Differential inhibition of mammalian aminopropyltransferase activities

Metabolomic characterization of the salt stress response in Streptomyces coelicolor

The humicolous actinomycete Streptomyces coelicolor routinely adapts to a wide variety of habitats and rapidly changing environments. Upon salt stress, the organism is also known to increase the levels of various compatible solutes. Here we report the results of the first high-resolution metabolomics time series analysis of various strains of S. coelicolor exposed to salt stress: the wild type, mutants with progressive knockouts of the ectoine biosynthesis pathway, and two stress regulator mutants (with disruptions of the sigB and osaB genes). Samples were taken from cultures at 0, 4, 8, and 24 h after salt stress treatment and analyzed by liquid chromatography-mass spectrometry with an LTQ Orbitrap XL mass spectrometer. The results suggest that a large fraction of amino acids is upregulated in response to the salt stress, as are proline/glycine-containing di- and tripeptides. Additionally we found that 5'-methylthioadenosine, a known inhibitor of polyamine biosynthesis, is downregulated upon salt stress. Strikingly, no major differences between the wild-type cultures and the two stress regulator mutants were found, indicating a considerable robustness of the metabolomic response to salt stress, compared to the more volatile changes in transcript abundance reported earlier.

Targeting polyamine metabolism and function in cancer and other hyperproliferative diseases

The polyamines spermidine and spermine and their diamine precursor putrescine are naturally occurring, polycationic alkylamines that are essential for eukaryotic cell growth. The requirement for and the metabolism of polyamines are frequently dysregulated in cancer and other hyperproliferative diseases, thus making polyamine function and metabolism attractive targets for therapeutic intervention. Recent advances in our understanding of polyamine function, metabolic regulation, and differences between normal cells and tumour cells with respect to polyamine biology, have reinforced the interest in this target-rich pathway for drug development.

Putrescine or spermidine binding site of aminopropyltransferases and competitive inhibitors

A model of the active site of aminopropyltransferases was proposed based on the study of a number of monoamino and diamino compounds as potential inhibitors and substrates, respectively, of spermidine synthase purified from pig liver. The active site seems to have a relatively large hydrophobic cavity adjacent to a negatively charged site, to which a protonated amino group of putrescine binds, with another amino group of putrescine being situated in the hydrophobic cavity as a free form to be aminopropylated by decarboxylated S-adenosylmethionine. On the basis of the above-mentioned model, another modified one was proposed for spermine synthase, and several compounds mentioned model, another modified one was proposed for spermine synthase, and several compounds designed according to the modified model were found to potently inhibit spermine synthase, purified from rat brain, in competition with spermidine. The newly developed inhibitors were about two orders of magnitude more potent in vitro than a known inhibitor of spermine synthase, dimethyl(5'-adenosyl)sulfonium perchlorate.

Inhibition of mammalian spermine synthase by N-alkylated-1,3-diaminopropane derivatives in vitro and in cultured rat hepatoma cells

A number of N-alkylated-1,3-diaminopropane derivatives [H2N-(CH2)3-NH-(CH2)nH, where n = 1-9] have been tested as potential inhibitors of partially purified rat hepatoma (HTC) cell or pure bovine spleen spermine synthase. Among the compounds described in this paper, the most potent competitive inhibitor of spermine synthase, with respect to spermidine, is N-butyl-1,3-diaminopropane with Ki values of 11.9 nM and 10.4 nM for the HTC cell and bovine spleen enzymes respectively. Inhibition of spermine synthase by this alkylated amine is selective since spermidine synthase activity is not affected up to 100 microM N-butyl-1,3-diaminopropane at a range of 5-200 microM putrescine. Added to the culture medium of growing HTC cells, N-butyl-1,3-diaminopropane causes the expected changes in the polyamine levels with a marked decrease of spermine and an increase of spermidine. Under these conditions cell growth continues unabated. Such N-alkylated-1,3-diaminopropane derivatives may have considerable potential as tools for studying the role of polyamines and in particular the functions of spermine in cell multiplication and differentiation.

