Phosphorylation of adenosine in renal brush-border membrane vesicles by an exchange reaction catalysed by adenosine kinase

Identification of agents that reduce renal hypoxia-reoxygenation injury using cell-based screening: purine nucleosides are alternative energy sources in LLC-PK1 cells during hypoxia

Acute tubular necrosis is a clinical problem that lacks specific therapy and is characterized by high mortality rate. The ischemic renal injury affects the proximal tubule cells causing dysfunction and cell death after severe hypoperfusion. We utilized a cell-based screening approach in a hypoxia-reoxygenation model of tubular injury to search for cytoprotective action using a library of pharmacologically active compounds. Oxygen-glucose deprivation (OGD) induced ATP depletion, suppressed aerobic and anaerobic metabolism, increased the permeability of the monolayer, caused poly(ADP-ribose) polymerase cleavage and caspase-dependent cell death. The only compound that proved cytoprotective either applied prior to the hypoxia induction or during the reoxygenation was adenosine. The protective effect of adenosine required the coordinated actions of adenosine deaminase and adenosine kinase, but did not requisite the purine receptors. Adenosine and inosine better preserved the cellular ATP content during ischemia than equimolar amount of glucose, and accelerated the restoration of the cellular ATP pool following the OGD. Our results suggest that radical changes occur in the cellular metabolism to respond to the energy demand during and following hypoxia, which include the use of nucleosides as an essential energy source. Thus purine nucleoside supplementation holds promise in the treatment of acute renal failure.

Enzymes of adenylate metabolism and their role in hibernation of the white-tailed prairie dog, Cynomys leucurus

AMP deaminase (AMPD) and adenylate kinase (AK) were purified from skeletal muscle of the white-tailed prairie dog, Cynomus leucurus, and enzyme properties were assayed at temperatures characteristic of euthermia (37 degrees C) and hibernation (5 degrees C) to analyze their role in adenylate metabolism during hibernation. Total adenylates decreased in muscle of torpid individuals from 6.97 +/- 0. 31 to 4.66 +/- 0.58 micromol/g of wet weight due to a significant drop in ATP but ADP, AMP, IMP, and energy charge were unchanged. The affinity of prairie dog AMPD for AMP was not affected by temperature and did not differ from that of rabbit muscle AMPD, used for comparison. However, both prairie dog and rabbit AMPD showed much stronger inhibition by ions and GTP at 5 degrees C, versus 37 degrees C, and inhibition by inorganic phosphate, NH(4)Cl, and (NH(4))(2)SO(4) was much stronger at 5 degrees C for the prairie dog enzyme. Furthermore, ATP and ADP, which activated AMPD at 37 degrees C, were strong inhibitors of prairie dog AMPD at 5 degrees C, with I(50) values of 1 and 14 microM, respectively. ATP also inhibited rabbit AMPD at 5 degrees C (I(50) = 103 microM). Strong inhibition of AMPD at 5 degrees C by several effectors suggests that enzyme function is specifically suppressed in muscle of hibernating animals. By contrast, AK showed properties that would maintain or even enhance its function at low temperature. K(m) values for substrates (ATP, ADP, AMP) decreased with decreasing temperature, the change in K(m) ATP paralleling the decrease in muscle ATP concentration. AK inhibition by ions was also reduced at 5 degrees C. The data suggest that adenylate degradation via AMPD is blocked during hibernation but that AK maintains its function in stabilizing energy charge.

Molecular cloning and expression of adenosine kinase from Leishmania donovani: identification of unconventional P-loop motif

The unique catalytic characteristics of adenosine kinase (Adk) and its stage-specific differential activity pattern have made this enzyme a prospective target for chemotherapeutic manipulation in the purine-auxotrophic parasitic protozoan Leishmania donovani. However, nothing is known about the structure of the parasite Adk. We report here the cloning of its gene and the characterization of the gene product. The encoded protein, consisting of 345 amino acid residues with a calculated molecular mass of 37173 Da, shares limited but significant similarity with sugar kinases and inosine-guanosine kinase of microbial origin, supporting the notion that these enzymes might have the same ancestral origin. The identity of the parasite enzyme with the corresponding enzyme from two other sources so far described was only 40%. Furthermore, 5' RNA mapping studies indicated that the Adk gene transcript is matured post-transcriptionally with the trans-splicing of the mini-exon (spliced leader) occurring at nt -160 from the predicted translation initiation site. The biochemical properties of the recombinant enzyme were similar to those of the enzyme isolated from leishmanial cells. The intrinsic tryptophan fluorescence of the enzyme was substrate-sensitive. On the basis of a multiple protein-alignment sequence comparison and ATP-induced fluorescence quenching in the presence or the absence of KI and acrylamide, the docking site for ATP has been provisionally identified and shown to have marked divergence from the consensus P-loop motif reported for ATP- or GTP-binding proteins from other sources.

