Studies on the metabolism of 5'-isobutylthioadenosine (SIBA): phosphorolytic cleavage by methylthioadenosine phosphorylase

Association of SIBA treatment and a Met-depleted diet inhibits in vitro growth and in vivo metastatic spread of experimental tumor cell lines

We have used 5'-deoxy-5'-S isobutyl-thioadenosine (SIBA), an analog of S-adenosylhomocysteine, alone or in association with a methionine-depleted diet in order to obtain an antitumoral effect in two different tumor models: a transplantable rat rhabdomyosarcoma (RMS-J1) induced by i.m. injection of nickel and the well-known Lewis lung carcinoma (3LL) of C57BL/6 mice. Since SIBA has been reported to inhibit the methyl group transfer from methionine to S-adenosylhomocysteine, among other activities, its association with a reduction of methyl donors, achieved by methionine depletion of the diet (in vivo) or the culture medium (in vitro), should logically lead to an additive effect. In vitro, 3LL and RMS-J1 were sensitive to the cytotoxic effect of SIBA and were methionine-dependent for their proliferation. Fibroblast proliferation was not affected by these two treatments alone or in association. In vivo, either SIBA treatment or a low methionine diet led to a significant decrease in the metastatic character of these two tumors; however, local tumor growth was not significantly affected. The median number of 3LL metastases counted in the lungs was reduced from 100 to 18 by SIBA treatment, and to 27 by the low methionine diet. No additive effect could be detected when the treatments were given simultaneously. RMS-J1-bearing rats treated with SIBA and fed a low Met diet underwent primary tumor excision. The median numbers of lung metastatic nodules were 27, 26, 14 and 8 for the control, SIBA-treated rats, methionine-deprived rats and rats receiving the combined therapy. Expressed as percentages 20 per cent were cured, 23 per cent showed a low number of lung metastases (P less than 10), whereas all the rats in the control group developed more than 10 pulmonary nodules. No cytotoxic effect could be observed on the treated rats. The role of SIBA and methionine depletion, as agents interfering with transmethylation processes, in regard to the control of tumor development, namely metastatic invasiveness, is discussed.

5'-Deoxy-5'-methylthioadenosine phosphorylase--III. Role of the enzyme in the metabolism and action of 5'-halogenated adenosine analogs

5'-Deoxy-5'-halogenated adenosines are alternative substrates for 5'-deoxy-5'-methylthioadenosine phosphorylase (MTAPase), an enzyme responsible for the metabolism of 5'-deoxy-5'-methylthioadenosine (MTA), a by-product of polyamine biosynthesis. The relative reactivity of these nucleosides with MTAPase from HL-60 human promyelocytic leukemia cells is MTA greater than 5'-deoxy-5'-fluoroadenosine (5'-FlAdo) greater than 5'-chloro-5'-deoxyadenosine (5'-ClAdo) greter than 5'-bromo-5'-deoxyadenosine (5'-BrAdo) greater than 5'-deoxy-5'-iodoadenosine (5'-IAdo). In MTAPase-containing cells, the adenine released from the 5'-halogenated adenosine was incorporated into adenine nucleotide pools; cleavage by (MTAPase appeared to be the rate-limiting step in this process. 5'-BrAdo and 5'-IAdo were growth inhibitors (EC50 values less than 10 microM) of MTAPase-containing cell lines (HL-60 human promyelocytic leukemia and the L5178Y murine lymphoblastic leukemia) but were much less active (EC50 values greater than 65 microM) against MTAPase-deficient cell lines (the CCRF-CEM human T cell leukemia and the L1210 murine leukemia). The full cytotoxicity of these compounds, therefore, appeared to be related to their phosphorolysis by MTAPase. Indirect evidence suggests that 5-halogenated ribose-1-phosphate derivatives of 5'-BrAdo or 5'-IAdo produced by the MTAPase reaction were the active metabolites of these 5'-halogenated adenosines.

Effect of analogues of 5'-methylthioadenosine on cellular metabolism. Inactivation of S-adenosylhomocysteine hydrolase by 5'-isobutylthioadenosine

