5'-deoxy-5'-methylthioadenosine phosphorylase--IV. Biological activity of 2-fluoroadenine-substituted 5'-deoxy-5'-methylthioadenosine analogs

Novel nucleoside-based antimalarial compounds

The malaria-causing parasite Plasmodium falciparum employs a salvage pathway for the biosynthesis of nucleotides, in contrast to de novo biosynthesis that is utilized by the human host. A series of twenty-two 2-, 6- and 5'-modified adenosine ribonucleosides was synthesized, with the expectation that these compounds would generate toxic metabolites instead of active nucleotides by the pathogen, while remaining inert in host cells. Bioassays with P. falciparum (K1 strain) indicated IC50 values as low as 110nM and a selectivity index with respect to cytotoxicity toward an L6 rat myoblast cell line of >1000 for the most potent analogue.

Isolation and substrate specificity of an adenine nucleoside phosphorylase from adult Schistosoma mansoni

An adenine nucleoside phosphorylase (ANP, EC none) activity was identified and partially purified from extracts of Schistosoma mansoni by chromatofocussing column chromatography and molecular sieving. The enzyme is distinct from purine nucleoside phosphorylase (PNP, EC 2.4.2.1). ANP is specific for adenine nucleosides which includes adenosine analogs modified in the aglycone, pentose or both moieties. (e.g. 2'-deoxyadenosine, 5'-deoxy-5'-methylthioadenosine, 5'-deoxy-5'-iodo-2-fluoroadenosine, etc.) The enzyme is also distinct from the mammalian 5'-deoxy-5'-methylthioadenosine phosphorylase (MTAP, EC 2.4.2.28) in that it is able of the phosphorolysis of 2'-deoxyadenosine while mammalian MTAP cannot. Because of ANP unique substrate specificity, the enzyme could play a role as a target for chemotherapy of these parasites. Cytotoxic analogs may be designed as subversive substrates that are selectively activated only by the schistosomal ANP.

Purine salvage pathways in the apicomplexan parasite Toxoplasma gondii

We have exploited a variety of molecular genetic, biochemical, and genomic techniques to investigate the roles of purine salvage enzymes in the protozoan parasite Toxoplasma gondii. The ability to generate defined genetic knockouts and target transgenes to specific loci demonstrates that T. gondii uses two (and only two) pathways for purine salvage, defined by the enzymes hypoxanthine-xanthine-guanine phosphoribosyltransferase (HXGPRT) and adenosine kinase (AK). Both HXGPRT and AK are single-copy genes, and either one can be deleted, indicating that either one of these pathways is sufficient to meet parasite purine requirements. Fitness defects suggest both pathways are important for the parasite, however, and that the salvage of adenosine is more important than salvage of hypoxanthine and other purine nucleobases. HXGPRT and AK cannot be deleted simultaneously unless one of these enzymes is provided in trans, indicating that alternative routes of functionally significant purine salvage are lacking. Despite previous reports to the contrary, we found no evidence of adenine phosphoribosyltransferase (APRT) activity when parasites were propagated in APRT-deficient host cells, and no APRT ortholog is evident in the T. gondii genome. Expression of Leishmania donovani APRT in transgenic T. gondii parasites yielded low levels of activity but did not permit genetic deletion of both HXGPRT and AK. A detailed comparative genomic study of the purine salvage pathway in various apicomplexan species highlights important differences among these parasites.

Bacterial variations on the methionine salvage pathway

BACKGROUND

The thiomethyl group of S-adenosylmethionine is often recycled as methionine from methylthioadenosine. The corresponding pathway has been unravelled in Bacillus subtilis. However methylthioadenosine is subjected to alternative degradative pathways depending on the organism.

