Regulation of S-adenosyl-L-methionine decarboxylase by 1-aminooxy-3-aminopropane: enzyme kinetics and effects on the enzyme activity in cultured cells

The kinetics of inactivation of adenosylmethionine decarboxylase of rat liver and of baby hamster kidney cells (BHK21/C31) by 1-aminooxy-3-aminopropane was studied. The apparent dissociation constants (Ki) for the hepatic and BHK21/C13 enzymes were 1.5 and 2.0 mM and the times of half-inactivation at infinite concentration of the inhibitor (tau 1/2) were 1.2 and 3.8 min, respectively. Treatment of BHK21/C13 with 0.5 mM 1-aminooxy-3-aminopropane prevented cell growth and depleted the cells of putrescine and spermidine within 1 day. The depletion of spermidine resulted in increased activity of S-adenosylmethionine decarboxylase which was due, at least partly, to the increase in the half-life of the enzyme activity. Because spermine levels were not significantly affected, it appears that spermidine is the principal feedback regulator of S-adenosylmethionine decarboxylase. So, 1-aminooxy-3-aminopropane is a very weak inhibitor of S-adenosylmethionine decarboxylase and the cellular effects can be correlated primarily with its inhibitory effects on ornithine decarboxylase and spermidine synthase. In cell-free systems, however, 1-aminooxy-3-aminopropane is likely to find use in unraveling the reaction mechanism of S-adenosylmethionine decarboxylase.     

The putrescine analogue 1-aminooxy-3-aminopropane perturbs polyamine metabolism in the phytopathogenic fungus<Emphasis Type="Italic"> Sclerotinia sclerotiorum</Emphasis>

The effects of the putrescine analogue 1-aminooxy-3-aminopropane on fungal polyamine metabolism were evaluated using Sclerotinia sclerotiorum as an experimental model. The compound inhibited ornithine decarboxylase, spermidine synthase, and S -adenosyl-methionine decarboxylase in mycelial extracts. Addition of 1-aminooxy-3-aminopropane at 1 mM to the culture medium did not reduce mycelial growth and caused a 29% decrease in free spermidine and a two-fold increase in free spermine. When added 4.5 h before the determination of ornithine decarboxylase, 1-aminooxy-3-aminopropane reduced in vivo activity of this enzyme by 40–50%. When added 48 h before the determination, 1-aminooxy-3-aminopropane at 0.01 and 0.1 mM caused a slight increase of in vivo ornithine decarboxylase activity, while it had no effect at 1 mM. Comparison of the action of 1-aminooxy-3-aminopropane with that of other inhibitors of polyamine biosynthesis suggested that its effects on in vivo ornithine decarboxylase activity resulted from a balance between direct inhibition of enzyme activity and indirect stimulation of enzyme synthesis and/or activity mediated by the decrease in spermidine levels, which in turn was due to inhibition of spermidine synthase and S -adenosyl-methionine decarboxylase. The potential of 1-aminooxy-3-aminopropane as a tool for studies on fungal polyamine metabolism and for the control of plant diseases of fungal origin is discussed.Abbreviations AdoMetDC S-Adenosyl-methionine decarboxylase - DFMO -Difluoromethylornithine - MGBG Methylglyoxal bis-[guanyl hydrazone] - ODC Ornithine decarboxylase     

Acetylation of decarboxylated S-adenosylmethionine by mammalian cells

Decarboxylated S-adenosylmethionine was found to be a substrate for the nuclear acetyltransferases that act on polyamines and on histones. The rate of acetylation of decarboxylated S-adenosylmethionine was more than twice that of spermidine at saturating substrate concentrations, and decarboxylated S-adenosylmethionine was an active inhibitor of the acetylation of histones by nuclear extracts from rat liver. The acetylation of decarboxylated S-adenosylmethionine occurred in vivo in SV-3T3 cells exposed to the ornithine decarboxylase inhibitor 2-(difluoromethyl)ornithine. The decline in putrescine and spermidine brought about by exposure to 2-(difluoromethyl)ornithine was found to be accompanied by a large rise in the content of both decarboxylated S-adenosylmethionine and acetylated decarboxylated S-adenosylmethionine. These results indicate that decarboxylated S-adenosylmethionine is metabolized not only in the well-known reactions in which it serves as an aminopropyl donor for polyamine biosynthesis but also by acetylation in reaction with acetyl coenzyme A. Furthermore, the inhibition of histone acetylation by decarboxylated S-adenosylmethionine could contribute to the biological effects brought about by inhibitors of ornithine decarboxylase.     

