Effects of polyamines on mitochondrial Ca transport

Mammalian mitochondria are able to enhance Ca²⁺ accumulation in the presence of polyamines by activating the saturable systems of Ca²⁺ inward transport and buffering extramitochondrial Ca²⁺ concentrations to levels similar to those in the cytosol of resting cells. This effect renders them responsive to regulate free Ca²⁺ concentrations in the physioloical range. The mechanism involved is due to a rise in the affinity of the Ca²⁺ transport system, induced by polyamines, most probably exhibiting allosteric behaviour. The regulatory site of this mechanism is the so-called S₁ binding site of polyamines, which operates in physiological conditions and is located in the energy well between the two peaks present in the energy profile of mitochondrial spermine transport. Spermine is bidirectionally transported across teh inner membrane by cycling, in which influx and efflux are driven by electrical and pH gradients, respectively. Most probably, polyamine affects the Ca²⁺ transport system when it acts from the outside-that is, in the direction of its uniporter channel, in order to reach the S₁ site. Important physiological functions are related to activation of Ca²⁺ transport systems by polyamines and their interactions with the S₁ site. These functions include a rise in the metabolic rate for energy supply and modulation of mitochondrial permeability transition induction, with consequent effects on the triggering of the apoptotic pathway.     

Polyamine uptake in carrot cell cultures

Putrescine and spermidine uptake into carrot (Daucus carota L.) cells in culture was studied. The time course of uptake showed that the two polyamines were very quickly transported into the cells, reaching a maximum absorption within 1 minute. Increasing external polyamine concentrations up to 100 millimolar showed the existence of a biphasic system with different affinities at low and high polyamine concentrations. The cellular localization of absorbed polyamines was such that a greater amount of putrescine was present in the cytoplasmic soluble fraction, while spermidine was mostly present in cell walls. The absorbed polyamines were released into the medium in the presence of increasing external concentrations of the corresponding polyamine or Ca²⁺. The effects of Ca²⁺ were different for putrescine and spermidine; putrescine uptake was slightly stimulated by 10 micromolar Ca²⁺ and inhibited by higher concentrations, while for spermidine uptake there was an increasing stimulation in the Ca²⁺ concentration range between 10 micromolar and 1 millimolar. La³⁺ nullified the stimulatory effect of 10 micromolar Ca²⁺ on putrescine uptake and that of 1 millimolar Ca²⁺ on spermidine uptake. La³⁺ at 0.5 to 1 millimolar markedly inhibited the uptake of both polyamines, suggesting that it interferes with the sites of polyamine uptake. Putrescine uptake was affected to a lesser extent by metabolic inhibitors than was spermidine uptake. It is proposed that the entry of polyamines into the cells is driven by the transmembrane electrical gradient, with a possible antiport mechanism between external and internal polyamine molecule.     

Uptake of spermine by rat liver mitochondria and its influence on the transport of phosphate

Spermine, a polyamine present in the mammalian cells at rather high concentration, has, among other actions, a remarkable stabilizing effect on mitochondria, functions which have generally been attributed to the capability of this and other polyamines to bind to membrane anionic sites. In the present paper evidence is provided that at physiological concentrations spermine may also be transported into rat liver mitochondrial matrix space, provided that mitochondria are energized and inorganic phosphate is simultaneously transported. The close dependence of spermine transport is also demonstrated by the concurrent efflux of spermine and inorganic phosphate when mitochondria preloaded with the two ionic species are deenergized either with uncouplers or respiratory chain inhibitors. Furthermore, Mersalyl, the known inhibitor of phosphate transport, prevents both spermine uptake and release. Mg2+ inhibits the transport of spermine conceivably by competing for the some binding sites on the mitochondrial membrane. The physiological significance of these results is discussed.     

