S-adenosylmethionine decarboxylase from human prostate. Activation by putrescine

Herpesvirus-induced spermidine synthesis and eIF5A hypusination for viral episomal maintenance

Spermidine is essential for cellular growth and acts as a prerequisite of hypusination, a post-translational modification of eukaryotic initiation factor 5A (eIF5A), allowing the translation of polyproline-containing proteins. Here, we show that oncogenic Kaposi's sarcoma-associated herpesvirus (KSHV) increases spermidine synthesis and eIF5A hypusination to enhance expression of polyproline-containing latency-associated nuclear antigen (LANA) for viral episomal maintenance. KSHV upregulates intracellular spermidine levels by dysregulating polyamine metabolic pathways in three-dimensional (3D) culture and 2D de novo infection conditions. Increased intracellular spermidine leads to increased eIF5A hypusination, ultimately enhancing LANA expression. In contrast, inhibition of spermidine synthesis or eIF5A hypusination alleviates LANA expression, decreasing viral episomal maintenance and KSHV-infected cell proliferation in vitro and in vivo, which is reversed by spermidine supplement. This demonstrates that KSHV hijacks spermidine synthesis and eIF5A hypusination pathways to enhance LANA expression for viral episomal maintenance, suggesting polyamine metabolism and eIF5A hypusination as therapeutic targets for KSHV-induced tumorigenesis.

The activator-binding site of Onchocerca volvulus S-adenosylmethionine decarboxylase, a potential drug target

S-Adenosylmethionine decarboxylase (AdoMetDC) is a key enzyme in polyamine biosynthesis. In many eukaryotes its activity is stimulated specifically by putrescine. The AdoMetDC of the filarial parasite Onchocerca volvulus, however, is not only stimulated by putrescine but also by the naturally occuring polyamines spermidine and spermine. Several diamines, acetylated polyamines and polyamine analogues were used to analyse what molecular prerequisites are needed to stimulate nematode AdoMetDC activity. In the absence of an activator, the O. volvulus enzyme exhibits an extremely low specific activity. This fact, together with the unspecificity of activator binding, was thought to be useful for a new strategy to inhibit nematode AdoMetDC activity. Therefore, different polyamine analogues were tested as competitive inhibitors towards the stimulatory effect putrescine has on the O. volvulus and, in comparison, on the Caenorhabditis elegans and human AdoMetDC. Bis(aralkyl)- and bis(alkyl)-substituted polyamine analogues with a 3-7-3 backbone were found to inhibit AdoMetDC activities, however, probably without interfering with the putrescine stimulation. The best inhibitor, BW-1, was about 10-fold more effective against O. volvulus AdoMetDC than against the human enzyme. Unexpectedly, BW-1 was determined to be a competitive inhibitor with respect to AdoMet, having a Ki value of 310 microM for the putrescine-stimulated human AdoMetDC. Furthermore, we show for the O. volvulus and the human enzyme that the degree of inhibition by BW-1 depends on the actual putrescine concentration.

Caenorhabditis elegans S-adenosylmethionine decarboxylase is highly stimulated by putrescine but exhibits a low specificity for activator binding

S-Adenosylmethionine decarboxylase (AdoMetDC) is a key enzyme of the polyamine synthetic pathway providing decarboxylated S-adenosylmethionine for the formation of spermidine and spermine, respectively. The catalytic activity of the AdoMetDC from the free-living nematode Caenorhabditis elegans highly depends on the presence of an activator molecule. Putrescine, a well-known stimulator of mammalian AdoMetDC activity, enhances the catalytic activity of the nematode enzyme 350-fold. Putrescine stimulation is discussed as a regulatory mechanism to relate putrescine abundance with the synthesis of spermidine and spermine. In contrast to any other known AdoMetDC, spermidine and spermine also represent significant activators of the nematode enzyme. However, the biological significance of the observed stimulation by these higher polyamines is unclear. Although C. elegans AdoMetDC exhibits a low specificity toward activator molecules, the amino acid residues that were shown to be involved in putrescine binding of the human enzyme are conserved in the nematode enzyme. Exchanging these residues by site-directed mutagenesis indicates that at least three residues, Thr192, Glu194 and Glu274, most likely contribute to activator binding in the C. elegans AdoMetDC. Interestingly, the mutant Glu194Gln exhibits a 100-fold enhanced basal activity in the absence of any stimulator, suggesting that this mutant protein mimics the conformational change usually induced by activator molecules. Furthermore, site-directed mutagenesis revealed that at least Glu33, Ser83, Arg91 and Lys95 are involved in posttranslational processing of C. elegans AdoMetDC.