Effect of 1-amino-oxy-3-aminopropane on polyamine metabolism and growth of L1210 cells.

1-Amino-oxy-3-aminopropane (AOAP) was reported to inhibit several mammalian polyamine-biosynthetic enzymes in vitro, including ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (AdoMetDC) [Khomutov, Hyvönen, Karvonen, Kauppinen, Paalanen, Paulin, Eloranta, Pajula, Andersson & Pösö (1985) Biochem. Biophys. Res. Commun. 130, 596-602]. In order to clarify its mechanism of action in intact cells, the inhibitory properties of AOAP on the growth and polyamine metabolism of L1210 cells were compared with those seen in a variant subline (D-R cells) which overproduces ODC. As little as 20 microM-AOAP completely blocked proliferation of L1210 cells, and this effect was reversed by the concomitant addition of exogenous putrescine or spermidine. Growth of D-R cells was not affected by AOAP at concentrations up to 0.5 mM. There was no difference in the uptake of AOAP between the L1210 and the D-R cells. Exposure of L1210 or D-R cells to AOAP greatly decreased ODC activity in undialysed cell extracts, but did not decrease AdoMetDC. Activities of both enzymes were increased severalfold by AOAP treatment when activity was measured in dialysed extracts. Treatment with AOAP depleted intracellular putrescine and spermidine contents of L1210 cells, while inducing a massive accumulation of decarboxylated AdoMet. The 8-fold higher putrescine pool present in untreated D-R cells was depleted in a dose-dependent manner by AOAP, but a significant decrease in spermidine and accumulation of decarboxylated AdoMet required 10 times higher drug concentrations, and the changes were much less dramatic than in L1210 cells. These results indicate that in L1210 cells AOAP behaves primarily as a reversible inhibitor of ODC.     

Prolactin stimulation of Nb 2 node lymphoma cell division is inhibited by polyamine biosynthesis inhibitors

The mitogenic action of prolactin in Nb 2 node lymphoma cells was inhibited by two drugs which interfere with polyamine biosynthesis. At concentrations of 0.5 mM and above alpha-difluoromethyl ornithine (DFMO), which inhibits ornithine decarboxylase and the conversion of ornithine to putrescine, significantly attenuated the mitogenic effect of prolactin. This inhibition was prevented by the addition of putrescine, spermidine, or spermine to the culture medium. At concentrations of 1 microM and above methylglyoxal bis(guanylhydrazone) (MGBG), which inhibits S-adenosylmethionine decarboxylase and hence the conversion of putrescine to spermidine and spermine, abolished the mitogenic action of prolactin. This inhibition was prevented by the addition of spermidine or spermine, but not putrescine, to the culture medium. These studies show that ongoing polyamine biosynthesis is essential for prolactin to express its mitogenic effect in this lymphoma cell line.     

Effects of S-adenosyl-1,8-diamino-3-thiooctane on polyamine metabolism

Exposure of mammalian cells (transformed mouse fibroblasts or rat hepatoma cells) to S-adenosyl-1,8-diamino-3-thiooctane produced profound changes in the intracellular polyamine content. Putrescine was increased and spermidine was decreased, consistent with the inhibition of spermidine synthase by this compound, which is a potent and specific "transition-state analogue inhibitor" of the isolated enzyme in vitro. The spermine content of the cells was increased by exposure to this drug presumably since spermine synthase was able to use a greater proportion of the available decarboxylated S-adenosylmethionine when spermidine synthase was inhibited. The decarboxylated S-adenosylmethionine content rose substantially because the activity of S-adenosylmethionine decarboxylase was increased in response to the decline in spermidine. These results indicate that S-adenosyl-1,8-diamino-3-thiooctane is taken up by mammalian cells and is an effective inhibitor of spermidine synthase in vivo and that S-adenosylmethionine decarboxylase is regulated by the content of spermidine, but not of spermine. The growth of SV-3T3 cells was substantially reduced in the presence of S-adenosyl-1,8-diamino-3-thiooctane at concentrations of 50 microM or greater. Such inhibition was reversed by the addition of spermidine but not by putrescine. When SV-3T3 cells were exposed to 5 mM alpha-(difluoromethyl)ornithine and 50 microM S-adenosyl-1,8-diamino-3-thiooctane, the content of all polyamines was reduced. Putrescine and spermidine declined by more than 90% and spermine by 80%. Such cells grew very slowly unless spermidine was added.     