Regulation of ground squirrel Na+K+-ATPase activity by reversible phosphorylation during hibernation

Maintenance of skeletal muscle energy status during hibernation in ground squirrels, Spermophilus lateralis, was accompanied by a decrease in Na+K+-ATPase pump activity. Energy charge was maintained (0.89) during hibernation at the expense of total adenylates (decreased by 41%). Muscle Na+K+-ATPase activity was 9.1 U/mg protein in euthermic controls but decreased by 60% during hibernation. Enzyme activity was similarly suppressed in vitro when extracts of control muscle were incubated with ATP plus second messengers of protein kinases A, G or C whereas stimulation of protein phosphatases in muscle extracts from hibernators increased Na+K+-ATPase activity. Additional studies confirmed that suppression and reactivation of the enzyme in euthermic muscle extracts can be achieved with protein kinase A and alkaline phosphatase treatments, respectively, and indicated that phosphorylation changes the ATP dependency of the enzyme. Thus, hibernation-induced suppression of Na+K+-ATPase activity in muscle and other organs of ground squirrels, which is a key part of the overall suppression of metabolic rate that constitutes torpor, appears to be regulated via reversible protein phosphorylation.

Cloning and characterization of cDNA for adenosine kinase from mammalian (Chinese hamster, mouse, human and rat) species. High frequency mutants of Chinese hamster ovary cells involve structural alterations in the gene

The enzyme adenosine kinase constitutes the major purine nucleoside phosphorylating activity in mammalian cells. In view of its central role in adenosine metabolism, which is an important physiological regulator, an understanding of the primary structure of adenosine kinase is of much interest. Using microsequence information from peptides derived from purified Syrian hamster liver enzyme, we have succeeded in isolating full length cDNA clones encoding adenosine kinase from Chinese hamster ovary cells and mouse 3T3 cells. The open reading frames in these clones consist of 334 and 335 amino acids and encode proteins of molecular masses 37364 Da and 37489 Da, respectively. In addition, the coding and upstream sequences for adenosine kinase from human (HeLa cells) and rat liver have also been cloned and sequenced. Transfection of an adenosine-kinase-deficient mutant (selected for resistance to the adenosine analog toyocamycin) of Chinese hamster ovary cells with a plasmid containing the cloned adenosine kinase cDNA, leads to regaining of adenosine kinase activity in the transformed cell. The adenosine kinase transformants also simultaneously lost their toyocamycin resistance and became similarly sensitive to the analog as the parental wild-type Chinese hamster ovary cells. The cloned adenosine kinase cDNA was also used to examine structural changes in mutants affected in adenosine kinase. In Chinese hamster ovary cells, one type of mutant that lacks adenosine kinase activity and displays high degree of resistance to various adenosine analogs, is obtained at an unusually high spontaneous frequency (10(-4)-10(-3)). Results of Southern and northern-blot analysis provide evidence that this group of mutants involves gross structural alterations affecting the adenosine kinase gene. Such structural alterations are not observed in another type of mutant which exhibits increased resistance only to C-adenosine analogs. Sequence similarity searches indicate that several of the bacterial and yeast sugar kinases (ribokinase, fructokinase and inosine-guanosine kinase) exhibit limited but significant similarity to the mammalian adenosine kinase. The sequence similarity data support the possibility that adenosine kinase shares a common evolutionary ancestor with these protein sequences.

Production of adenosine and nucleoside analogs by the exchange reaction catalyzed by rat liver adenosine kinase

We have previously shown [8] that rat liver adenosine kinase can produce [14C]AMP from [14C]adenosine (Ado) and unlabelled adenosine monophosphate (AMP), in the absence of ATP, by an exchange reaction. In this study, we investigated whether Ado or AMP could be replaced in this exchange reaction by other nucleosides or nucleoside monophosphates (NMP), respectively. In the presence of 1 mM of the unlabelled NMP analogs 7-deazaadenosine (tubercidin) 5'-monophosphate, 6-chloropurine riboside 5'-monophosphate, or N6-methyl-AMP, [14C]AMP was formed from 20 microM [14C]Ado at up to 50% of the rate recorded with 1 mM unlabelled AMP. In the presence of 0.2 mM of the unlabelled analog nucleosides tubercidin, N6-methyladenosine, or 6-methylmercaptopurine riboside, [14C]Ado was generated from 1 mM [14C]AMP at up to 60% of the rate recorded with 0.2 mM unlabeled Ado. Small amounts of [14C]Ado were also formed from the natural nucleosides 5-amino-4-imidazolecarboxamide (AICA) riboside or 2'-deoxyadenosine. Administration of therapeutic anticancer and antiviral nucleosides that can serve as substrates for the exchange reaction catalyzed by adenosine kinase might, thus, result in a net production of Ado, a potent autacoid with physiological effects in numerous tissues.