Putrescine activation of Trypanosoma cruzi S-adenosylmethionine decarboxylase

S-Adenosylmethionine decarboxylase (AdoMetDC) is a pyruvoyl-dependent enzyme that is processed from a single polypeptide into two subunits creating the cofactor. In the human enzyme, both the proenzyme processing reaction and enzyme activity are stimulated by the polyamine putrescine. The processing reaction of Trypanosoma cruzi AdoMetDC was studied in an in vitro translation system. The enzyme was fully processed in the absence of putrescine, and the rate of this reaction was not stimulated by addition of the polyamine. Residues in the putrescine binding site of the human enzyme were evaluated for their role in processing of the T. cruzi enzyme. The E15A, I80K/S178E, D174A, and E256A mutant T. cruzi enzymes were fully processed. In contrast, mutation of R13 to Leu (the equivalent residue in the human enzyme) abolished processing of the T. cruzi enzyme, demonstrating that Arg at position 13 is a major determinant for proenzyme processing in the parasite enzyme. This amino acid change is a key structural difference that is likely to be a factor in the finding that putrescine has no role in processing of the T. cruzi enzyme. In contrast, the activity of T. cruzi AdoMetDC is stimulated by putrescine. Equilibrium sedimentation experiments demonstrated that putrescine does not alter the oligomeric state of the enzyme. The putrescine binding constant for binding to the T. cruzi enzyme (K(d) = 150 microM) was measured by a fluorescence assay and by ultrafiltration with a radiolabeled ligand. The mutant T. cruzi enzyme D174V no longer binds putrescine, and is not activated by the diamine. In contrast, mutation of E15, S178, E256, and I80 had no effect on putrescine binding. The k(cat)/K(m) values for E15A and E256A mutants were stimulated by putrescine to a smaller extent than the wild-type enzyme (2- and 4-fold vs 11-fold, respectively). These data suggest that the putrescine binding site on the T. cruzi enzyme contains only limited elements (D174) in common with the human enzyme and that the diamine plays different roles in the function of the mammalian and parasite enzymes.

Single-turnover kinetic analysis of Trypanosoma cruzi S-adenosylmethionine decarboxylase

Trypanosoma cruzi S-adenosylmethionine decarboxylase (AdoMetDC) catalyzes the pyruvoyl-dependent decarboxylation of S-adenosylmethionine (AdoMet), which is an important step in the biosynthesis of polyamines. The time course of the AdoMetDC reaction under single-turnover conditions was measured to determine the rate of the slowest catalytic step up to and including decarboxylation. Analysis of this single-turnover data yields an apparent second-order rate constant for this reaction of 3300 M(-1) s(-1) in the presence of putrescine, which corresponds to a catalytic rate of >6 s(-1). This rate is minimally 100-fold faster than the steady-state rate suggesting that product release, which includes Schiff base hydrolysis, limits the overall reaction. AdoMetDC exhibits an inverse solvent isotope effect on the single-turnover kinetics, and the pH profile predicts a pK(a) of 8.9 for the basic limb. These results are consistent with a Cys residue functioning as a general acid in the rate-determining step of the single-turnover reaction. Mutation of Cys-82 to Ala reduces the rate of the single turnover reaction to 11 M(-1) s(-1) in the presence of putrescine. Further, a solvent isotope effect is not observed for the mutant enzyme. Reduction of the wild-type enzyme with cyanoborohydride traps the Schiff base between the enzyme and decarboxylated substrate, while little Schiff base species of either substrate or product was trapped with the C82A mutant. These data suggest that Cys-82 functions as a general acid/base to catalyze Schiff base formation and hydrolysis. The solvent isotope and pH effects are mirrored in single-turnover analysis of reactions without the putrescine activator, yielding an apparent second-order rate constant of 150 M(-1) s(-1). The presence of putrescine increases the single-turnover rate by 20-fold, while it has relatively little effect on the affinity of the enzyme for product. Therefore, putrescine likely activates the T. cruzi AdoMetDC enzyme by accelerating the rate of Schiff base exchange.