An early enlargement of the putrescine pool is required for growth in L1210 mouse leukemia cells under hypoosmotic stress

Hypoosmotic stress is a potent inducer of ornithine decarboxylase (ODC) activity in a variety of mammalian cells, but the physiological relevance of this response has not been determined. To test whether an increased putrescine content confers a growth advantage at lower osmolarities, we compared the ability of L1210 mouse leukemia cells and of ODC-overproducing variants obtained from this cell line (D-R cells) to proliferate after a hypotonic shock (325----130 mosmol/kg). The growth rate of D-R cells at 130 mosmol/kg was greater than or equal to 5-fold higher than in L1210 cells; and unlike the ODC-overproducing strain, L1210 cells underwent up to a 90% loss of viability over time as seen after restoration of normosmotic growth conditions and by trypan blue exclusion tests. The addition of putrescine or L-ornithine stimulated the proliferation of both cell sublines up to 5-fold in a concentration-dependent manner, with a maximal effect observed at about 10 and 100 microM, respectively. Putrescine restored virtually normal growth rates in both sublines at osmolarities as low as 190 mosmol/kg. No other alpha,omega-diamine was active in that respect whereas spermidine was markedly inhibitory. Furthermore, D-R cells incubated at 130 mosmol/kg showed a marked growth inhibition by 1-aminooxy-3-aminopropane (potent ODC inhibitor to which they are resistant in isotonic media) as a result of putrescine but not spermidine depletion. Whereas ODC was strongly and rapidly induced by hypotonic shock there was a precipitous decline in S-adenosylmethionine decarboxylase activity. Putrescine synthesis and accumulation were nevertheless reduced in D-R cells incubated at 130 mosmol/kg because of a decreased availability of L-ornithine. When either putrescine or L-ornithine was added to hypotonic media, D-R cells accumulated putrescine massively for extended periods together with a reduction in spermidine and spermine contents. Putrescine transport patterns were altered by hypotonic shock, net excretion of the diamine being reduced by about 80%, with a concurrent enlargement of the intracellular pool. Finally, parental L1210 cells incubated with an irreversible inhibitor of S-adenosylmethionine decarboxylase for 24 h until hypotonic shock and supplemented with putrescine in the presence of the drug thereafter exhibited a greatly exaggerated growth stimulation by the diamine. These results demonstrate an essential role for an early increase in putrescine content in the growth adaptation of a mammalian cell line to a lower osmolarity.     

Uptake of 3H-labeled 1-aminooxy-3-aminopropane by baby hamster kidney cells

The uptake, catabolism, and release of H-labeled 1-aminooxy-3-aminopropane, a new putrescine analog shown to be a potent polyamine antimetabolite, into and from baby hamster kidney cells (BHK21/C13) were studied. The results show that [3H]-1-aminooxy-3-aminopropane (APA) is not concentrated in the cell, does not compete with polyamines for transport and reveals no difference in uptake between polyamine-depleted and control cells. After a 12-h culture, 60% of APA was recovered intact in the culture media. At this time point, only 30% of the intracellular radioactivity was intact APA, showing that the drug is catabolized in the cells. This intracellular ratio persisted throughout the 4-day culture period. The metabolites of APA were not characterized further. The results indicate that the drug is not recognized as a polyamine by the cells and does not replace or interfere with the polyamines in cellular functions. Thus, its potent affinity to ornithine decarboxylase and spermidine synthase is likely to be due to close structural similarity with the intermediates formed in these reactions. This has implications for the mechanisms involved.     

1-Aminooxy-3-aminopropane, a new and potent inhibitor of polyamine biosynthesis that inhibits ornithine decarboxylase, adenosylmethionine decarboxylase and spermidine synthase

1-Aminooxy-3-aminopropane was shown to be a potent competitive inhibitor (Ki = 3.2 nM) of homogenous mouse kidney ornithine decarboxylase, a potent irreversible inhibitor (Ki = 50 microM) of homogeneous liver adenosylmethionine decarboxylase and a potent competitive (Ki = 2.3 microM) of homogeneous bovine brain spermidine synthase. It did not inhibit homogeneous bovine brain spermine synthase and it did not serve as a substrate for spermidine synthase. The compound did not inhibit tyrosine aminotransferase, alanine aminotransferase or aspartate aminotransferase, which are pyridoxal phosphate-containing enzymes like ornithine decarboxylase. The inactivation of adenosylmethionine decarboxylase was partially prevented by pyruvate, which is the coenzyme of adenosylmethionine decarboxylase, and by the substrate, adenosylmethionine. 1-Aminooxy-3-aminopropane at 0.5 mM concentration inhibited the growth of HL-60 promyelocytic leukemia cells and this inhibition was prevented by spermidine but not by putrescine.     