Effects of polyamines on mitochondrial Ca(2+) transport

Mammalian mitochondria are able to enhance Ca(2+) accumulation in the presence of polyamines by activating the saturable systems of Ca(2+) inward transport and buffering extramitochondrial Ca(2+) concentrations to levels similar to those in the cytosol of resting cells. This effect renders them responsive to regulate free Ca(2+) concentrations in the physioloical range. The mechanism involved is due to a rise in the affinity of the Ca(2+) transport system, induced by polyamines, most probably exhibiting allosteric behaviour. The regulatory site of this mechanism is the so-called S(1) binding site of polyamines, which operates in physiological conditions and is located in the energy well between the two peaks present in the energy profile of mitochondrial spermine transport. Spermine is bidirectionally transported across teh inner membrane by cycling, in which influx and efflux are driven by electrical and pH gradients, respectively. Most probably, polyamine affects the Ca(2+) transport system when it acts from the outside-that is, in the direction of its uniporter channel, in order to reach the S(1) site. Important physiological functions are related to activation of Ca(2+) transport systems by polyamines and their interactions with the S(1) site. These functions include a rise in the metabolic rate for energy supply and modulation of mitochondrial permeability transition induction, with consequent effects on the triggering of the apoptotic pathway.     

Agmatine is transported into liver mitochondria by a specific electrophoretic mechanism

Agmatine, a divalent diamine with two positive charges at physiological pH, is transported into the matrix of liver mitochondria by an energy-dependent mechanism the driving force of which is DeltaPsi (electrical membrane potential). Although this process showed strict electrophoretic behaviour, qualitatively similar to that of polyamines, agmatine is most probably transported by a specific uniporter. Shared transport with polyamines by means of their transporter is excluded, as divalent putrescine and cadaverine are ineffective in inhibiting agmatine uptake. Indeed, the use of the electroneutral transporter of basic amino acids can also be discarded as ornithine, arginine and lysine are completely ineffective at inducing the inhibition of agmatine uptake. The involvement of the monoamine transporter or the existence of a leak pathway are also unlikely. Flux-voltage analysis and the determination of activation enthalpy, which is dependent upon the valence of agmatine, are consistent with the hypothesis that the mitochondrial agmatine transporter is a channel or a single-binding centre-gated pore. The transport of agmatine was non-competitively inhibited by propargylamines, in particular clorgilyne, that are known to be inhibitors of MAO (monoamine oxidase). However, agmatine is normally transported in mitoplasts, thus excluding the involvement of MAO in this process. The I2 imidazoline receptor, which binds agmatine to the mitochondrial membrane, can also be excluded as a possible transporter since its inhibitor, idazoxan, was ineffective at inducing the inhibition of agmatine uptake. Scatchard analysis of membrane binding revealed two types of binding site, S1 and S2, both with mono-co-ordination, and exhibiting high-capacity and low-affinity binding for agmatine compared with polyamines. Agmatine transport in liver mitochondria may be of physiological importance as an indirect regulatory system of cytochrome c oxidase activity and as an inducer mechanism of mitochondrial-mediated apoptosis.     

Cell proliferation,potassium channels,polyamines and their interactions: a mini review

Polyamines, which are obligatory molecules involved in cell cycling and proliferation, are subject to a change in their free intracellular concentrations during the cell cycle. Potassium (K⁺) channels are also considered, but less well recognized, to be necessary for cell proliferation by either hyperpolarizing or depolarizing cells during the cell cycle. A block of polyamine synthesis as well as block or knockout of K⁺ channels can halt cell proliferation. K⁺ channels like BK (maxi calcium (Ca²⁺)-activated K⁺), Kir (inward rectifier), M-type K⁺-and TASK (two-pore domain K⁺) channels or the delayed rectifier K⁺ channels are modulated in their electrical properties by polyamines. Polyamines are most effective in blocking these channels when applied to the intracellular face of these channels except for TASK channels where they act only from the extracellular side. Quinidine, a general K⁺ channel blocker, was found to reduce putrescine concentrations, to block the ornithine decarboxylase and halt cell proliferation. From these results, the question arises if there is an interaction between polyamines, K⁺ channels and proliferation. It might be speculated that a decrease of intracellular polyamines allows more K⁺ channels to be active, thus inducing hyperpolarization, while an increase of the polyamine concentration may block K⁺ channel activity leading to depolarization of the membrane potential. On the other hand, a block or a deletion of K⁺ channels may cause a decrease of the polyamine concentration in cells. More research is needed to test these hypotheses.     

