ribose, albeit at a rate ca. 100-fold slower than for the cyclization of NAD+ and also, in the presence of methanol, into its methanolysis product beta-1' '-O-methyl 2-fluoro-ADP-ribose with a preference for methanolysis over hydrolysis of ca. 100:1. CD38 likely converted 2-fluoro-NAD+ exclusively into the same product. We conclude that A. californica ADP-ribosyl cyclase can indeed be classified as a multifunctional enzyme that also exhibits a classical NAD+ glycohydrolase function. This alternative pathway that remains, however, kinetically cryptic when using NAD+ as substrate can be unmasked with a dinucleotide analogue whose conversion into the cyclic derivative is blocked. 2-Fluoro-NAD+ is therefore a useful molecular tool allowing dissection of the kinetic scheme for this enzyme.     

Probing Aplysia californica adenosine 5'-diphosphate ribosyl cyclase for substrate binding requirements: design of potent inhibitors.

Readily synthesized nicotinamide adenine dinucleotide (NAD(+)) analogues have been used to investigate aspects of the cyclization of NAD(+) to cyclic adenosine 5'-O-diphosphate ribose (cADPR) catalyzed by the enzyme adenosine 5'-O-diphosphate (ADP) ribosyl cyclase and to produce the first potent inhibitors of this enzyme. In all cases, inhibition of Aplysia californica cyclase by various substrate analogues was found to be competitive while inhibition by nicotinamide exhibited mixed-behavior characteristics. Nicotinamide hypoxanthine dinucleotide (NHD(+)), nicotinamide guanine dinucleotide (NGD(+)), C1'-m-benzamide adenine dinucleotide (Bp(2)A), and C1'-m-benzamide nicotinamide dinucleotide (Bp(2)N) were found to be nanomolar potency inhibitors with inhibition constants of 70, 143, 189, and 201 nM, respectively. However, NHD(+) and NGD(+) are also known substrates and are slowly converted to cyclic products, thus preventing their further use as inhibitors. The symmetrical bis-nucleotides, bis-adenine dinucleotide (Ap(2)A), bis-hypoxanthine dinucleotide (Hp(2)H), and bis-nicotinamide dinucleotide (Np(2)N), exhibited micromolar competitive inhibition, with Ap(2)A displaying the greatest affinity for the enzyme. 2',3'-Di-O-acetyl nicotinamide adenine dinucleotide (AcONAD(+)) was not a substrate for the A. californica cyclase but also displayed some inhibition at a micromolar level. Finally, inhibition of the cyclase by adenosine 5'-O-diphosphate ribose (ADPR) and inosine 5'-O-diphosphate ribose (IDPR) was observed at millimolar concentration. The nicotinamide aromatic ring appears to be the optimal motif required for enzymatic recognition, while modifications of the 2'- and 3'-hydroxyls of the nicotinamide ribose seem to hamper binding to the enzyme. Stabilizing enzyme/inhibitor interactions and the inability of the enzyme to release unprocessed material are both considered to explain nanomolar inhibition. Recognition of inhibitors by other ADP ribosyl cyclases has also been investigated, and this study now provides the first potent nonhydrolyzable sea urchin ADP ribosyl cyclase and cADPR hydrolase inhibitor Bp(2)A, with inhibition observed at the micromolar and nanomolar level, respectively. The benzamide derivatives did not inhibit CD38 cyclase or hydrolase activity when NGD(+) was used as substrate. These results emphasize the difference between CD38 and other enzymes in which the cADPR cyclase activity predominates.     

NMR solution structure and conformational analysis of the calcium release agent cyclic adenosine 5'-diphosphate ribose (cADPR)

A high-resolution NMR study of the solution structure of the calcium release agent cADPR has been performed. Pseudorotationals analysis reveals that in solution both sugar rings in cADPR adopt predominantly (approximately 75%) South conformations, with the A and N rings adopting approximately 2T3 (C2'-endo(major)-C3'-exo(minor) and 4(3)T (C3'-exo-C4'-endo) conformations, respectively. The backbone torsion angles beta and gamma have also been determined. While the minor North conformers were not observed in the crystal structure of cADPR, the solution values of the major South conformers compare well to those found in crystal structure.     

Mapping adenosine cyclic 3',5'-phosphate binding sites on type I and type II adenosine cyclic 3',5'-phosphate dependent protein kinases using ribose ring and cyclic phosphate ring analogues of adenosine cyclic 3',5'-phosphate

A series of adenosine cyclic 3',5'-phosphate (cAMP) derivatives containing modifications or substitutions in either the 2',3',4', or 5' position or the phosphate were examined for their abilities to activate type I isozymes of cAMP-dependent protein kinase (PK I) from rabbit or porcine skeletal muscle and type II isozymes of cAMP-dependent protein kinase (PK II) from bovine brain and heart. The studies revealed that the activation of both PK I and PK II isozymes requires a 2'-hydroxyl group in the ribo configuration, a 3' oxygen in the ribo configuration, and a charged cyclic phosphate. The two isozymes appeared to differ in those portions of their respective cAMP-binding sites that are adjacent to the 4' position of the ribose ring and the 3' position, 5' position, and phosphate portion of the cyclic phosphate ring.     