Aminooxypropylamine--an effective inhibitor of ornithine decarboxylase in vitro and in vivo

Hydroxylamine-containing analogues of putrescine and cadaverine have been found effective in inhibiting the mouse liver ornithine decarboxylase, the best among synthesized were 1-aminooxy-3-aminopropane (I50 2.10(-8) M) and 1-aminooxy-4-aminobutane (I50 2.10(-7) M). The inhibitory effect of these substances on the mouse liver ornithine-transaminase and S-adenosylmethionine decarboxylase from E. coli was displayed at concentrations higher by several orders of magnitude, that demonstrated the specificity of the compounds of this type. 1-Aminooxy-3-aminopropane in experiments in vivo suppressed the ornithine decarboxylase activity in mouse liver at 16 mg/kg by 75%, the toxic effect being insignificant.     

The role of polyamine depletion and accumulation of decarboxylated S-adenosylmethionine in the inhibition of growth of SV-3T3 cells treated with alpha-difluoromethylornithine.

The effects of alpha-difluoromethylornithine, a specific inhibitor of ornithine decarboxylase, on cell growth rate, polyamine content and the content of decarboxylated S-adenosylmethionine in SV-3T3 transformed mouse fibroblasts were studied. DL-alpha-Difluoromethylornithine at 1 mM or higher concentrations decreased the growth rate by over 90% after 2 or more days of exposure, but the cells remained viable, although quiescent for at least 9 days. Addition of 10 microM-spermidine or -spermine or 50 microM-putrescine at any time throughout this period completely reversed the inhibition of growth. Treatment with alpha-difluoromethylornithine decreased putrescine and spermidine contents by more than 98% and that of spermine by 60%, but cells exposed to exogenous polyamines did not require complete replenishment of the polyamine pools to resume growth. In fact, a virtually normal growth rate was obtained in cells lacking putrescine, having 2% of normal spermidine content and 156% of normal spermine. These results suggest that the well-known increase in putrescine and spermidine in cells stimulated for growth is not essential for this to occur and that mammalian cells can utilize spermine as their only polyamine. A substantial reversal of the growth-inhibitory effect of alpha-difluoromethylornithine was produced by a number of polyamines not normally found in mammalian cells, including the spermidine analogues aminopropylcadaverine and sym-homospermidine, which were partially converted into their respective spermine analogues by addition of an aminopropyl group within the cell. The spermine analogue sym-norspermine was also effective, but the maximal growth rate produced by these unphysiological polyamines was only 60-70% of that produced by the normal polyamines. These results indicate that spermidine and spermine have the optimal length for activation of the cellular processes critically dependent on polyamines and should help in identifying these processes. Exposure to alpha-difluoromethylornithine leads to an enormous rise in the concentration of decarboxylated S-adenosylmethionine, which reached a peak at 530-fold after 3 days of exposure and steadily declined to 140-fold after 11 days. This increase was abolished by addition of exogenous polyamines, which rapidly decreased the activity of S-adenosylmethionine decarboxylase. The increase in decarboxylated S-adenosylmethionine is unlikely to be solely responsible for the decrease to the same extent by spermine, sym-norspermidine and sym-homospermidine, which produce 97%, 16% and 60% of the control growth rate, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)     

Validation of spermidine synthase as a drug target in African trypanosomes

The trypanocidal activity of the ODC (ornithine decarboxylase) inhibitor DFMO (difluoromethylornithine) has validated polyamine biosynthesis as a target for chemotherapy. As DFMO is one of only two drugs used to treat patients with late-stage African trypanosomiasis, the requirement for additional drug targets is paramount. Here, we report the biochemical properties of TbSpSyn (Trypanosoma brucei spermidine synthase), the enzyme immediately downstream of ODC in this pathway. Recombinant TbSpSyn was purified and shown to catalyse the formation of spermidine from putrescine and dcSAM (decarboxylated S-adenosylmethionine). To determine the functional importance of TbSpSyn in BSF (bloodstream form) parasites, we used a tetracycline-inducible RNAi (RNA interference) system. Down-regulation of the corresponding mRNA correlated with a decrease in intracellular spermidine and cessation of growth. This phenotype could be complemented by expressing the SpSyn (spermidine synthase) gene from Leishmania major in cells undergoing RNAi, but could not be rescued by addition of spermidine to the medium due to the lack of a spermidine uptake capacity. These results therefore genetically validate TbSpSyn as a target for drug development and indicate that in the absence of a functional biosynthetic pathway, BSF T. brucei cannot scavenge sufficient spermidine from their environment to meet growth requirements.     