Cytoplasmic polyamines block the fast-activating vacuolar cation channel

The fast-activating vacuolar (FV) channel dominates the electrical characteristics of the tonoplast at physiological free Ca²⁺ concentrations. Since polyamines are known to increase in plant cells in response to stress, the regulation of FV channels by polyamines was investigated. Patch-clamp measurements were performed on whole barley ( Hordeum vulgare ) mesophyll vacuoles and on excised tonoplast patches. The trivalent polyamine spermidine and the tetravalent polyamine spermine blocked FV channels with K_d≈ 100 μM and K_d≈ 5 μM, respectively. Increasing cytosolic and vacuolar Ca²⁺ had no effect on putrescine and spermidine binding to FV channels but slightly decreased the affinity for spermine. The inhibition of FV channels by all three polyamines was not voltage-dependent. This points to a different mode of binding compared to inward rectifier K⁺ channels and Ca²⁺-permeable glutamate receptor channels from animal cells, which show rectification due to a voltage-dependent block by polyamines. In plant cells, the common polyamines (putrescine, spermidine and spermine) are likely to mediate a salt stress-induced decrease of ion flux across the vacuolar membrane by blocking FV channels.     

Spermidine Uptake by Mitochondria of Helianthus tuberosus

In the present work evidence is provided that spermidine, a polyamine largely present in plant tissues, may be transported, at physiological concentrations, into the matrix space of mitochondria isolated from tubers of Helianthus tuberosus L. cv OB1 (Jerusalem artichoke). It is concluded that the movement of spermidine strictly depends on membrane potential, since it is drastically blocked by valinomycin and only slightly sensitive to nigericin. Mg²⁺ and K⁺ inhibit the transport of spermidine in line with the general concept that these cations compete for the same binding sites on the mitochondrial membrane. In contrast to previous data on mammalian mitochondria, spermidine uptake by plant mitochondria does not depend on the presence of inorganic phosphate. This latter result, along with evidence that Ca²⁺ does not affect accumulation of spermidine, indicates that the control of the polyamine uptake mechanism in plant mitochondria is distinct from that of mammalian systems.     

Insulin-like effects of polyamines spermine binding to fat cells and fat cell membranes

Two naturally occurring polyamines, spermine and spermidine, mimic the action of insulin on lipid and glucose metabolism in adipocytes. To evaluate the role of cell membranes in the action of polyamines, studies of [¹⁴C] spermine binding using an oil separation method were conducted in isolated rat adipocytes and adipose cell membranes. Spermine binding and dissociation in fat cells and fat cell membranes were rapid and complete within 3–6 min. Following a 30-min incubation of [¹⁴C] spermine with fat cell membranes, over 90% of bound [¹⁴C] spermine was dissociable while under similar conditions only 25% of bound [¹⁴C] spermine was dissociable in cells. The non-dissociable fractions in cells likely represented intracellular accumulation. Binding and stimulation of glucose oxidation were demonstrated at similar concentrations. Bound spermine was displaced by spermine, spermidine and 1,8-diaminooctane with greater efficacy than putrescine (a polyamine devoid of insulin-like properties) or insulin. Similarly, polyamines did not complete with insulin for binding to isolated adipocytes. It appears, therefore, that polyamines initiate their insulin-like effects by interacting with the cell membrane at sites which are common to biologically active polyamines and which are distinct from the insulin receptor.     

Regulations of thyroid decarboxylase by the polyamines Induction of a protein inhibitor of ornithine decarboxylase by the end-products of the reaction

When spermdine, putrescine or 1,3-diaminopropane was injected (12.5 μmol/100 g body weight) into rats i h before thyrotropin, ornithine decarboxylase activity was increased by 75–150% over control levels. However, when 75 μmol polyamine/100 g body weight was injected, thyrotropin-activated activity was inhibited by 70–95%. Multiple polyamine injections inhibited goitrogen-induced activity and gland weight increase by approx. 35%.The polyamines also inhibited thyrotrophin-activated rat thyroid ornithine decarboxylase in vitro in a dose-related fashion, with 50% inhibition occurring at 2–5 · 10⁻⁴ M. The inhibition was not due to a direct effect on the enzyme. No stimulation was seen with low concentration of polyamine. The polyamines had no effect on in vitro thyroid protein/RNA synthesis or glucose oxidation but had a biphasic effect on plasma membrane adenylate cyclase activity.A protein inhibitor to thyroid ornithine decarboxylase was generated in vivo by multiple injections of the polyamines into rats, and in vitro by incubating bovine thyroid slices with 2–10 mM polyamine. The inhibitor was non-dialyzable, destroyed by boiling, and its formation was blocked in a dose-related fashion by cycloheximide.We conclude that: (1) thyroid ornithine decarboxylase is subject not only to positive control, but is also negatively regulated by its end-products, the polyamines, which induce a protein inhibitor to ornithine decarboxylase; (2) since gland growth is also inhibited under these conditions, the polyamine effect on thyroid ornithine decarboxylase may be biologically significant.     