2-Mercaptoethylamine, a competitive inhibitor of spermidine synthase in mammalian cells

Spermidine synthase from rat ventral prostate was inhibited by 2-mercaptoethylamine (MEA). Inhibition of spermidine synthase by MEA was competitive with respect to one of the substrates putrescine, but not competitive with respect to the other substrate decarboxylated S-adenosylmethionine. MEA markedly depressed spermidine and spermine contents in human erythroid leukemia K562 cells, suggesting that these changes resulted from the inhibitory effect of MEA on spermidine synthase in situ.

Inhibition of ornithine decarboxylase activity and cell growth by diamines: a comparison between the effects of two homologs, 1,3-diaminopropane and 1,4-diaminobutane (putrescine)

The ornithine decarboxylase (ODC) activity of Ehrlich ascites tumor cells was almost completely inhibited by treatment with either putrescine (10 mM) or 1,3-diaminopropane (5 mM). 1,3-Diaminopropane treatment eradicated the cellular content of putrescine and reduced that of spermidine and spermine. Putrescine treatment caused a dramatic increase in cellular putrescine content and a temporary decrease in spermidine and spermine content. Despite the fact that 1,3-diaminopropane and putrescine inhibited the ODC activity more effectively than did alpha-difluoromethylornithine (DFMO), an enzyme-activated irreversible inhibitor of ODC, they were considerably less antiproliferative in action. However, as compared to DFMO the diamines were less effective in reducing the total polyamine (putrescine + spermidine + spermine) content of the cells.

5'-deoxy-5'-methylthioadenosine phosphorylase--II. Role of the enzyme in the metabolism and antineoplastic action of adenine-substituted analogs of 5'-deoxy-5'-methylthioadenosine

The biological activities of several previously synthesized [J. A. Montgomery et al., J. med. Chem. 17, 1197 (1974)] adenine-substituted analogs of 5'-deoxy-5'-methylthio- or 5'-deoxy-5'-ethyl-thioadenosine, including the 2-fluoroadenine, 2-chloroadenine, 2,6-diaminopurine, 8-azaadenine, and 4-aminopyrazolo [3,4-d]pyrimidine-containing derivatives, have been reexamined. It is demonstrated that many of these analogs are cleaved to their respective free base analogs by 5'-deoxy-5'-methyl-thioadenosine phosphorylase (MTAPase), an enzyme associated with polyamine biosynthesis, and that this reaction is necessary for the cytotoxic action of these MTA analogs to be fully expressed. Evidence to support this includes: (1) the growth of two MTAPase-containing human colon carcinoma cell lines (the HCT-15 and DLD-1 lines) was inhibited by these analogs, whereas an MTAPase-deficient cell line, the CCRF-CEM human T-cell leukemia, was relatively insensitive to their cytotoxic action; (2) extracts of the MTAPase-containing colon carcinoma cell lines were able to cleave these analogs to their respective free base analogs; in contrast, extracts of MTAPase-deficient CCRF-CEM cells were unable to cleave these analogs; (3) intact colon carcinoma cells converted these MTA analogs to their corresponding 5'-phosphorylated analog nucleotides, whereas CCRF-CEM cells did not, at least to detectable levels; and (4) the MTA analog, 5'-deoxy-5'-ethylthio-4-aminopyrazolo [3,4-d]pyrimidine ribonucleoside, which is not a substrate of MTAPase, did not form analog nucleotides and was essentially noncytotoxic to all cell lines tested, whereas the corresponding adenine analog, 4-aminopyrazolo [3,4-d]pyrimidine, readily formed analog nucleotides and was highly cytotoxic to all the lines. It is postulated that the corresponding adenine analog 5'-phosphorylated nucleotides are the primary active metabolites of these MTA analogs, having been formed by the cleavage of these nucleosides to free adenine analogs by MTAPase, followed by the conversion of these base analogs to analog nucleotides by adenine phosphoribosyltransferase and the enzymes of adenine nucleotide phosphorylation. This pathway represents a novel drug-activation system for the synthesis of analog nucleotides and has the potential to be exploited chemotherapeutically.

