Alpha-difluoromethylornithine (DFMO) as a potent arginase activity inhibitor in human colon carcinoma cells

Alpha-difluoromethylornithine (DFMO) is commonly used as a specific ornithine decarboxylase (ODC, EC4.1.1.17) irreversible inhibitor. ODC is the enzyme responsible for polyamine biosynthesis, which has been shown to be strictly necessary for cell proliferation. In HT-29 Glc-/+ cells, L-arginine is the major precursor of these molecules through the sequential actions of arginase, which leads to L-ornithine generation and ODC. L-ornithine, a substrate for ODC, retroinhibits arginase. Since DFMO is an ornithine analogue, we searched for a direct effect of this agent upon arginase. The flux of L-arginine through arginase in intact cells was inhibited by 51+/-11% by 10 mM of DFMO whereas 10 mM of L-valine, a known potent arginase inhibitor, inhibited this flux by 73+/-6%. DFMO equilibrated between extracellular and intercellular spaces and, when used at 10-mM concentration, was without effect on L-arginine net uptake. Measurement of arginase activity in HT-29 cell homogenates with increasing concentrations of DFMO and L-arginine led to an inhibition with a calculated Ki (inhibitory constant) equal to 3.9+/-1.0 mM. L-ornithine was less effective than DFMO in inhibiting arginase activity. Bovine liver arginase, used as another source of the enzyme, was also severely inhibited by DFMO. The inhibitory effect of DFMO upon arginase, one step upstream of the ODC reaction in the metabolic conversion of L-arginine to polyamines, is of potential physiological importance, since it could alter the production of ornithine and thus its metabolism in pathways other than the ODC pathway.     

Membrane permeability transition as induced by dysfunction of the electron transport chain

A novel activity producing gamma-aminobutyric acid (GABA) from L-ornithine in the presence of NAD(P)+ was found in the crude extract of L-ornithine-induced Hafnia alvei, in addition to L-ornithine decarboxylase (ODC) activity. The reaction system for the former activity consisted of two enzymes, L-ornithine oxidase (decarboxylating, OOD) and gamma-aminobutyraldehyde (GABL) dehydrogenase (GDH). OOD catalyzed the conversion of L-ornithine into GABL, CO2, NH3, and H2O2 in the presence of O2, and GDH dehydrogenated GABL to GABA in the presence of NAD(P)+. OOD, purified to homogeneity, had a high ODC activity and the activity ratio of ODC to OOD was almost constant throughout the purification (ODC/ OOD=160:1). The molecular mass of the OOD was about 230 kDa, probably consisting of three identical subunits of a 77 kDa peptide, and OOD had an absorption maximum at 420 nm as well as at 278 nm, the specific absorption for an enzyme containing pyridoxal phosphate (PLP). The content of PLP was estimated at about 1 mol per subunit. OOD was specific to L-ornithine, and other L-amino acids and polyamines including putrescine were inert. The enzyme was activated by PLP, but not by pyridoxamine 5'-phosphate, FAD, FMN, or pyrroloquinoline quinone, and it was inactivated by hydrazine, semicarbazide, and hydroxylamine. The holoenzyme can be resolved to the apoenzyme by incubation with hydroxylamine, and reconstituted with PLP. These properties of OOD were almost the same as those of ODC separately purified to homogeneity from H. alvei. Zn2+ and Cu2+, butanedione, and sodium borohydride inhibited both OOD and ODC in a similar manner. The OOD reaction required O2 and only the ODC reaction proceeded under anaerobic conditions. The substitution of air for oxygen in the reaction vessel and the addition of catalase-H2O, enhanced only the OOD reaction, resulting in an increase of the ratio of OOD/ODC to 1:30 and 1:4.1, respectively. These results suggested that OOD and ODC are identical and that the former is a side reaction of the latter in the presence of O2.     

Polyamine deficiency impairs proliferation and differentiation of cultured enterocytes (CaCo-2)

The polyamine dependence of enterocyte growth and differentiation was studied in the human intestinal cell line CaCo-2 using a specific inhibitor of the key enzyme ornithine decarboxylase (ODC), difluoromethylornithine (DFMO). ODC was highest during the initial phase of rapid growth and was inhibited in a dose dependent fashion by DFMO at 0.06-2 mM. At low levels DFMO only delayed cell replication without affecting final cell count whereas at concentrations of 0.125 mM and above the final cell number was diminished by at least 53% compared to controls. In contrast, DFMO even at 0.03 mM reduced sucrase activity to 44% of controls when added at day 2 but was ineffective when supplemented at day 7 of culture or later. The inhibitor also diminished the number and length of microvilli in a dose dependent fashion, although this effect required higher DFMO levels than the reduction of sucrase activity. The DFMO mediated suppression of cell replication, enzymatic and morphologic differentiation was reversible in the presence of the ODC product putrescine. Putrescine alone did not affect any of the above parameters. In conclusion, the present data suggest that ODC and polyamines are involved both in enterocyte growth and differentiation.     

