Regulation of the interaction of purified human erythrocyte AMP deaminase and the human erythrocyte membrane

Surface AMP deaminase 2 as a novel regulator modifying extracellular adenine nucleotide metabolism

Adenine nucleotides represent crucial immunomodulators in the extracellular environment. The ectonucleotidases CD39 and CD73 are responsible for the sequential catabolism of ATP to adenosine via AMP, thus promoting an anti-inflammatory milieu induced by the "adenosine halo". AMPD2 intracellularly mediates AMP deamination to IMP, thereby both enhancing the degradation of inflammatory ATP and reducing the formation of anti-inflammatory adenosine. Here, we show that this enzyme is expressed on the surface of human immune cells and its predominance may modify inflammatory states by altering the extracellular milieu. Surface AMPD2 (eAMPD2) expression on monocytes was verified by immunoblot, surface biotinylation, mass spectrometry, and immunofluorescence microscopy. Flow cytometry revealed enhanced monocytic eAMPD2 expression after TLR stimulation. PBMCs from patients with rheumatoid arthritis displayed significantly higher levels of eAMPD2 expression compared with healthy controls. Furthermore, the product of AMPD2-IMP-exerted anti-inflammatory effects, while the levels of extracellular adenosine were not impaired by an increased eAMPD2 expression. In summary, our study identifies eAMPD2 as a novel regulator of the extracellular ATP-adenosine balance adding to the immunomodulatory CD39-CD73 system.

Purine Nucleotides in the Regulation of Brown Adipose Tissue Activity

Non-shivering thermogenesis in mammalian brown adipose tissue is a powerful mechanism to defend normothermia in cold climates. To minimize the loss of chemical energy, the central functional component, mitochondrial uncoupling protein 1, UCP1, must be tightly regulated. The canonical pathway of UCP1 activation includes lipolytic release of free fatty acids in response to an adrenergic signal. Activating fatty acids overcome constitutive inhibition of UCP1 by the di- and triphosphate forms of purine nucleotides, i.e., ATP, ADP, GTP, and GDP. Cellular concentrations of inhibitory, free nucleotides are subject to significant, adrenergically induced alterations. The regulatory components involved may constitute novel drug targets to manipulate brown fat thermogenesis and thereby organismic energy balance. We here review evidence for and against a dominant role of nucleotides in thermogenic control, describe conceptual routes to endogenously and pharmacologically alter free nucleotide pool size, speculate on a signaling role of degradation products released from active brown fat, and highlight gaps in our understanding of signaling and metabolic pathways involved.

Role of the HPRG Component of Striated Muscle AMP Deaminase in the Stability and Cellular Behaviour of the Enzyme

Multiple muscle-specific isoforms of the Zn²⁺ metalloenzyme AMP deaminase (AMPD) have been identified based on their biochemical and genetic differences. Our previous observations suggested that the metal binding protein histidine-proline-rich glycoprotein (HPRG) participates in the assembly and maintenance of skeletal muscle AMP deaminase (AMPD1) by acting as a zinc chaperone. The evidence of a role of millimolar-strength phosphate in stabilizing the AMPD-HPRG complex of both AMPD1 and cardiac AMP deaminase (AMPD3) is suggestive of a physiological mutual dependence between the two subunit components with regard to the stability of the two isoforms of striated muscle AMPD. The observed influence of the HPRG content on the catalytic behavior of the two enzymes further strengthens this hypothesis. Based on the preferential localization of HPRG at the sarcomeric I-band and on the presence of a Zn²⁺ binding motif in the N-terminal regions of fast TnT and of the AMPD1 catalytic subunit, we advance the hypothesis that the Zn binding properties of HPRG could promote the association of AMPD1 to the thin filament.