The comparative effects of 5'-methylthioadenosine and some of its analogs on cells containing, and deficient in, 5'-methylthioadenosine phosphorylase

The antiproliferative effects of 5'-methylthioadenosine and the 5'-methylthioadenosine analogs, 5'-isobutylthioadenosine, 5'-deoxyadenosine and 5'-methylthiotubercidin were examined using two mouse cell lines, one 5'-methylthioadenosine phosphorylase-deficient the other containing 5'-methylthioadenosine phosphorylase. All of the compounds were found to be growth inhibitory to both cell lines, demonstrating that these compounds need not be degraded to exert their inhibitory effects. A correlation was observed between the potency of the growth inhibitory effect and the ability of the cells to degrade these compounds. 5'-Methylthioadenosine, 5'-deoxyadenosine and 5'-isobutylthioadenosine, all of which are substrates for the 5'-methylthioadenosine phosphorylase in vitro, were more growth inhibitory to the 5'-methylthioadenosine phosphorylase-deficient cells than to the 5'-methylthioadenosine phosphorylase-containing cells, whereas, the 7-deaza analog, 5'-methylthiotubercidin, a nondegradable inhibitor of the 5'-methylthioadenosine phosphorylase, was a more potent inhibitor of the 5'-methylthioadenosine phosphorylase-containing cell line. Due to the inhibition by 5'-methylthiotubercidin on 5'-methylthioadenosine phosphorylase in vitro the disposition of cellularly-synthesized 5'-methylthioadenosine was explored using both cell types. 5'-Methylthiotubercidin inhibited the accumulation of exogenous 5'-methylthioadenosine from 5'-methylthioadenosine phosphorylase-deficient cells with no effect on intracellular 5'-methylthioadenosine. In contrast, 5'-methylthiotubercidin caused a large accumulation of extracellular 5'-methylthioadenosine with a concomitant smaller increase intracellularly in 5'-methylthioadenosine phosphorylase-containing cells. That cellularly-synthesized 5'-methylthioadenosine as well as the cellular excretion of this nucleoside are altered in response to treatment with 5'-methylthiotubercidin suggests two possible sites at which 5'-methylthiotubercidin may exert its effect.

Effect of inhibition of polyamine synthesis on the content of decarboxylated S-adenosylmethionine

1. The content of decarboxylated S-adenosylmethionine (AdoMet) in transformed mouse fibroblasts (SV-3T3 cells) was increased 500-fold to about 0.4fmol/cell when ornithine decarboxylase was inhibited by alpha-difluoromethylornithine. This increase was due to the absence of putrescine and spermidine, which serve as substrates for aminopropyltransferases with decarboxylated AdoMet as an aminopropyl donor, and to the enhanced activity of AdoMet decarboxylase brought about by depletion of spermidine. The increase in decarboxylated AdoMet content was abolished by addition of putrescine, but not by 1,3-diaminopropane. 2. 5'-Methylthiotubercidin also increased decarboxylated AdoMet content, presumably by direct inhibition of aminopropyl-transferase activities, but the increase in its content and the decline in spermidine content were much less than those produced by alpha-difluoromethylornithine. 3. Decarboxylated AdoMet content of regenerating rat liver was measured in rats treated with inhibitors of ornithine decarboxylase. The content was increased by 60% 32h after partial hepatectomy in control rats, by 90% when alpha-difluoromethylornithine was given to the partially hepatectomized rats, and by 330% when 1,3-diaminopropane was used to inhibit putrescine and spermidine synthesis. After 48h of exposure to 1,3-diaminopropane, which completely prevented the increase in spermidine after partial hepatectomy, there was a 5-fold rise in hepatic decarboxylated AdoMet concentration. These increases were prevented by treatment with putrescine or with methylglyoxal bis(guanylhydrazone), an inhibitor of AdoMet decarboxylase. 4. These results show that changes in AdoMet metabolism result from the administration of specific inhibitors of polyamine synthesis. The possible consequences of the accumulation of decarboxylated AdoMet, which could, for example, interfere with normal cellular methylation or lead to depletion of cellular adenine nucleotides, should be considered in the interpretation of results obtained with such inhibitors.