Polyamine depletion induces apoptosis through mitochondria-mediated pathway

Polyamines, namely putrescine, spermidine, and spermine, are essential for cell survival and proliferation. A decrease in intracellular polyamine levels is associated with apoptosis. In this study, we used inhibitors of polyamine biosynthesis to examine the effect of polyamine depletion. A combination of inhibitors of ornithine decarboxylase, S-adenosylmethionine decarboxylase, or spermidine synthase decreased intracellular polyamine levels and induced cell death in a WEHI231 murine B cell line. These cells exhibited apoptotic features including chromatin condensation and oligonucleosomal DNA fragmentation. Addition of exogenous polyamines reversed the observed features of apoptotic cell death. Similar effects were also observed in other cell lines: a human B cell line Ramos and a human T cell line Jurkat. Depletion of polyamines induced activation of caspase-3 and disruption of the mitochondrial membrane potential (Delta psi m). Inhibition of caspase activities by an inhibitor prevented the apoptotic nuclear changes but not Delta psi m disruption induced by polyamine depletion. Overexpression of Bcl-xl, an anti-apoptotic Bcl-2 family protein, completely inhibited Delta psi m disruption, caspase activation, and cell death. These results indicate that the depletion of intracellular polyamines triggers the mitochondria-mediated pathway for apoptosis, resulting in caspase activation and apoptotic cell death.     

Bidirectional fluxes of spermine across the mitochondrial membrane

The polyamine spermine is transported into the mitochondrial matrix by an electrophoretic mechanism having as driving force the negative electrical membrane potential (ΔΨ). The presence of phosphate increases spermine uptake by reducing ΔpH and enhancing ΔΨ. The transport system is a specific uniporter constituted by a protein channel exhibiting two asymmetric energy barriers with the spermine binding site located in the energy well between the two barriers. Although spermine transport is electrophoretic in origin, its accumulation does not follow the Nernst equation for the presence of an efflux pathway. Spermine efflux may be induced by different agents, such as FCCP, antimycin A and mersalyl, able to completely or partially reduce the ΔΨ value and, consequently, suppress or weaken the force necessary to maintain spermine in the matrix. However this efflux may also take place in normal conditions when the electrophoretic accumulation of the polycationic polyamine induces a sufficient drop in ΔΨ able to trigger the efflux pathway. The release of the polyamine is most probably electroneutral in origin and can take place in exchange with protons or in symport with phosphate anion. The activity of both the uptake and efflux pathways induces a continuous cycling of spermine across the mitochondrial membrane, the rate of which may be prominent in imposing the concentrations of spermine in the inner and outer compartment. Thus, this event has a significant role on mitochondrial permeability transition modulation and consequently on the triggering of intrinsic apoptosis.     

Membrane binding and transport of N-aminoethyl-1,2-diamino ethane (dien) and N-aminopropyl-1,3-diamino propane (propen) by rat liver mitochondria and their effects on membrane permeability transition

This investigation is aimed at defining the structural requirements for aliphatic polyamines to interact with mitochondrial binding sites, which are relevant for the regulation of the permeability transition and for mitochondrial polyamine uptake. The triamines N-aminoethyl-1,2-diaminoethane (dien) and N-aminopropyl-1,3-diaminopropane (propen), both symmetric polyamines, are accumulated to differing extents by an energy-dependent mechanism in liver mitochondria. Propen is also able completely to inhibit the permeability transition of mitochondria, induced by Ca2+ plus phosphate, with the same efficacy as the asymmetric ubiquitary triamine spermidine, whereas dien fails to exhibit this effect. The competitive inhibition of both triamines on spermidine transport demonstrates that they bind to the same site(s) of this polyamine and exploit its transport system. The binding of dien and propen to mitochondrial membrane was studied by applying a thermodynamic model of ligand-receptor interactions developed both for equilibrium and far-from-equilibrium binding processes. Results show the presence of two mono-coordinated binding sites, S1 and S2, for propen, and one monocoordinated binding site for dien, all exhibiting high capacity and low affinity. Comparisons of the binding parameters of these polyamines with those of other natural polyamines reveal that, besides flexibility and hydrophilicity, as previously suggested, protonation of the imino group and the symmetry of the molecules for S1, and the presence of an aminobutyl group for S2, also contribute to the polyamine interactions observed in the two sites.     