Exploring human adenosine A3 receptor complementarity and activity for adenosine analogues modified in the ribose and purine moiety

In this paper we investigated the influence on affinity, selectivity and intrinsic activity upon modification of the adenosine agonist scaffold at the 3'- and 5'-positions of the ribofuranosyl moiety and the 2- and N6-positions of the purine base. This resulted in the synthesis of various analogues, that is, 3-12 and 24-33, with good hA3AR selectivity and moderate-to-high affinities (as in 32, Ki=27 nM). Interesting was the ability to tune the intrinsic activity depending on the substituent introduced at the 3'-position.     

Possible role of adenosine cyclic 3':5'-monophosphate phosphodiesterase in the morphological transformation of Chinese hamster ovary cells mediated by N6,O2-dibutyryl adenosine cyclic 3':5"-monophosphate.

We have demonstrated that in Chinese hamster ovary (CHO) cells, N6,O2'-dibutyryl adenosine cyclic 3':5'-monophosphate (dibutyryl cyclic AMP) has a remarkable morphogenetic effect in converting cells of a compact, epithelial-like morphology into a spindle-shaped, fibroblast-like form. Homogenates of CHO cells were found to contain two adenosine cyclic 3':5'-monophosphate (cyclic AMP) phosphodiesterase (EC 3.1.4.c) activities, which differ in apparent Km with respect to their substrate, cyclic AMP. These were designated cyclic AMP phosphodiesterase I, with a low Km of 2 to 5 muM and cyclic AMP phosphodiesterase II, with a high Km of 1 to 3 mM. Cyclic AMP phosphodiesterase I was competitively inhibited by N6-monobutyryl and dibutyryl cyclic AMP, with apparent Ki values of 40 to 60 muM and 0.25 to 0.35 mM, respectively. Experimental evidence demonstrates that the effect of exogenous dibutyryl cyclic AMP on cell morphology is a result of an increase in the endogenous level of cyclic AMP. This increase appears to be due largely to the inhibitory action of intracellular N6-monobutyryl cyclic AMP on cyclic AMP phosphodiesterase I, which results in a decreased rate of degradation of intracellular cyclic AMP.     

AdoHcy hydrolase of Trichomonas vaginalis: studies of the effects of 5'-modified adenosine analogues and related 6-N-cyclopropyl derivatives

Trypanosoma brucei and Trichomonas vaginalis are both parasitic protozoans that are known to share many similar biochemical pathways. Aristeromycin, as well as 5'-iodovinyl and 5'-oxime analogues of adenosine, are potent inhibitors of AdoHcy hydrolase in T. brucei, an enzyme that catalyses the hydrolysis of AdoHcy to adenosine and L-homocysteine. To help determine the role of this enzyme in T. vaginalis, we have tested a library of 5'-modified adenosine derivatives, including 5'-deoxy-5'-(iodomethylene)-adenosine and related 6-N-cyclopropyl analogues. Our results indicate that these inhibitors are effective at inhibiting the growth of T. vaginalis, by as much as 95%.     

Functional characterization of a novel serotonin receptor (5-HTap2) expressed in the CNS of Aplysia californica

Serotonin has been shown to be a neuromodulator in the Aplysia californica CNS. The diversity of serotonin actions is due to the existence of several different receptor subtypes. In this study we report the cloning of a full-length cDNA, coding for a novel serotonin receptor (5-HTap2). The receptor protein bears the characteristics of G protein-coupled receptors. It shares 68% and 34% of its amino acid sequence identity with the 5-HTlym receptor from Lymnaea stagnalis and the mammalian 5-HT1A receptor, respectively. When transfected in HEK 293 cells, 5-HTap2 was negatively coupled to adenylate cyclase. Ligand binding analysis indicated that the order of potencies of various drugs for the inhibition of [3H]LSD binding was: methiothepin > metergoline > 5-CT > PAPP > 5-HT > ketanserin > NAN-190 > 8-OH-DPAT > clozapine. RT-PCR amplification of RNA isolated from different tissues indicated that this receptor is expressed in the CNS and in bag cells. The expression of 5-HTap2 restricted to the CNS suggests an important role for this receptor in the modulation of neuronal functions in Aplysia. Moreover, the high expression of 5-HTap2 in the bag cells, associated with its pharmacological profile, suggests that this receptor may be implicated in modulating the afterdischarge during the egg-laying behavior.     