Specificity of mammalian spermidine synthase and spermine synthase

1. The specificity of rat prostatic spermidine synthase and spermine synthase with respect to the amine acceptor of the propylamine group was studied. 2. Spermidine synthase could use cadaverine (1,5-diaminopentane) instead of putrescine, but the Km for cadaverine was much greater and the rate with 1mM-cadaverine was only 10% of that with putrescine. 1,3-Diaminopropane was even less active (2% of the rate with putrescine) and no other compound tested (including longer alpha,omega-diamines, spermidine and its homologues and monoacetyl derivatives) was active. 3. Spermine synthase was equally specific. The only compounds tested that showed any activity were 1,8-diamino-octane, sym-homospermidine, sym-norspermidine and N-(3-aminopropyl)-cadaverine, which at 1mM gave rates 2, 17, 3 and 4% of the rate with spermidine respectively. 4. The formation of polyamine derivatives of cadaverine and to a very small extent of 1,3-diaminopropane was confirmed by exposing transformed mouse fibroblasts to these diamines when synthesis of putrescine was prevented by alpha-difluoromethylornithine. Under these conditions the cells accumulated significant amounts of N-(3-aminopropyl)cadaverine and NN'-bis(3-aminopropyl)cadaverine when exposed to cadaverine and small amounts of sym-norspermidine and sym-norspermine when exposed to 1,3-diaminopropane.

Indirect evidence for a strict negative control of S-adenosyl-L-methionine decarboxylase by spermidine in rat hepatoma cells

1. Direct or indirect inhibitors of l-ornithine decarboxylase (EC 4.1.1.17), structurally related or unrelated to l-ornithine, including dl-alpha-difluoromethylornithine, alpha-methylornithine and 1,3-diaminopropane, used alone or in combination, decreased polyamine concentrations in rat hepatoma tissue culture (HTC) cells and increased S-adenosyl-l-methionine decarboxylase activity (EC 4.1.1.50). 2. Comparison of the catalytic properties of S-adenosyl-l-methionine from cells with elevated and normal activities revealed no apparent modification of the catalytic site as judged by affinity for the substrate, stimulation by di- and tri-amines and inhibition by methylglyoxal bis-(guanylhydrazone). 3. Actinomycin D and cycloheximide, and RNA and a proteinsynthesis inhibitor respectively, blocked the increase of S-adenosyl-l-methionine decarboxylase activity elicited by alpha-difluoromethylornithine. In polyamine-depleted cells the apparent half-life of elevated S-adenosyl-l-methionine decarboxylase activity, determined by inhibition of protein synthesis, was 2.5-fold longer than in control cells. The present results suggest that elevation of S-adenosyl-l-methionine decarboxylase activity by alpha-difluoromethylornithine is due to stabilization of the enzyme. 4. Restoration of the normal intracellular putrescine content, by addition of putrescine to the medium of polyamine-deficient cells, transiently increased S-adenosyl-l-methionine decarboxylase activity. Thereafter, intracellular conversion of putrescine into spermidine was accompanied by inactivation of the enzyme at a rate that was similar to that found on addition of spermidine itself. No relationship between total intracellular spermine content and S-adenosyl-l-methionine decarboxylase activity could be established. 5. Addition of 1mm-1,3-diaminopropane to polyamine-deficient cells did not cause a decrease in the activity of S-adenosyl-l-methionine decarboxylase, whereas addition of 1,5-diaminopentane (cadaverine) did. 1,3-Diamino-N-(3-aminopropyl)propane did not accumulate in cells treated with alpha-difluoromethylornithine and 1,3-diaminopropane, whereas addition of 1,5-diaminopentane led to the accumulation of 1,5-diamino-N-(3-aminopropyl)pentane. 1,3-Diamino-N-(3-aminopropyl)propane (10mum) was as effective as spermidine in decreasing S-adenosyl-l-methionine decarboxylase activity. Thus effectiveness of a diamine in decreasing enzyme activity is related to its capability of being converted into a closely structurally related homologue of spermidine by spermidine synthase. 6. The spermidine site of action appears to be post-translational since (a) the spermidine-induced decrease of S-adenosyl-l-methionine activity was not prevented by actinomycin D and (b) spermidine in the presence of cycloheximide led to a synergistic inactivation of the enzyme with a decay rate that progressively approached control values. Altogether these results are indirect evidence for a strict negative control of S-adenosyl-l-methionine decarboxylase by spermidine and substantiate previous findings [Mamont, Duchesne, Grove & Tardif (1978) Exp. Cell Res.115, 387-393]. Spermidine appears to act on some processes involved in denaturation and/or degradation of the enzyme protein. Putrescine appears to decrease the rate of these processes. The physiological significance of the regulatory control of S-adenosyl-l-methionine decarboxylase is discussed.