Calcium activates erythrocyte AMP deaminase [isoform E (AMPD3)] through a protein-protein interaction between calmodulin and the N-terminal domain of the AMPD3 polypeptide

Erythrocyte AMP deaminase [isoform E (AMPD3)] is activated in response to increased intracellular calcium levels in Tarui's disease, following exposure of ionophore-treated cells to extracellular calcium, and by the addition of calcium to freshly prepared hemolysates. However, the assumption that Ca(2+) is a positive effector of isoform E is inconsistent with the loss of sensitivity to this divalent cation following dilution of erythrocyte lysates or enzyme purification. Ca(2+) regulation of isoform E was studied by examining in vitro effects of calmodulin (CaM) on this enzyme and by monitoring the influence of CaM antagonists on purine catabolic flow in freshly prepared erythrocytes under various conditions of energy imbalance. Erythrocyte and recombinant isoform E both adsorb to immobilized Ca(2+)-CaM, and relative adsorption across a series of N-truncated recombinant enzymes localizes CaM binding determinants to within residues 65-89 of the AMPD3 polypeptide. Ca(2+)-CaM directly stimulates isoform E catalytic activity through a K(mapp) effect and also antagonizes the protein-lipid interaction between this enzyme and intracellular membranes that inhibits catalytic activity. AMP is the predominant purine catabolite in erythrocytes deprived of glucose or exposed to A23187 ionophore alone, whereas IMP accumulates when Ca(2+) is included under the latter conditions and also during autoincubation at 37 degrees C. Preincubation with a CaM antagonist significantly slows the accumulation of erythrocyte IMP under both conditions. The combined results reveal a protein-protein interaction between Ca(2+)-CaM and isoform E and identify a mechanism that advances our understanding of erythrocyte purine metabolism. Ca(2+)-CaM overcomes potent isoform E inhibitory mechanisms that function to maintain the total adenine nucleotide pool in mature erythrocytes, which are unable to synthesize AMP from IMP because of a developmental loss of adenylosuccinate synthetase. This may also explain why Tarui's disease erythrocytes exhibit accelerated adenine nucleotide depletion in response to an increase in intracellular Ca(2+) concentration. This regulatory mechanism could also play an important role in purine metabolism in other human tissues and cells where the AMPD3 gene is expressed.

N-terminal sequence and distal histidine residues are responsible for pH-regulated cytoplasmic membrane binding of human AMP deaminase isoform E

Mammalian AMP deaminase 3 (AMPD3) enzymes reportedly bind to intracellular membranes, plasma lipid vesicles, and artificial lipid bilayers with associated alterations in enzyme conformation and function. However, proteolytic sensitivity of AMPD polypeptides makes it likely that prior studies were performed with N-truncated enzymes. This study uses erythrocyte ghosts to characterize the reversible cytoplasmic membrane association of human full-sized recombinant isoform E (AMPD3). Membrane-bound isoform E exhibits diminished catalytic activity whereas low micromolar concentrations of the cationic antibiotic, neomycin, disrupt this protein-lipid interaction and relieve catalytic inhibition. The cytoplasmic membrane association of isoform E also displays an inverse correlation with pH in the physiological range. Diethyl pyrocarbonate (DEPC) modification of isoform E nearly abolishes its cytoplasmic membrane binding capacity, and this effect can be reversed by hydroxylamine. Difference spectra reveal that 18 of 29 histidine residues in each isoform E subunit are N-carbethoxylated by DEPC. These combined data demonstrate that protonated imidazole rings of histidine residues mediate a pH-responsive association of isoform E with anionic charges on the surface of the cytoplasmic membrane, possibly phosphatidylinositol 4,5-bisphosphate, a pure noncompetitive inhibitor of the enzyme. Finally, AMPD1 and a series of N-truncated AMPD3 enzymes are used to show that these behaviors are specific to isoform E and require up to 48 N-terminal amino acids, even though this stretch of sequence contains no histidine residues. The pH-responsive cytosol-membrane partitioning of isoform E may be an important mechanism for branch point regulation of adenylate catabolism.