The effects of a number of nucleosides related to 5'-methylthioadenosine on the activities of S-adenosylhomocysteine hydrolase, 5'-methylthioadenosine phosphorylase, spermidine synthase and spermine synthase were investigated. Both 5'-methylthioadenosine and 5'-isobutylthioadenosine gave rise to an enzyme-activated irreversible inhibition of S-adenosylhomocysteine hydrolase, but 5'-methylthiotubercidin (5'-methylthio-7-deaza-adenosine), 5'-deoxy-5'-chloroformycin, 5'-ethylthio-2-fluoro-adenosine and 1,N6-etheno-5'-methylthioadenosine were totally ineffective in producing this inactivation. Of the nucleosides tested, only 5'-methylthioadenosine, 5'-methylthiotubercidin and 5'-isobutylthioadenosine were inhibitory towards the aminopropyltransferases responsible for the synthesis of spermine and spermidine. 5'-Methylthiotubercidin, 5'-deoxy-5'-chloroformycin and 5'-isobutylthioadenosine were inhibitors of the degradation of 5'-methylthioadenosine by 5'-methylthioadenosine phosphorylase, but only 5'-isobutylthioadenosine was also a substrate for this enzyme. These results suggest that the effects of 5'-isobutylthioadenosine of the cell may result from the combination of inhibitory actions on polyamine synthesis, 5'-methylthioadenosine degradation and S-adenosylhomocysteine degradation. The resulting increased concentrations of S-adenosylhomocysteine could bring about inhibition of methyltransferase reactions. A new convenient method for the assay of S-adenosylhomocysteine hydrolase in the direction of synthesis is described.

Structural analogs of 5'-methylthioadenosine as substrates and inhibitors of 5'-methylthioadenosine phosphorylase and as inhibitors of human lymphocyte transformation

5'-Deoxy-5'-methylthioadenosine (MTA) phosphorylase was purified 13.4-fold from human peripheral lymphocytes. The enzyme demonstrated normal Michaelis-Menten kinetics with Km values of 26 microM and 7.5 mM for the two substrates, MTA and phosphate, respectively. The rate of MTA degradation was temperature dependent, 47 degrees being the optimum temperature. Five structural analogs served as alternative substrates with Km values ranging from 31 to 53 microM while two compounds, 5'-deoxy-5'-methylthiotubercidin (MTT) (Ki = 31 microM) and adenine (Ki = 172 microM), were inhibitory. These same analogs were examined as inhibitors of mitogen-induced human lymphocyte blastogenesis. MTT was found to be the most effective inhibitor of lymphocyte transformation with an I50 of 80 microM.

The effect of adenosine analogues on the in vitro growth of Trypanosoma cruzi

Trypanosoma cruzi (Brazil strain) was maintained in liver infusion tryptose and medium supplemented with 5 or 10% foetal calf serum at 27 degrees C on a rotating shaker platform. 85 to 95% of the organisms under these conditions are epimastigotes and the medium supported logarithmic growth for up to 24 hours. The effect of S-isobutyl adenosine and Sinefungin against cultured T. cruzi epimastigotes was studied: growth rate was slowed by both in a dose-dependent fashion; 500 micrometer Sinefungin caused complete inhibition which was irreversible after 24-hour exposure but the effect of S-isobutyl adenosine (100 micrometer) was reversible. Motility and morphology appeared to be unaffected.

5'-Methylthioadenosine nucleosidase. Purification and characterization of the enzyme from Lupinus luteus seeds

5'-Methylthioadenosine nucleosidase (EC 3.2.2.9), the enzyme which catalyzes hydrolytic cleavage of 5'-methylthioadenosine with the formation of adenine and 5'-methylthioribose, has been purified to homogeneity from Lupinus luteus seeds. The nucleosidase has a native molecular weight of 62 000 and consists of two identical subunits, as judged by gel filtration and dodecylsulfate/polyacrylamide gel electrophoresis. The nucleosidase exhibits highest specificity towards the natural substrate with a Km of 4.1 X 10(-7) M for 5'-methylthioadenosine. It does not cleave adenine from S-adenosylhomocysteine. Among the synthetic analogs of 5'-methylthioadenosine tested, eleven compounds appear to be able to substitute as substrates. Furthermore, the enzyme can liberate hypoxanthinine from six inosyl (deaminated) derivatives obtained by enzymatic deamination of 5'-methylthioadenosine and its synthetic analogs. The Km for 5'-methylthioinosine is 55 microM, and the maximal velocity about 50-times lower than for 5'-methylthioadenosine. The reaction catalyzed by the nucleosidase can be inhibited by adenine (Ki = 11 microM), 3-deazaadenine (Ki = 19 microM), and 9-erythro(2-hydroxyl-3-nonyl)adenine (ki = 37 microM).