Regulation of SIRT 1 mediated NAD dependent deacetylation: a novel role for the multifunctional enzyme CD38

The SIRT 1 enzyme is a NAD dependent deacetylase implicated in ageing, cell protection, and energy metabolism in mammalian cells. How the endogenous activity of SIRT 1 is modulated is not known. The enzyme CD38 is a multifunctional enzyme capable of synthesis of the second messenger, cADPR, NAADP, and ADPR. However, the major enzymatic activity of CD38 is the hydrolysis of NAD. Of particular interest is the fact that CD38 is present on the inner nuclear membrane. Here, we investigate the modulation of the SIRT 1 activity by CD38. We propose that by modulating availability of NAD to the SIRT1 enzyme, CD38 may regulate SIRT1 enzymatic activity. We observed that in CD38 knockout mice, tissue levels of NAD are significantly increased. We also observed that incubation of purified recombinant SIRT1 enzyme with CD38 or nuclear extracts of wild-type mice led to a significant inhibition of its activity. In contrast, incubation of SIRT1 with cellular extract from CD38 knockout mice was without effect. Furthermore, the endogenous activity of SIRT1 was several time higher in nuclear extracts from CD38 knockout mice when compared to wild-type nuclear extracts. Finally, the in vivo deacetylation of the SIRT1 substrate P53 is increased in CD38 knockout mice tissue. Our data support the novel concept that nuclear CD38 is a major regulator of cellular/nuclear NAD level, and SIRT1 activity. These findings have strong implications for understanding the basic mechanisms that modulate intracellular NAD levels, energy homeostasis, as well as ageing and cellular protection modulated by the SIRT enzymes.     

Synthesis of 5'-substituted fluoro-neplanocin A analogues: importance of a hydrogen bonding donor at 5'-position for the inhibitory activity of S-adenosylhomocysteine hydrolase

Four 5'-substituted fluoro-neplanocin A analogues 1a-d were designed and synthesized, using cyclopentenone derivative 2 as a key intermediate. The inhibitory activity against SAH was in the following order: NH(2)>SH>F, N(3), indicating a hydrogen bonding donor such as OH or NH(2) was essential for inhibitory activity. All the final compounds showed much less decreased cytotoxicity in two cancer cell lines (Col2 and A549), implying that phosphorylation of the 5'-hydroxyl group of fluoro-neplanocin A is closely related to its high cytotoxicity.     

Evidence for a role for cellular alkalinization in the cyclic adenosine 3',5'-monophosphate-mediated initiation of motility in bovine caput spermatozoa

Bicarbonate ion, the local anesthetics procaine and dibucaine, and the ionophores monensin and nigericin have been shown to markedly increase the ability of agents that elevate cyclic adenosine monophosphate (cAMP) levels to initiate motility in bovine caput spermatozoa. A number of other weak bases, including theophylline, D-600 and dipyridamole, elevate cAMP levels maximally in caput sperm at low levels but induce motility only at high levels. These compounds thus appear to have a dual role in the initiation of motility, i.e., they elevate both cAMP levels and internal pH. Confirmation of this view was provided by the demonstration that bicarbonate ion and procaine permit initiation of motility by theophylline, D-600 and dipyridamole at markedly reduced levels. Also, forskolin (a neutral adenylate cyclase activator) elevates cyclic AMP levels in caput sperm but initiates motility only in the presence of bicarbonate or procaine, and the membrane-permeant cAMP analogue 8-bromo-cAMP is capable of inducing motility only in the presence of bicarbonate. Thus, motility in caput sperm is induced only under conditions that elevate both intracellular cAMP and pH, whereas caudal sperm motility is stimulated by an elevation of either cAMP or pH. These data suggest that the epididymal development of motility requires a maturational increase in internal pH. This suggestion was confirmed by direct measurement of the internal pH of caput and caudal sperm; the internal pH of the former was found to be 5.84 +/- 0.1 and the latter 6.27 +/- 0.05.     

Aspartyl peptide labeled by 2-(4-bromo-2,3-dioxobutylthio)adenosine 5'-diphosphate in the allosteric ADP site of pig heart NAD+-dependent isocitrate dehydrogenase