Decreased transmethylation of biogenic amines after in vivo elevation of brain S-adenosyl-l-homocysteine

The ability of S-adenosyl-L-homocysteine (AdoHcy) to inhibit biologic transmethylation reactions in vitro has led us to explore the possibility of pharmacologically manipulating AdoHcy levels in vivo and examining the consequences of these alterations on the transmethylation of some biogenic amines. Swiss-Webster mice were injected intraperitoneally with different doses of adenosine (Ado) and D, L-homocysteine thiolactone (Hcy) and were killed at various times thereafter. S-Adenosyl-methionine (AdoMet) and AdoHcy concentrations were determined by using a modified isotope dilution-ion exchange chromatography-high pressure liquid chromatography technique sensitive to less than 10 pmol. Increasing doses of Ado + Hcy (50-1000 mg/kg of each) produced a dose-related increase in blood, liver, and brain AdoHcy levels. At a dose level of 200 mg/kg Ado + Hcy, AdoHcy levels were markedly elevated, with minimal concomitant perturbations of AdoMet. This elevation was maximal 40 min after giving Ado + Hcy, returning to control values within 6 h. Ado + Hcy treatment resulted in decreased activities of catechol-O-methyltransferase, histamine-N-methyltransferase, and AdoHcy hydrolase in vitro. The cerebral catabolism of intraventricularly administered [(3)H]histamine (HA) was decreased in a dose-related manner by Ado + Hcy treatment as evidenced by higher amounts of nonutilized [(3)H]HA in brain, concurrent decreases in [(3)H]methylhistamine formation, and decreases in the transmethylation conversion index. Steady state levels of HA also showed dose-related increases after Ado + Hcy treatment. It is concluded that injections of Ado + Hcy can markedly elevate AdoHcy levels in vivo, which can, in turn, decrease the rate of transmethylation reactions.

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.

Antiproliferative effects of inhibitors of polyamine synthesis in tumors of neural origin

The antiproliferative properties of inhibitors of polyamine synthesis were evaluated in cultured neuroblastoma and glioma cells. The diamines (1,3-propanediamine, 1,5-pentanediamine, and 1,6-hexanediamine) dramatically decreased neuroblastoma replication and inhibited the rate-limiting enzyme, ornithine decarboxylase. Glioma cells were less sensitive to the diamines in spite of significant drug-induced decreases in enzyme activity. The fact that ornithine decarboxylase was inhibited in both cell lines with different effects on proliferation suggests that the activity of other enzymes in polyamine biosynthesis may be altered selectively by these inhibitors.

Studies of inhibition of rat spermidine synthase and spermine synthase

1. S-Adenosyl-l-methionine, S-adenosyl-l-homocysteine, 5'-methylthioadenosine and a number of analogues having changes in the base, sugar or amino acid portions of the molecule were tested as potential inhibitors of spermidine synthase and spermine synthase from rat ventral prostate. 2. S-Adenosyl-l-methionine was inhibitory to these reactions, as were other nucleosides containing a sulphonium centre. The most active of these were S-adenosyl-l-ethionine, S-adenosyl-4-methylthiobutyric acid, S-adenosyl-d-methionine and S-tubercidinylmethionine, which were all comparable in activity with S-adenosylmethionine itself, producing 70-98% inhibition at 1mm concentrations. Spermine synthase was somewhat more sensitive than spermidine synthase. 3. 5'-Methylthioadenosine, 5'-ethylthioadenosine and 5'-methylthiotubercidin were all powerful inhibitors of both enzymes, giving 50% inhibition of spermine synthase at 10-15mum and 50% inhibition of spermidine synthase at 30-45mum. 4. S-Adenosyl-l-homocysteine was a weak inhibitor of spermine synthase and practically inactive against spermidine synthase. Analogues of S-adenosylhomocysteine lacking either the carboxy or the amino group of the amino acid portion were somewhat more active, as were derivatives in which the ribose ring had been opened by oxidation. The sulphoxide and sulphone derivatives of decarboxylated S-adenosyl-l-homocysteine and the sulphone of S-adenosyl-l-homocysteine were quite potent inhibitors and were particularly active against spermidine synthase (giving 50% inhibition at 380, 50 and 20mum respectively). 5. These results are discussed in terms of the possible regulation of polyamine synthesis by endogenous nucleosides and the possible value of some of the inhibitory substances in experimental manipulations of polyamine concentrations. It is suggested that 5'-methylthiotubercidin and the sulphone of S-adenosylhomocysteine or of S-adenosyl-3-thiopropylamine may be particularly valuable in this respect.