RESULTS

This work uses genome in silico analysis to propose methionine salvage pathways for Klebsiella pneumoniae, Leptospira interrogans, Thermoanaerobacter tengcongensis and Xylella fastidiosa. Experiments performed with mutants of B. subtilis and Pseudomonas aeruginosa substantiate the hypotheses proposed. The enzymes that catalyze the reactions are recruited from a variety of origins. The first, ubiquitous, enzyme of the pathway, MtnA (methylthioribose-1-phosphate isomerase), belongs to a family of proteins related to eukaryotic intiation factor 2B alpha. mtnB codes for a methylthioribulose-1-phosphate dehydratase. Two reactions follow, that of an enolase and that of a phosphatase. While in B. subtilis this is performed by two distinct polypeptides, in the other organisms analyzed here an enolase-phosphatase yields 1,2-dihydroxy-3-keto-5-methylthiopentene. In the presence of dioxygen an aci-reductone dioxygenase yields the immediate precursor of methionine, ketomethylthiobutyrate. Under some conditions this enzyme produces carbon monoxide in B. subtilis, suggesting a route for a new gaseous mediator in bacteria. Ketomethylthiobutyrate is finally transaminated by an aminotransferase that exists usually as a broad specificity enzyme (often able to transaminate aromatic aminoacid keto-acid precursors or histidinol-phosphate).

CONCLUSION

A functional methionine salvage pathway was experimentally demonstrated, for the first time, in P. aeruginosa. Apparently, methionine salvage pathways are frequent in Bacteria (and in Eukarya), with recruitment of different polypeptides to perform the needed reactions (an ancestor of a translation initiation factor and RuBisCO, as an enolase, in some Firmicutes). Many are highly dependent on the presence of oxygen, suggesting that the ecological niche may play an important role for the existence and/or metabolic steps of the pathway, even in phylogenetically related bacteria. Further work is needed to uncover the corresponding steps when dioxygen is scarce or absent (this is important to explore the presence of the pathway in Archaea). The thermophile T. tengcongensis, that thrives in the absence of oxygen, appears to possess the pathway. It will be an interesting link to uncover the missing reactions in anaerobic environments.

Expression of methylthioadenosine phosphorylase cDNA in p16-, MTAP- malignant cells: restoration of methylthioadenosine phosphorylase-dependent salvage pathways and alterations of sensitivity to inhibitors of purine de novo synthesis

5'-Deoxy-5'-methylthioadenosine phosphorylase (MTAP) is involved in the salvage of adenine and methylthio moieties of 5'-deoxy-5'-methylthioadenosine, a byproduct of polyamine synthesis, to adenine nucleotides and methionine, respectively. The gene encoding MTAP, MTAP, is frequently codeleted along with the tumor suppressor gene p16 in malignant cells bearing homozygous deletions in the chromosome 9p21 region. p16-, MTAP- malignant cells have been shown to be more susceptible to the purine de novo inhibitory actions of antifolates such as methotrexate than are p16+, MTAP+ cells. To understand the underlying mechanism, we reintroduced MTAP activity into two p16-, MTAP- cell model systems, the MiaPaCa-2 and PANC-1 human pancreatic carcinoma cell lines, by transfection with MTAP cDNA. It was found that transfection with MTAP cDNA (i) restored both the MTAP-dependent adenine and methionine salvage pathways, (ii) decreased the rates of purine de novo synthesis (18-47% lower than the wild-type or sham-transfected counterparts), and (iii) decreased cellular sensitivity to the antipurine-related growth-inhibitory actions of methotrexate and azaserine. These data support the hypothesis that operation of the MTAP-dependent adenine salvage pathway renders MTAP+ cells less dependent on de novo purine synthesis and hence less susceptible than MTAP- malignant cells to the growth-inhibitory actions of agents (e.g. antifolates) whose mechanism of action in part involves the de novo purine pathway. These findings provide a theoretical basis for the relatively selective action certain antifolates may have against MTAP-deficient malignancies.