Localization of N-terminal sequences in human AMP deaminase isoforms that influence contractile protein binding

The reversible association of AMP deaminase (AMPD, EC 3.5.4.6) with elements of the contractile apparatus is an identified mechanism of enzyme regulation in mammalian skeletal muscle. All three members of the human AMPD multigene family contain coding information for polypeptides with divergent N-terminal and conserved C-terminal domains. In this study, serial N-terminal deletion mutants of up to 111 (AMPD1), 214 (AMPD2), and 126 (AMPD3) residues have been constructed without significant alteration of catalytic function or protein solubility. The entire sets of active enzymes are used to extend our understanding of the contractile protein binding of AMPD. Analysis of the most truncated active enzymes demonstrates that all three isoforms can associate with skeletal muscle actomyosin and suggests that a primary binding domain is located within the C-terminal 635-640 residues of each polypeptide. However, discrete stretches of N-terminal sequence alter this behavior. Residues 54-83 in the AMPD1 polypeptide contribute to a high actomyosin binding capacity of both isoform M spliceoforms, although the exon 2- enzyme exhibits significantly greater association compared to its exon 2+ counterpart. Conversely, residues 129-183 in the AMPD2 polypeptide reduce actomyosin binding of isoform L. In addition, residues 1-48 in the AMPD3 polypeptide dramatically suppress contractile protein binding of isoform E, thus allowing this enzyme to participate in other intracellular interactions.

Towards an understanding of the functional significance of N-terminal domain divergence in human AMP deaminase isoforms

Human AMP deaminase (AMPD; EC 3.5.4.6) isoforms are encoded by a multigene family and have conserved C-terminal domains that contain catalytic residues and an ATP-binding site. N-terminal domains diverge dramatically, yet are conserved when compared across mammalian species. Cross-species conservation of entire gene-specific polypeptides (e.g., rat versus human AMPD1) suggests that divergent N-terminal domains may play a role in isoform-specific properties of the enzyme. It now has become evident that the majority of published data used to characterize purified AMPD isoforms were likely derived from preparations lacking significant portions of their N-terminal domains (up to nearly 100 residues). Accumulating evidence indicates that divergent N-terminal sequences do influence catalytic behavior, protein-protein interactions, and intracellular distributions of this enzyme.

Regulation of AMP deaminase by phosphoinositides

AMP deaminase (AMPD) converts AMP to IMP and is a diverse and highly regulated enzyme that is a key component of the adenylate catabolic pathway. In this report, we identify the high affinity interaction between AMPD and phosphoinositides as a mechanism for regulation of this enzyme. We demonstrate that endogenous rat brain AMPD and the human AMPD3 recombinant enzymes specifically bind inositide-based affinity probes and to mixed lipid micelles that contain phosphatidylinositol 4,5-bisphosphate. Moreover, we show that phosphoinositides specifically inhibit AMPD catalytic activity. Phosphatidylinositol 4,5-bisphosphate is the most potent inhibitor, effecting pure noncompetitive inhibition of the wild type human AMPD3 recombinant enzyme with a K(i) of 110 nM. AMPD activity can be released from membrane fractions by in vitro treatment with neomycin, a phosphoinositide-binding drug. In addition, in vivo modulation of phosphoinositide levels leads to a change in the soluble and membrane-associated pools of AMPD activity. The predicted human AMPD3 sequence contains pleckstrin homology domains and (R/K)X(n)(R/K)XKK sequences, both of which are characterized phosphoinositide-binding motifs. The interaction between AMPD and phosphoinositides may mediate membrane localization of the enzyme and function to modulate catalytic activity in vivo.

Interaction of AMP deaminase with RNA

tRNA, 18 S and 28 S ribosomal RNAs were found to activate muscle AMP deaminase (AMP aminohydrolase, EC 3.5.4.6) but inhibit liver and heart AMP deaminases. The macromolecular structures are essential for modulation of enzyme activity, since the effects of RNA disappeared after RNAase treatment. Sucrose density centrifugation experiments clearly demonstrated the binding of purified muscle AMP deaminase to tRNA, 18 S and 28 S RNAs. The binding is reversible and responsive to alterations of pH and KCl concentration. The binding was stable at pH 5.1-7.0 in 0.1 M KCl, but most of the enzyme dissociated at pH 7.5. KCl below 0.1 M concentration had no effect on dissociation of enzyme-RNA complex, but in 0.15 M KCl the complex was partially dissociated and in 0.2 M KCl most of the enzyme was released. Various nucleotides were also effective in dissociation of the enzyme from complex. The binding is saturable and the maximum number of muscle AMP deaminase molecules bound per mol 28 S RNA was calculated to be approx. 30. Liver and heart AMP deaminases were also found to interact with RNA.