Inhibition of ornithine decarboxylase potentiates nitric oxide production in LPS-activated J774 cells

We have examined whether modulation of the polyamine biosynthetic pathway, through inhibition by alpha-difluoromethylornithine (DFMO) of the rate limiting enzyme, ornithine decarboxylase (ODC), modulates NO synthesis in J774 macrophages. DFMO potentiated LPS-stimulated nitrite production in both a concentration- and time-dependent manner, increasing nitrite levels by 48+/-5% at 10 mM. This effect was observed in cells pre-treated with DFMO for 24 h prior to stimulation with LPS. Addition of DFMO 12 h after LPS failed to potentiate LPS-induced nitrite production. Supplementation of the culture medium with horse serum (10%) in place of foetal calf serum (10%) caused no significant change in either LPS-induced nitrite production or in the ability of DFMO (10 mM) to potentiate LPS-induced NO synthesis. Metabolism of L-[3H]arginine to L-[3H]citrulline by partially purified inducible nitric oxide synthase (iNOS) was not significantly altered by either DFMO (1-10 mM) or by putrescine (0.001-1 mM), spermidine (0.001-1 mM) or spermine (0.001-1 mM). iNOS activity was also unaffected by 1 mM EGTA but was markedly attenuated (70+/-0.07%) by L-NMMA (100 microM). Pre-incubation of cells with DFMO (10 mM; 24 h) prior to activation with LPS resulted in enhanced (approximately 2 fold) iNOS protein expression. These results show that DFMO potentiates LPS-induced nitrite production in the murine macrophage cell line J774. Since the only known mechanism of action of DFMO is inhibition of ODC, and thus polyamine biosynthesis, we conclude that expression of iNOS can be critically regulated by endogenous polyamines.     

Polyamines regulate human medullary thyroid carcinoma TT-cell proliferation and secretion of calcitonin and calcitonin gene-related peptide

The significance of polyamines for the neoplastic proliferation and secretion of calcitonin (CT) and calcitonin-gene-related peptide (CGRP) by the human medullary thyroid carcinoma TT cell line was investigated. TT cells were cultured in vitro for 6 days with or without additions of pathway inhibitors of polyamine biosynthetic enzymes. Treatment of the cells with 1 mM of the specific L-ornithine decarboxylase (ODC) inhibitor DL-alpha-difluoromethylornithine (DFMO) resulted in a 97% decrease in ODC activity, lowered contents of putrescine (96%) and spermidine (85%) and cell proliferation rates (90%) along with a compensatory 15-fold increase in S-adenosyl-L-methionine decarboxylase (SAMDC) activity. DFMO treatment also led to a decrease in cellular content of CT (33%) and CGRP (26%), while the drug enhanced secretion of CT (31%) but depressed that of CGRP (26%), and elevated the ratio of CT to CGRP secreted into the medium by 74%. Ethylglyoxal bis(guanylhydrazone) (EGBG), a SAMDC inhibitor, at 100 microM evoked a similar reduction of cell proliferation and lowered the content of spermine by 81%. Furthermore, EGBG treatment caused a 34-fold increase in ODC activity and a subsequent 35-fold build-up of putrescine, but also seemed to stabilize SAMDC as evidenced by a highly enhanced SAMDC activity (approximately 200-fold) during enzyme assays in the absence of the inhibitor. EGBG exposure resulted in an increase in cellular CT content (110%) and secretion of the hormone (82%), while not affecting CGRP content or release.2+ EGBG effects were partially counteracted by DFMO.(ABSTRACT TRUNCATED AT 250 WORDS)     

Polyamines are essential for cell transformation by pp60v-src: delineation of molecular events relevant for the transformed phenotype