Effects of adenosine analogues on ATP concentrations in human erythrocytes. Further evidence for a route independent of adenosine kinase

Adenosine derivatives are frequently used in chemotherapy because of their potent antitumor, antiviral and antiparasitic activity. We investigated the metabolism of some adenosine analogues in adenosine deaminase inhibited normal and adenine phosphoribosyltransferase (APRT) deficient human erythrocytes. The ATP and GTP concentrations and the formation of unusual nucleotides were measured. Some of the analogues studied (tubercidin, 9 beta-D-arabinofuranosyladenine, 2'-deoxyadenosine, 2-chloroadenosine, neplanocin A) were phosphorylated to the corresponding nucleoside triphosphates and this process was abolished by iodotubercidin--an adenosine kinase inhibitor. With the exception of 2'-deoxyadenosine, nucleotide analogue formation was accompanied by ATP depletion. ATP decrease was not observed after adenosine kinase inhibition and ATP concentration even increased in the presence of 2'-deoxyadenosine, neplanocin A and 5'-iodo-5'-deoxyadenosine. However, the latter increment was not observed in APRT deficient erythrocytes. Bredinin, S-adenosylhomocysteine, deoxycoformycin and adenosine dialdehyde did not form nucleotide derivatives or exert any effects on ATP concentration. It is concluded that adenosine analogues can either enter the nucleotide pool via phosphorylation mechanisms, or may be converted to ATP by the pathways involving the intermediate formation of adenine.

Metabolism and action of purine nucleoside analogs

Recent investigations have identified many new purine nucleoside analogs that act as antimetabolites. This article focuses on the metabolism and mechanisms of action of tiazofurin, 3-deazaguanosine, neplanocin A, arabinosyladenine in combination with inhibitors of adenosine deaminase, arabinosyl-2-fluoroadenine, and 2-chloro-2'-deoxyadenosine, drugs that are either currently being evaluated in clinical trials or are close to that stage. The diverse metabolic requirements for activation, unique mechanisms of action, and differential biological activities of these compounds are characterized and evaluated for prospective therapeutic application.

5'-deoxy-5'-methylthioadenosine phosphorylase--V. Acycloadenosine derivatives as inhibitors of the enzyme

Various adenosine acyclonucleoside derivatives were tested as inhibitors of 5'-deoxy-5'-methylthioadenosine (MeSAdo) phosphorylase, an enzyme involved in the salvage of adenine and methionine from MeSAdo. The 2-halogenated derivatives of acyloadenosine [9-(2-hydroxyethoxy-methyl)adenine], including the chloro-, bromo- and iodo-congeners, all inhibited murine Sarcoma 180 (S180) MeSAdo phosphorylase, with Ki values in the range of 10(-6) to 10(-5) M. Halogenated derivatives of 9-(1,3-dihydroxy-2-propoxymethyl)adenine, which more closely resemble the natural substrate, were substantially more potent inhibitors of the enzyme, with Ki values in the range of 2-7 x 10(-7) M. 5'-Methylthio and 5'-halogenated analogs of 2'-deoxy-1',2'-seco-adenosine were weak inhibitors, with Ki values of 10(-4) M or greater. 9-[(1-Hydroxy-3-iodo-2-proxy)methyl]adenine. (HIPA), the derivative with the lowest Ki values among these analogs, was a competitive inhibitor of S180 MeSAdo phosphorylase. In preliminary studies, HIPA inhibited MeSAdo phosphorylase in intact HL-60 human promyelocytic leukemia cells, as it limited the incorporation of [8-14C]MeSAdo into cellular adenine nucleotide pools. In addition, 9-(phosphonoalkyl)adenines, representing potential multisubstrate inhibitors of MeSADo phosphorylase, were synthesized. Of these the heptyl derivative was the most potent inhibitor, with a Ki of 1.5 x 10(-5) M at low (3.5 mM) phosphate concentrations. The inhibitory effects of these analogs could be ablated at high phosphate concentrations (50 mM), suggesting that they interact with the phosphate binding site on the enzyme. Some of these novel MeSAdo phosphorylase inhibitors may have a role in cancer chemotherapy as potentiators of agents that block purine de novo synthesis, e.g. antifolates and 6-methylmercaptopurine ribonucleoside.