Polyamine transport in parasites: a potential target for new antiparasitic drug development

The metabolism of the naturally occurring polyamines-putrescine, spermidine and spermine-is a highly integrated system involving biosynthesis, uptake, degradation and interconversion. Metabolic differences in polyamine metabolism have long been considered to be a potential target to arrest proliferative processes ranging from cancer to microbial and parasitic diseases. Despite the early success of polyamine inhibitors such as alpha-difluoromethylornithine (DFMO) in treating the latter stages of African sleeping sickness, in which the central nervous system is affected, they proved to be ineffective in checking other major diseases caused by parasitic protozoa, such as Chagas' disease, leishmaniasis or malaria. In the use and design of new polyamine-based inhibitors, account must be taken of the presence of up-regulated polyamine transporters in the plasma membrane of the infectious agent that are able to circumvent the effect of the drug by providing the parasite with polyamines from the host. This review contains information on the polyamine requirements and molecular, biochemical and genetic characterization of different transport mechanisms in the parasitic agents responsible for a number of the deadly diseases that afflict underdeveloped and developing countries.     

Selective regulation of polyamine metabolism with methylated polyamine analogues

Polyamine metabolism is intimately linked to the physiological state of the cell. Low polyamines levels promote growth cessation, while increased concentrations are often associated with rapid proliferation or cancer. Delicately balanced biosynthesis, catabolism, uptake and excretion are very important for maintaining the intracellular polyamine homeostasis, and deregulated polyamine metabolism is associated with imbalanced metabolic red/ox state. Although many cellular targets of polyamines have been described, the precise molecular mechanisms in these interactions are largely unknown. Polyamines are readily interconvertible which complicate studies on the functions of the individual polyamines. Thus, non-metabolizable polyamine analogues, like carbon-methylated analogues, are needed to circumvent that problem. This review focuses on methylated putrescine, spermidine and spermine analogues in which at least one hydrogen atom attached to polyamine carbon backbone has been replaced by a methyl group. These analogues allow the regulation of both metabolic and catabolic fates of the parent molecule. Substituting the natural polyamines with methylated analogue(s) offers means to study either the functions of an individual polyamine or the effects of altered polyamine metabolism on cell physiology. In general, gem-dimethylated analogues are considered to be non-metabolizable by polyamine catabolizing enzymes spermidine/spermine-N ¹-acetyltransferase and acetylpolyamine oxidase and they support short-term cellular proliferation in many experimental models. Monomethylation renders the analogues chiral, offering some advantage over gem-dimethylated analogues in the specific regulation of polyamine metabolism. Thus, methylated polyamine analogues are practical tools to meet existing biological challenges in solving the physiological functions of polyamines.     

An Improved Isolation Procedure for the Gas Chromatographic Analysis of Urinary Polyamines

The isolation of polyamines from urinary hydrolysates in a sufficiently pure state for subsequent analysis by gas chromatography has proved to be difficult. However, by using columns of Porapak-Q and ion-exchange resins, urinary hydrolysates are readily purified and formation of trifluoroacetyl derivatives of polyamines proceeds in high yield without carryover of artifacts in the gas chromatographic elution profile. Good yields from the trifluoroacetylation reaction are not achieved if large quantities of salts or urinary pigments are present. By obtaining the polyamine carbonates in the final stages of the method described, the trifluoroacetylation reaction yields excellent derivatives of nanogram or microgram amounts, particularly after standing over-night at room temperature. The procedure described in detail should permit routine urinary polyamine analysis where rapidity, ease of handling many samples, freedom from complications and artifacts are a consideration. The recent reports by Russell^{1, 2} that the urinary excretion of polyamines are greatly elevated in cancer patients has stimulated interest in these compounds as possible biological “markers” for the diagnosis of cancer. The polyamines usually considered are: putrescine, 1, 4-diaminobutane; cadaverine, 1, 5-diaminopentane; spermidine, and spermine. An extensive literature has developed over the last 50 years concerning the isolation and determination of polyamines including many excellent reviews. ^3–5 However, the isolation and determination of small quantities of polyamines from biological sources has proven to be difficult. This has led to conflicting conclusions among investigators as to which polyamine is the major excretion product in the urine of cancer patients. ^{2, 6, 7, 8} The following report presents in detail a new procedure of isolation of urinary polyamines in high yield and pure state that facilitates quantitation of these amines by gas chromatography.     