Ornithine decarboxylase (ODC), the key enzyme of polyamine biosynthesis, becomes upregulated during cell proliferation and transformation. Here we show that intact ODC activity is needed for the acquisition of a transformed phenotype in rat 2R cells infected with a temperature-sensitive mutant of Rous sarcoma virus. Addition of the ODC inhibitor alpha-difluoromethyl ornithine (DFMO) to the cells (in polyamine-free medium) before shift to permissive temperature prevented the depolymerization of filamentous actin and morphological transformation. Polyamine supplementation restored the transforming potential of pp60v-src. DFMO did not interfere with the expression of pp60v-src or its in vitro tyrosine kinase activity. The tyrosine phosphorylation of most cellular proteins, including ras GAP, did not either display clear temperature- or DFMO-sensitive changes. A marked increase was, however, observed in the tyrosine phosphorylation of phosphatidylinositol 3-kinase and proteins of 33 and 36 kD upon the temperature shift, and these hyperphosphorylations were partially inhibited by DFMO. A DFMO-sensitive increase was also found in the total phosphorylation of calpactins I and II. The well-documented association of GAP with the phosphotyrosine-containing proteins p190 and p62 did not correlate with transformation, but a novel 42-kD tyrosine phosphorylated protein was complexed with GAP in a polyamine- and transformation-dependent manner. Further, tyrosine phosphorylated proteins of 130, 80/85, and 36 kD were found to coimmunoprecipitate with pp60v-src in a transformation-related manner. Altogether, this model offers a tool for sorting out the protein phosphorylations and associations critical for the transformed phenotype triggered by pp60v-src, and implicates a pivotal role for polyamines in cell transformation.     

Trypanosomatidae produce acetate via a mitochondrial acetate:succinate CoA transferase

Hydrogenosome-containing anaerobic protists, such as the trichomonads, produce large amounts of acetate by an acetate:succinate CoA transferase (ASCT)/succinyl CoA synthetase cycle. The notion that mitochondria and hydrogenosomes may have originated from the same alpha-proteobacterial endosymbiont has led us to look for the presence of a similar metabolic pathway in trypanosomatids because these are the earliest-branching mitochondriate eukaryotes and because they also are known to produce acetate. The mechanism of acetate production in these organisms, however, has remained unknown. Four different members of the trypanosomatid family: promastigotes of Leishmania mexicana mexicana, L. infantum and Phytomonas sp., and procyclics of Trypanosoma brucei were analyzed as well as the parasitic helminth Fasciola hepatica. They all use a mitochondrial ASCT for the production of acetate from acetyl CoA. The succinyl CoA that is produced during acetate formation by ASCT is recycled presumably to succinate by a mitochondrial succinyl CoA synthetase, concomitantly producing ATP from ADP. The ASCT of L. mexicana mexicana promastigotes was further characterized after partial purification of the enzyme. It has a high affinity for acetyl CoA (Km 0.26 mM) and a low affinity for succinate (Km 6.9 mM), which shows that significant acetate production can occur only when high mitochondrial succinate concentrations prevail. This study identifies a metabolic pathway common to mitochondria and hydrogenosomes, which strongly supports a common origin for these two organelles.     

Site-specific cleavage of tRNA by imidazole and/or primary amine groups bound at the 5''-end of oligodeoxyribonucleotides

Thrombin, a serine protease, is a potent mitogen for vascular smooth muscle cells (SMCs), but its mechanism of action is not known. Since L-ornithine is metabolized to growth-stimulatory polyamines, we examined whether thrombin regulates the transcellular transport and metabolism of L-ornithine by vascular SMCs. Treatment of SMCs with thrombin initially (0 to 2 hours) decreased L-ornithine uptake, whereas longer exposures (6 to 24 hours) progressively increased transport. Kinetic studies indicated that thrombin-induced inhibition was associated with a decrease in affinity for L-ornithine, whereas stimulation was mediated by an increase in transport capacity. Thrombin induced the expression of both cationic amino acid transporter (CAT)-1 and CAT-2 mRNA. Furthermore, thrombin stimulated L-ornithine metabolism by inducing ornithine decarboxylase (ODC) mRNA expression and activity. The stimulatory effect of thrombin on both L-ornithine transport and ODC activity was reversed by hirudin, a thrombin inhibitor, and was mimicked by a 14-amino acid thrombin receptor-activating peptide. Thrombin also markedly increased the capacity of SMCs to generate putrescine, a polyamine, from extracellular L-ornithine. The thrombin-mediated increase in putrescine production was reversed by N(G)-methyl-L-arginine, a competitive inhibitor of cationic amino acid transport, or by alpha-difluoromethylornithine (DFMO), an ODC inhibitor. DFMO also inhibited thrombin-induced SMC proliferation. These results demonstrate that thrombin stimulates polyamine synthesis by inducing CAT and ODC gene expression and that thrombin-stimulated SMC proliferation is dependent on polyamine formation. The ability of thrombin to upregulate L-ornithine transport and direct its metabolism to growth-stimulatory polyamines may contribute to postangioplasty restenosis and atherosclerotic lesion formation.     