Feedback regulation of S-adenosylmethionine decarboxylase synthesis.

Treatment of Ehrlich ascites-tumour cells with 1-amino-oxy-3-aminopropane (AOAP), a potent inhibitor of ornithine decarboxylase, resulted in a marked decrease in cellular contents of putrescine and spermidine, concomitant with an arrest of cell growth. The activity of S-adenosylmethionine decarboxylase (AdoMetDC) was greatly increased in cells treated with AOAP. This increase in AdoMetDC activity was shown to be, at least partly, caused by enhanced synthesis of the enzyme, which most likely was induced by the change in cellular polyamine content.     

Effects of inhibitors of ornithine and S-adenosylmethionine decarboxylases on L6 myoblast proliferation

The role of polyamines in myoblast proliferation was studied by treating cells of Yaffe's L6 line of rat myoblasts with inhibitors of polyamine synthesis. Both an irreversible inhibitor of ornithine decarboxylase--difluoromethyl-ornithine (DFMO)--and a competitive inhibitor of S-adenosyl-methionine decarboxylase--methylglyoxal-bis(guanylhydrazone) (MGBG)--depressed spermidine levels and inhibited myoblast proliferation. Spermine levels were not significantly depressed by either inhibitor and putrescine levels were decreased only by DFMO. Putrescine and spermidine, but not magnesium, prevented inhibition of myoblast proliferation by DFMO and MGBG; determination of 14C-DFMO uptake in the presence and absence of these compounds demonstrated that they did not reduce the rate or extent of inhibitor uptake and thus prevent its inhibition of ornithine decarboxylase. Thus it seems likely that these inhibitors reduce cell proliferation by inhibiting polyamine formation. Addition of spermidine to the cells led to a substantial reduction in the activity of S-adenosyl-methionine-decarboxylase, suggesting that the enzyme is subject to negative regulation by the products of the polyamine biosynthetic pathway. Unexpectedly, addition of spermidine also increased intracellular putrescine levels; this apparently resulted from conversion of spermidine to putrescine. Addition of putrescine or spermidine in the absence of serum did not increase the rate of myoblast proliferation although it did elevate intracellular polyamine levels as expected. We conclude that some threshold level of one or more polyamines (probably spermidine) is necessary but not sufficient for initiation and maintenance of myoblast proliferation in culture.     

Polyamine metabolism in potassium-deficient bacteria

The metabolism of polyamines was studied in K(+)-dependent strains of Escherichia coli. When these stringent organisms were in a medium containing Na(+) instead of K(+), protein synthesis was arrested, but synthesis of ribonucleic acid continued as it would in a relaxed organism. The Na(+) medium inhibited synthesis of spermidine and S-adenosylmethionine. However, the synthesis of putrescine was accelerated at least five- to eightfold. Exogenous ornithine doubled even this rate of putrescine synthesis but did not increase the low level of putrescine synthesis in the K(+) medium. In K(+) or Na(+) media, with or without 0.3 mm arginine, putrescine was derived almost entirely from ornithine via ornithine decarboxylase. Addition of spermidine (5 mm) to a Na(+) culture markedly inhibited putrescine synthesis. The ornithine decarboxylase of an extract of a K(-)-dependent strain prepared at low ionic strength was separated from ribosomes, deoxyribonucleic acid, and associated polyamines by centrifugation, and from many ions by ultrafiltration and fractionation on Sephadex G-100. Addition of Na(+) and K(+) salts to 200 mm was markedly inhibitory. The combined reductions both in synthesis of the inhibitor spermidine and in intracellular ionic strength may explain the in vivo activation of this enzyme.     