Novel gating mechanism of polyamine block in the strong inward rectifier K channel Kir2.1

Inward rectifying K channels are essential for maintaining resting membrane potential and regulating excitability in many cell types. Previous studies have attributed the rectification properties of strong inward rectifiers such as Kir2.1 to voltage-dependent binding of intracellular polyamines or Mg to the pore (direct open channel block), thereby preventing outward passage of K ions. We have studied interactions between polyamines and the polyamine toxins philanthotoxin and argiotoxin on inward rectification in Kir2.1. We present evidence that high affinity polyamine block is not consistent with direct open channel block, but instead involves polyamines binding to another region of the channel (intrinsic gate) to form a blocking complex that occludes the pore. This interaction defines a novel mechanism of ion channel closure.     

Regulation of thyroid ornithine decarboxylase by the polyamines. Induction of a protein inhibitor of ornithine decarboxylase by the end-products of the reaction

When spermidine, putrescine or 1,3-diaminopropane was injected (12.5 mumol/100 g body weight) into rats 1 h before thyrotropin, ornithine decarboxylase activity was increased by 75--150% over control levels. However, when greater than or equal to 75 mumol polyamine/100 g body weight was injected, thyrotropin-activated activity was inhibited by 70--95%. Multiple polyamine injections inhibited goitrogen-induced activity and gland weight increase by approx 35%. The polyamines also inhibited thyrotropin-activated rat thyroid ornithine decarboxylase in vitro in a dose-related fashion, with 50% inhibition occurring at 2--5 . 10(-4)M. The inhibition was not due to a direct effect on the enzyme. No stimulation was seen with low concentrations of polyamine. The polyamines had no effect on in vitro thyroid protein/RNA synthesis or glucose oxidation but had a biphasic effect on plasma membrane adenylate cyclase activity. A protein inhibitor to thyroid ornithine decarboxylase was generated in vivo by multiple injections of the polyamines into rats and in vitro by incubating bovine thyroid slices with 2--10 mM polyamine. The inhibitor was non-dialyzable, destroyed by boiling, and its formation was blocked in a dose-related fashion by cycloheximide. We conclude that: (1) thyroid ornithine decarboxylase is subject not only to positive control, but is also negatively regulated by its end-products, the polyamines, which induce a protein inhibitor to ornithine decarboxylase; (2) since gland growth is also inhibited under these conditions, the polyamine effect on thyroid ornithine decarboxylase may be biologically significant.     

Effects of polyamines on nucleosidediphosphate kinase activity.

Effects of naturally occurring polyamines were tested on the activity of bovine liver nucleosidediphosphate kinase with ATP as phosphoryl donor and eight nucleosidediphosphates as phosphoryl acceptors. The enzyme was either stimulated, inhibited, or unaffected depending upon the nucleosidediphosphate substrate and the polyamine, indicating that a general cation effect is not a sole mechanism of polyamine action. This selectivity and specificity of the effects with respect to both the polyamines and the nucleosidediphosphates leads us to speculate that an action of polyamines on nucleosidediphosphate kinase may play a significant role in the regulation of specific nucleosidetriphosphate synthesis $\text{in vivo}$.     

Active transport and metabolic characteristics of polyamines in the rat lens

Putrescine, spermidine and spermine were transported into the rat lens against a concentration gradient. This process appeared to be energy-dependent and involved a carrier system different from those for amino acids. Competition experiments suggested that the three polyamines were transported by the same system or very similar systems. Incorporated spermine was converted to spermidine and putrescine, and spermidine was converted to putrescine. In contrast, the conversion of putrescine to spermidine and spermine, or the conversion of spermidine to spermine was not observed. Furthermore, ornithine was not utilized for the synthesis of putrescine. These metabolic characteristics of the polyamines in the rat lens were correlated with the extremely low activities of ornithine decarboxylase and S-adenosylmethionine decarboxylase. Other enzymes of polyamine metabolisms, however, were relatively active. In conclusion, the lens has a very low ability for the de novo synthesis of polyamines. The polyamines in the lens are considered to be supplied form the surrounding intraocular fluid by an active transport system specific for polyamines.