Cloning and kinetic characterization of the Trypanosoma cruzi S-adenosylmethionine decarboxylase

The gene for S-adenosylmethionine decarboxylase (AdoMetDC), a rate-limiting enzyme in the biosynthesis of polyamines, has been cloned from a Trypanosoma cruz cDNA library. The cDNA clone contains a 1.1 kb open reading frame predicted to encode a 42 kDa protein that shares 31% sequence identity to the human proenzyme. T. cruzi AdoMetDC expressed and purified from E. coli is auto-catalytically processed into two subunits of 32 kDa (alpha) and 10 kDa (beta). The catalytic activity of the purified recombinant enzyme is activated by the addition of putrescine to the reaction. To determine the effect of putrescine on the kinetics of the reaction, the velocity data collected at various substrate and putrescine concentrations were fit to the rate equation describing a non-essential activator. In the presence of fully saturating putrescine, k(cat) increases by 9-fold over the unactivated rate to 0.06 s(-1). The model derived Km for AdoMet is 0.05 mM in the absence of putrescine and the model-derived Kd for putrescine binding to free enzyme is 2.5 mM. The Km for AdoMet increases by alpha 2-fold when the enzyme is fully saturated with putrescine. Unlike human AdoMetDC, cadaverine activates the T. cruzi enzyme to a similar extent as putrescine.

S-adenosylmethionine decarboxylase in human brain. Regional distribution and influence of aging

Recent experimental animal studies have implicated brain polyamines as having roles in both brain development and human brain neurodegenerative conditions. In order to provide baseline information, in normal human brain, on one of the key polyamine synthesising enzymes, S-adenosylmethionine decarboxylase (SAMDC), we examined the sensitivity of this enzyme to various cofactors/inhibitors, its regional distribution, and influence of aging in neurologically normal autopsied human brain. SAMDC in normal human brain is similar to that reported in other mammalian cells with regard to substrate affinity (Km = 39 microM), marked sensitivity to putrescine activation (+600%), inhibition (methylglyoxalbisguanidine and MDL 73811), and pH optimum (7.2). There was an uneven distribution of enzyme activity in human brain, and of the 12 brain regions examined, the highest activity was observed in occipital, parietal, frontal and temporal cortices (36-58 pmol/h/mg protein); intermediate activity in cerebellar and insular cortex, pulvinar thalamus, caudate and putamen (12-27 pmol/h/mg protein); and lowest activity in medial-dorsal thalamus, lateral globus pallidus and white matter (< 11 pmol/h/mg protein). The influence of aging (1 day to 103 years) on SAMDC activity in occipital cortex, the region showing the highest activity in human brain (n = 59) was also determined. Enzyme activity increased by approximately 600% from age 6 months to near maximal levels at age 10 years, then remained generally unchanged up to 103 years. Since SAMDC is a key regulatory enzyme in the synthesis of spermidine and spermine, the marked increase in SAMDC activity in the neonate and the sustained high enzyme levels throughout adulthood, imply a role for these polyamines in both development and mature brain function.

S-adenosylmethionine decarboxylase from the thermophilic archaebacterium Sulfolobus solfataricus. Purification, molecular properties and studies on the covalently bound pyruvate

S-Adenosylmethionine decarboxylase from Sulfolobus solfataricus, a thermoacidophilic archaebacterium optimally growing at 87 degrees C, has been purified to homogeneity. The specific activity of the homogeneous enzyme is 12 nmol CO2 formed min-1 (mg protein)-1 and the overall yield 8%. The enzyme is thermophilic with an optimum at 75 degrees C, is thermostable, and does not require divalent cations or putrescine for activity. It has a molecular mass of 32 kDa, and appears to be a monomeric protein. S-Adenosylmethionine decarboxylase from S. solfataricus contains covalently linked pyruvate as prosthetic group and is inactivated in a time-dependent process by NaCNBH3, in the presence of both the substrate and the product. Incubation with decarboxylated S-adenosyl[Me-3H]methionine and NaCNBH3 resulted in the labeling of the protein at the active site.

Methylthioadenosine nucleosidase in normal and dystrophic human muscle

Human skeletal muscle homogenate was found to contain a nucleosidase that catalyzes the hydrolysis of 5'-methylthioadenosine, a known inhibitor of many methyl transfer reactions. When the levels of methylthioadenosine nucleosidase in muscle of patients were compared with those of controls, no significant alterations in its activity were noted in patients with various forms of muscular dystrophies, polymyositis and certain denervating diseases.