Utilization of putrescine in tobacco cell lines resistant to inhibitors of polyamine synthesis

Three tobacco cell lines have been analyzed which are resistant to lethal inhibitors of either putrescine production or conversion of putrescine into polyamines. Free and conjugated putrescine pools, the enzymic activities (arginine, ornithine, and S-adenosylmethionine decarboxylases), and the growth characteristics during acidic stress were measured in suspension cultures of each cell line. One cell line, resistant to difluoromethylornithine (Dfr1) had a very low level of ornithine decarboxylase activity which was half insensitive to the inhibitor in vitro. Intracellular free putrescine in Dfr1 was elevated 10-fold which was apparently due to a 20-fold increase in the arginine decarboxylase activity. The increased free putrescine titer was not reflected in an increased level of spermidine, spermine, or putrescine conjugation. Dfr1 cultures survived acidic stress at molarities which were lethal to wild type cultures. Two other mutants, resistant to methylglyoxal bis(guanylhydrazone) (Mgr3, Mgr12), had near normal levels of the three decarboxylases and normal titers of free putrescine, spermidine, and spermine. Both mutants however had elevated levels of conjugated putrescine. Mgr12 had an increased sensitivity to acidic medium. These results suggest that increased levels of free putrescine production may enhance the ability of tobacco cells to survive acid stress. This was supported by the observation that cytotoxic effects of inhibiting arginine decarboxylase in wild type cell lines were dependent on the acidity of the medium.     

Control of ornithine decarboxylase in Chinese hamster ovary cells by polyamines. Translational inhibition of synthesis and acceleration of degradation of the enzyme by putrescine, spermidine, and spermine

We have recently isolated, without using any inhibitors, a mutant of Chinese hamster ovary cell line which greatly overproduces ornithine decarboxylase in serum-free culture. Addition of polyamines (putrescine, spermidine, or spermine, 10 microM) or ornithine (1 mM), the precursor of polyamines, to the culture medium of these cells caused a rapid and extensive decay of ornithine decarboxylase activity. At the same time the activity of S-adenosylmethionine decarboxylase showed a less pronounced decrease. Notably, the polyamine concentrations used were optimal for growth of the cells and caused no perturbation of general protein synthesis. Spermidine and spermine appeared to be the principal regulatory amines for both enzymes, but also putrescine, if accumulated at high levels in the cells, was capable of suppressing ornithine decarboxylase activity. The amount of ornithine decarboxylase protein (as measured by radioimmunoassay) declined somewhat more slowly than the enzyme activity, but no more than 10% of the loss of activity could be ascribed to post-translational modifications or inhibitor interaction. Some evidence for inactivation through ornithine decarboxylase-antizyme complex formation was obtained. Gel electrophoretic determinations of the [35S]methionine-labeled ornithine decarboxylase revealed a rapid reduction in the synthesis and acceleration in the degradation of the enzyme after polyamine additions. No decrease in the amounts of the two ornithine decarboxylase-mRNA species, hybridizable to a specific cDNA, was detected, suggesting that polyamines depressed ornithine decarboxylase synthesis by selectively inhibiting translation of the message.     

Purification of S-adenosylmethionine decarboxylase from soybean.

S-Adenosylmethionine decarboxylase (EC 4.1.1.19) was purified to homogeneity from the cytosol of soybean (Glycine max) axes by ammonium sulfate fractionation, DEAE-Sepharose and methylglyoxalbis(guanylhydrazone)-Sepharose 6B chromatographies. The enzyme was free from diamine oxidase activity. The molecular weight of the enzyme estimated by gel filtration and sodium dodecyl sulfate polyacrylamide gel electrophoresis was 66,000. The Km value for S-adenosylmethionine was 0.26 mM. The optimum pH and temperature were 7.5 and 40 degrees C. Neither putrescine nor Mg2+ affected the enzyme activity, but the enzyme was inhibited by spermidine, spermine, methylglyoxalbis(guanylhydrazone), sodium borohydride and phenylhydrazine. Agmatine was a novel inhibitor which inhibited S-adenosylmethionine decarboxylase and arginine decarboxylase, preventing the accumulation of decarboxylated S-adenosylmethionine and putrescine, respectively.     

Changes in the polyamine and nucleotide metabolism of P388 leukemia cells treated with DL-alpha-difluoromethylornithine in culture

Influence of DL-alpha-difluoromethylornithine (DFMO) treatment on the growth kinetics, labelling index, extra- and intracellular polyamine and nucleotide concentrations was monitored in cultured P388 leukemia cells. A substantial decrease of cell proliferation was observed when the cells were continuously treated with 1-5 mM DFMO. Depletion of cellular polyamines, mostly of putrescine and spermidine, was seen with a concomitant but delayed increase of spermidine and spermine levels in the culture medium. Changes of DNA content and of labelling index of untreated and treated cells seem to indicate that DFMO arrested cells in G1/S transition. The results presented here provide additional in vitro evidence on the characteristic changes in the metabolic imbalance of ornithine in tumor cells induced by DFMO via inhibition of ornithine decarboxylase and ornithine carbamoyl transferase activities.