Effects of inhibitors of spermidine and spermine synthesis on polyamine concentrations and growth of transformed mouse fibroblasts

1. A number of compounds known to inhibit polyamine biosynthesis at various steps in the biosynthetic pathway were tested for their ability to inhibit growth and decrease polyamine concentrations in virally transformed mouse fibroblasts (SV-3T3 cells). 2. Virtually complete inhibition of growth was produced by the inhibitors of ornithine decarboxylase alpha-methylornithine and alpha-difluoromethylornithine and by the inhibitors of S-adenosylmethionine decarboxylase 1,1'-[(methylethanediylidene)dinitrilo]diguanidine and 1,1'-[(methylethanediylidene)dinitrilo]bis-(3-aminoguanidine). The former inhibitors decreased putrescine and spermidine contents in the cells to very low values, whereas the latter substantially increased putrescine but decreased spermidine concentrations. The inhibitory effects of all of these inhibitors on cell growth could be prevented by the addition of spermidine, suggesting that spermidine depletion is the underlying cause of their inhibition of growth. 3. alpha-Difluoromethylornithine, which is an irreversible inhibitor of ornithine decarboxylase, was a more potent inhibitor of growth and polyamine production (depleting spermidine almost completely and spermine significantly) than alpha-methylornithine, which is a competitive inhibitor. This was not the case with the inhibitors of S-adenosylmethionine decarboxylase where 1,1'-[(methylethanediylidene)dinitrilo]diguanidine, a reversible inhibitor, was more active than 1,1'-[(methylethanediylidene)dinitrilo]bis-(3-aminoguanidine), an irreversible inhibitor. It is suggested that this effect may be due to the lesser uptake and/or greater chemical reactivity of the latter compound. 4. Various nucleoside derivatives of S-adenosylhomocysteine that inhibited spermidine synthase in vitro did not have significant inhibitory action against polyamine accumulation in the cell. These compounds, which included S-adenosylhomocysteine sulphone, decarboxylated S-adenosylhomocysteine sulphone, decarboxylated S-adenosylhomocysteine sulphoxide and S-adenosyl-4-thio-butyric acid sulphone did not inhibit cell growth or polyamine content until cytotoxic concentrations were added. 5. 5'-Methylthioadenosine, 5'-isobutylthioadenosine and 5'-methylthiotubercidin, which inhibit aminopropyltransferase activity in vitro, all inhibited cell growth and decreased spermidine content. Although these compounds were most active against spermine synthase in vitro, they acted in the cell primarily to decrease spermidine content. Cell growth could not be restored to normal values by addition of spermidine, suggesting that these nucleosides have another inhibitory action towards cellular proliferation. 6. 5'-Methylthioadenosine and 5'-isobutylthioadenosine are degraded by a phosphorylase present in SV3T3 cells, yielding 5-methylthioribose-1-phosphate and 5-isobutylthioribose-1-phosphate respectively, and adenine. This degradation appears to decrease the inhibitory action towards cell growth, suggesting that the nucleosides themselves are exerting the inhibitory action. 5'-Methylthiotubercidin, which is not a substrate for the phosphorylase and is a competitive inhibitor of it, was the most active of these nucleosides in inhibiting cell growth and spermidine content. 5'-Methylthiotubercidin and alpha-difluoromethylornithine had additive effects on retarding cell growth, but not on cellular spermine accumulation, also suggesting that the primary growth-inhibiting action of the nucleoside was not on polyamine production. 7. These results support the concept that 5'-methylthioadenosine phosphorylase plays an important role in permitting cell growth to continue by preventing the build-up of inhibitory intracellular concentrations of 5'-methylthioadenosine.

5'-Methylthioadenosine phosphorylase from Caldariella acidophila. Purification and properties

The occurrence of 5'-methylthioadenosine phosphorylase in Caldariella acidophila, a thermophilic bacterium growing optimally at 87 degrees C, is reported. It represents the first example in prokaryotes of a phosphoryolytic cleavage of the thioether. The reaction products, purified by ion-exchange chromatography, have been identified as 5-methylthioribose-1-phosphate and adenine by several analytical procedures. The enzyme has been purified to homogeneity in 32% yield by using DEAE-cellulose and hydroxyapatite chromatography, gel filtration and isoelectric focusing. The enzyme shows a high degree of thermophilicity, its temperature optimum being at 93 degrees C; furthermore no loss of activity is observable after exposure for 1 h at 100 degrees C. The kinetic data indicate a sequential mechanism of the reaction. The apparent Km values are 0.095 mM for 5'-methylthioadenosine and 6.1 mM for phosphate. The specificity of the reaction is rather strict. Experiments performed with analogues of the substrate, i.e. 5'-methylthioinosine, 5'-dimethylthioadenosine sulfonium salt, 5'-n-butylthioadenosine, 5'-isobutylthioadenosine, 5'-isobutylthioinosine, adenosylhomocysteine, 5'-thioethanoladenosine, adenosine, indicate the relevance of the adenine amino group and the sulfur in thioether form in the binding to the enzyme protein.