The nucleotide affinity label 2-(4-bromo-2,3-dioxobutylthio)adenosine 5'-diphosphate (2-BDB-TADP) reacts covalently with pig heart NAD+-dependent isocitrate dehydrogenase with a limiting value of 75% inactivation and loss of ADP activation concomitant with incorporation of about 1 mol of reagent/mol of average enzyme subunit (Huang, Y.-C., Bailey, J. M., and Colman, R. F. (1986) J. Biol. Chem. 251, 14100-14107). Complete protection against the functional changes is provided by ADP + Mn2+, and reagent incorporation is decreased to about 0.5 mol/mol of average enzyme subunit. We have now identified the critical modified peptide by comparison of the peptides labeled by 2-BDB-TADP at pH 6.8 in the absence and presence of ADP + Mn2+. After removal of excess reagent, modified enzyme was treated with [3H]NaBH4 to reduce the keto groups of the reagent and introduce a radioactive tracer into the reagent which is covalently linked to the protein. Following carboxymethylation and digestion with trypsin, the specific modified peptide was isolated using two successive high performance liquid chromatography steps: 1) 0.1% trifluoroacetic acid with an acetonitrile gradient; and 2) 20 mM ammonium acetate, pH 5.8, with an acetonitrile gradient. Gas phase sequencing gave the modified peptide Leu-Gly-Asp-Gly-Leu-Phe-Leu-Gln in which aspartic acid is the target of 2-BDB-TADP. Isolation of the corresponding tryptic peptide from unmodified enzyme yielded the sequence Leu-Gly-Asp-Gly-Leu-Phe-Leu-Gln-CmCys-CmCys-Lys. Isocitrate dehydrogenase is composed of three distinct subunits (alpha, beta, and gamma), separable by chromatofocusing in urea and identified by analytical gel isoelectric focusing. The evidence indicates that the specific peptide labeled by 2-BDB-TADP, which is at or near the ADP site, can be derived from the gamma subunit.     

The cyclic GMP-induced inward current in neuron R14 of Aplysia californica: similarity to a FMRFamide-induced inward current

Injecting cGMP into Aplysia neuron R14 induced an inward current similar to one elicited by application of FMRFamide to the outside of that cell. In contrast, injection of cAMP into R14 caused a long-lasting outward current and conductance increase. Phosphodiesterase inhibitors increased the cGMP and FMRFamide-induced inward currents in R14. The cGMP-induced inward current is voltage dependent and is largely carried by Na+. It is also strongly and inversely dependent on both external [Ca2+] and [Cl-], although these ions are not significant current carriers. Changing external [K+] had no effect. Voltage and ion dependencies of the cGMP-induced inward current are similar to those of an inward current induced by FMRFamide. Thus cGMP may be a second messenger to FMRFamide in producing a slow inward current in R14. cGMP does not appear to be a second messenger to FMRFamide in most Aplysia neurons.     

Kinetic mechanism of the adenosine 3',5'-monophosphate dependent protein kinase catalytic subunit in the direction of magnesium adenosine 5'-diphosphate phosphorylation.

In order to define the overall kinetic mechanism of adenosine 3',5'-monophosphate dependent protein kinase catalytic subunit and also to elaborate the kinetic mechanism in the direction of peptide phosphorylation, we have determined its kinetic mechanism in the direction of MgADP phosphorylation. Studies of initial velocity as a function of uncomplexed Mg2+ (Mgf) in the absence and presence of dead-end inhibitors were used to define the kinetic mechanism. Data are consistent with the overall kinetic mechanism in the direction of MgADP phosphorylation being random with both the pathways allowed, i.e., the pathway in which MgADP binds to the enzyme prior to phosphorylated peptide and the pathway in which phosphorylated peptide binds to enzyme prior to MgADP. In addition, depending on the concentration of Mgf, one or the other pathway predominates. At low (0.5 mM) Mgf, the mechanism is steady-state ordered with the pathway in which phosphorylated peptide binds first being preferred; at high (10 mM) Mgf, the kinetic mechanism is equilibrium ordered, and the pathway in which MgADP binds first is preferred. This change in mechanism to equilibrium ordered at higher concentration of Mgf is due to an increase in affinity of the enzyme for MgADP and a decrease in affinity for the phosphorylated peptide. The Haldane relationship gives a Keq of 2 +/- 1 x 10(3) at pH 7.2, in agreement with the values obtained from 31P NMR (1.6 +/- 0.8 x 10(3)) and direct determination of reactant concentrations at equilibrium (3.5 +/- 0.6 x 10(3)).     

2',3'-isopropylidene group, a molecular scaffold to study the activity of adenosine and adenylate deaminase on adenosine analogues modified in the ribose moiety

2 ',3 '-Isopropylidene group can be used as a molecular scaffold for the introduction of modifications at 5 ' and 1 ' positions of adenosine and these modified nucleosides are used to evaluate the biocatalytic activity of adenosine and adenylate deaminase.     

Differential binding of cyclic adenosine 3'' ,5''-monophosphate to the cyclic adenosine 3'' ,5''-monophosphate receptor protein in Escherichia coli.

Binding of cyclic adenosine 3' ,5'-monophosphate (cAMP) by the cAMP receptor protein in crude cell-free extracts of Escherichia coli was characterized. When cell were grown in glucose, binding was inhibited 50% relative to extracts from cells grown with succinate as carbon source . This inhibition could be relieved by dialysis.