ADP-ribose cyclase

The intracellular NAD glycohydrolase is now known as ADP-ribose cyclase there is also a cell surface ectozyme, identical with the lymphocyte CD38 antigen. CD38 also occurs intracellularly, in endosomes. Both enzymes catalyze the formation of both cADP-ribose and NAADP, as well as the glycohydrolase reaction. The cyclase reaction predominates at neutral pH, and the nicotinamide/nicotinic acid exchange reaction at acid pH, suggesting that in cytosol the main product of the soluble enzyme is cADP-ribose. In endosomes,... 

AU-trans-retinoic acid (Section 2.2.S.2) stimulates the synthesis of cADP-ribose in kidney cells in culture, apparendy as a result of the induction of CD38 (Beers et al., 1995 Takahashi et al., 1995) in ovariectomized rats, estradiol induces cytosolic ADP-ribosyl cyclase in the uterus, but not in estrogen unresponsive tissues (Chini et al., 1997). If this induction of ADP-ribose cyclase by estrogens leads to significant depletion of nicotinamide nucleotides, it may provide an additional explanation for the 2 1 excess of females to males in the incidence of pellagra (Section 8.5). 

F ure 8.7. Reactions catalyzed by ADP ribose cyclase (NAD glycohydiolase, EC 3.2.2.5). When the substrate is NADP, the base exchange reaction leads to the formation of nicotinic acid adenine dinucleotide phosphate. 

Pertussis toxin is produced by the bacterium Bordetella pertussis. It covalently modifies G-proteins of the G/Go family (transfer of a ADP-ribose moiety of NAD onto G-protein a-subunits). ADP-ribosylated G-proteins are arrested in their inactive state and, as a consequence, functionally uncoupled from their respective effectors. Examples for pertussis toxin-sensitive cellular responses include the hormonal inhibition of adenylyl cyclases, stimulation ofK+ channels, inhibition of Ca2+ channels and stimulation ofthe cGMP-phosphodiesterase in retinal rods. 

G proteins can be modified by ADP-ribosylation catalyzed by certain bacterial toxins. Among the tools that facilitated the discovery and characterization of G proteins were the bacterial toxins cholera and pertussis, which were known to influence adenylyl cyclase activity. Subsequently, it was shown that the actions of these toxins are achieved by their ability to catalyze the addition of an ADP-ribose group donated from nicotinamide adenine dinucleotide (NAD) to specific amino acid residues in certain heterotrimeric G protein a subunits [ 1 ]. 

Wilson HL, Dipp M, Thomas JM, Lad C, Galione A, Evans AM 2001 ADP-ribosyl cyclase and cyclic ADP-ribose hydrolase act as a redox sensor. A primary role for cyclic ADP-ribose in hypoxic pulmonary vasoconstriction. J Biol Chem 276 11180-11188... 

In some cell types (including cardiac muscle cells, pancreatic cells), another second messenger", the cyclic ADP-ribose (Fig. 6.8), is involved in opening the ryanodin receptors (Lee et al., 1994). The cADP-ribose is formed from NAD by an enzymatic pathway with the help of an ADP-ribosyl cyclase. 

Galione A (1994) Cyclic ADP-ribose, the ADP-ribosyl cyclase pathway and calcium signalling. Mol Cell Endocrinol 98 125-31... 

G-protein a-subunits also possess specific residues that can be covalently modified by bacterial toxins. Cholera toxin catalyzes the transfer of ADP-ribose moiety of NAD to a specific arginine residue in certain a-subunits, whereas pertussis toxin ADP-ribosylates those a-subunits that contain a specific cysteine residue near the carboxy-terminus. Modification of the a-subunit by cholera toxin persistently activates these protein by inhibiting their GTPase activity, whereas pertussis toxin inactives Gia protein and thereby results in the uncoupling of receptor from the effector. G-protein a-subunits are regulated by covalent modifications by fatty acids myristate and palmate. These lipid modifications serve to anchor the subunits to the membrane and increase the interaction with other protein and also increase the affinity of the a-subunit for 3y. In this regard, the myristoylation of Gia is required for adenylyl cyclase inhibition in cell-free assay (Taussig et al. 1993). 

Chini EN, de Toledo EG, Thompson MA, and Dousa TP (1997) Effect of estrogen upon cyclic ADP ribose metabolism beta-estradiol stimulates ADP ribosyl cyclase in rat uterus. Proceedings of the National Academy of ScierKes of the USA 94, 5872-6. 

Whereas cholera is a result of a G protein trapped in the active conformation, causing the signal-transduction pathway to be perpetually stimulated, pertussis, or whooping cough, is a result of the opposite situation. Pertussis toxin also adds an ADP-ribose moiety,— in this case, to a G j protein, a G protein that inhibits adenyl cyclase, closes Ca + channels, and... 

NAD serves as a donor of an ADP-ribose group for the synthesis of cyclic ADP-ribose. ADP-ribosyl cyclase catalyzes the synthesis of cyclic ADP-ribose (Figure 9,651 J). This molecule works within the cell and provokes the release of calcium ions from an internal storage site (Jacobson cf al., 1997). The overall concentration of cyclic ADP-ribose in various organs is about 2.0micromolar (Dousa etai, 1996. The action of cyclic ADP-ribose In the cell is thought to be similar to that of another signaling molecule, 1P3 (see Calcium section). 

Fig. 6.9 Reactions of ADP ribosyl cyclase. Structures of NADP, nicotinic acid adenine dinucleotide phosphate (NAADP) and cyclic ADP-ribose phosphate (cADPRP). ADP-ribosyl cyclase, in base ex-...

FI G U R E 2 Metabolism of cyclic ADP-ribose. cADPR is synthesized from p-NAD+ by ADP-ribosyl cyclases and hydrolyzed to its inactive metabolite, ADP-ribose, by cADPR hydrolases. In mammalian cells, cyclase and hydrolase activities have been shown to be expressed on one bifunctional protein. 

ADP-ribosyl cyclase and cADPR hydrolase activities have been shown to be widespread in mammalian and other tissues (see Sections III.B and III.C). However, the demonstration that these enzymes are bifunctional in that they are resident on the same protein has led to the suggestion that many common ecto-NAD" glycohydrolases, which covert NAD+ to ADP-ribose, may do so via cADPR as an intermediate (Kim et al., 1993b). This might suggest that many of these... 

The secretory product of Bordetella pertussis interferes with the ability of agonists to inhibit adenylate cyclase. It catalyzes the transfer of the ADP-ribose moiety of NAD+ to a cysteine residue close to the carboxy terminus of Gi,. 

Forskolin, which is isolated from Coleus forskohlii, stimulates adenylate cyclase. Choleragen, the secretory product of Vibrio cholerae, can persistently activate adenylate cyclase by catalyzing the transfer of the ADP-ribose moiety of nicotinamide adenine dinucleotide (NAD+) to (see also Figure 52). 

Cholera and pertussis toxins catalyze the covalent addition of ADP-ribose to specific sites in the ot subunits of Gs and Gi, respectively. This modification inhibits GTPase action in the ot subunits and converts them to irreversible activators of adenylate cyclase. As a result, cAMP accumulates. In the intestine, the response to this is an uncontrollable secretion of water and sodium—causing severe diarrhea and dehydration. 

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