The role of 5'-methylthioadenosine phosphorylase in 5'-methylthioadenosine-mediated inhibition of lymphocyte transformation

To determine if increased 5'-methylthioadenosine phosphorylase activity in activated lymphocytes may be responsible for the decreased inhibitory effect noted when 5'-methylthioadenosine is added after stimulation, the activity of this enzyme was monitored during lymphocyte transformation. A direct correlation existed between the transformation process and 5'-methylthioadenosine phosphorylase activity; the longer the stimulation process progressed, the phosphorylase activity; the longer the stimulation process progressed, the greater the enzyme activity. The 7-deaza analog of 5'-methylthioadenosine, 5'-methylthiotubercidin, was utilized to explore further the role that the phosphorylase may play in the reversal process. 5'-Methylthiotubercidin acted as a potent inhibitor, but not a substrate, of the 5'-methylthioadenosine phosphorylase, and was an even more potent inhibitor of lymphocyte transformation than 5'-methylthioadenosine. However, in direct contrast to the 5'-methylthioadenosine effect, inhibition by 5'-methylthiotubercidin could not be completely reversed. These data suggest the 5'-methylthioadenosine phosphorylase plays an important role in reversing 5'-methylthioadenosine-mediated inhibition and that the potent, nonreversible inhibitory effects of 5'-methylthiotubercidin are due to its resistance to 5'-methylthioadenosine phosphorylase degradation.

Studies on polyamine biosynthesis in Euglena gracilis

Euglene gracilis (strain Z) was found to contain five polyamines which could be separated by high-pressure cation-exchange chromatography. 1,3-Diaminopropane, putrescine, norspermidine (N-(3-aminopropyl)-1,3-diaminopropane), spermidine and norspermine (N,N'-bis(aminopropyl)-1,3-diaminopropane) were identified. Biosynthesis of putrescine in E. gracilis proceeds through decarboxylation of L-ornithine, no arginine decarboxylase (EC 4.1.1.19) activity could be detected. The properties of the enzymes ornithine decarboxylase (EC 4.1.1.17) and S-adenosylmethionine decarboxylase (EC 4.1.1.50) in this alga were found to be similar to those of the enzymes isolated from animal tissues or yeast cells. A bioxynthetic scheme is proposed which relates the different polyamines occurring in E. gracilis.

Polyamine synthesis in mammalian tissues. Isolation and characterization of spermine synthase from bovine brain

Spermine synthase, a propylamine transferase, which catalyses the biosynthesis of spermine from S-methyladenosylhomocystemine and spermidine has been purified to an apparent homogeneity (about 6000-fold) from bovine brain using spermine-Sepharose affinity chromatography. The enzyme preparation was free from S-adenosylmethionine decarboxylase and spermidine synthase activities. The molecular Stokes radius of the enzyme was calculated to be 4.16 nm. The enzyme has an apparent molecular weight of approximately 88 000, composing of two subunits of equal size. The enzyme showed a broad pH optimum between 7.0 and 8.0 and an acidic isoelectric point at pH 5.10. The apparent Km values for S-methyladenosylhomocysteamine was 0.6 microM and about 60 microM for spermidine. The enzyme showed strict specificity to spermidine as the propylamine acceptor. Both the reaction products, spermine and 5'-methylthioadenosine inhibited the enzyme activity, methylthioadenosine being a powerful competitive inhibitor with respect to S-methyladenosylhomocysteamine (Ki value of about 0.3 microM). Putrescine also inhibited competitively with respect to spermidine (Ki value of about 1.7 mM). Spermine synthase had no requirements for metal or other cofactors.