Interaction of 4-aminobutyrate-transaminase from swine kidneys with 5'- and 6'-methyl derivatives of pyridoxal-5'-phosphate

The study of interaction of 4-aminobutyrate transaminase with 5'- 6'-methyl derivates of PLP demonstrated that only the former was capable of forming a catalytically active holoenzyme possessing 0.37 activity of the native holoenzyme and a low affinity substrates. This compound interacts with the apoenzyme at a slower rate than does PLP; it has a reduced affinity towards apotransaminase (Km = 1.10(-4) M) and is replaced from the active site by native coenzyme. The other analog of pyridoxal-5'-phosphate forms a catalytically inactive complex with the apoenzyme; the other analog is not replaced from the active center by native coenzyme and non-competitively inhibits the reconstruction of apotransaminase (Ki = 2.10(-5) M).     

Phase I chemoprevention study of piroxicam and alpha-difluoromethylornithine

A two-step Phase I study of piroxicam (PXM) and a-difluoromethylornithine (DFMO) alone and in combination was initiated to assess toxicity and the impact of these drugs on several biological markers. In step 1, 12 subjects with a history of skin cancers were assigned to receive PXM 10 mg every day (q.d.) or 10 mg every other day (q.o.d.). The dosage of PXM 10 mg q.o.d. was tolerated. No changes were seen in 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced ornithine decarboxylase (ODC) or urinary polyamine levels. Steady-state serum levels of PXM were consistent with the oral dose level. In step 2, 31 subjects with stage 0 or I nonmelanoma skin cancers, stage A or B prostate or colon cancer, or stage I breast cancer or who had a family history of cancer were randomized to receive DFMO 0.5 g/m2, PXM 10 mg q.o.d., or the combination of DFMO and PXM. In addition to the biological markers of TPA-induced ODC activity in skin biopsies and urinary polyamine levels, we measured urinary 11-dehydrothromboxane B2, a specific metabolite of thromboxane A2. Of the 12 subjects on DFMO/PXM, 2 dropped out for non-drug-related reasons. Three developed grade-2 drug-related toxicities. One subject developed dyspnea that resolved and was able to continue on the study for 6 months. One subject who developed diarrhea that resolved after 5 days was also able to restart the drug without a recurrence. A third subject described intermittent episodes of tinnitus starting 4 h after taking PXM that lasted only 5 s and did not progress on treatment. Comparing the 6-month measurements with pretreatment, DFMO/PXM or DFMO significantly reduced TPA-induced ODC levels (Ps, 0.03 and 0.05). Urinary polyamine levels of spermidine decreased slightly with the DFMO/PXM or DFMO alone, whereas putrescine decreased with PXM alone. Levels of 11-dehydrothromboxane B2 were depressed by PXM and PXM/DFMO. The doses of DFMO/PXM determined in step 2 are potential starting dosages for Phase IIa and IIb chemoprevention trials.     

The role of phenylalanine in structure-function relationships of phenylalanine hydroxylase revealed by radiation target analysis

The activity of rat liver phenylalanine hydroxylase (PAH; phenylalanine 4-monooxygenase, EC 1.14.16.1) is regulated by interaction with its substrate, phenylalanine, and its coenzyme, BH4 [tetrahydrobiopterin (6R-dihydroxypropyl-L-erythro-5,6,7,8-tetrahydropterin)]. The structural changes accompanying these interactions have been studied by radiation target analysis. PAH purified from rat liver was incubated with 2 mM phenylalanine to achieve complete activation of the enzyme. Frozen samples were irradiated with various doses of high energy electrons; samples were subsequently thawed, and several surviving properties of the enzyme were determined. Each parameter decreased as a single exponential function of radiation dose. Radiation target analysis of enzymatic activity yielded a dimeric target size. Similar radiation effects on subunit monomers and on tetrameric structure were observed. Together with results from unactivated enzyme, these data show that phenylalanine increases the interactions between the subunits in a dimer and weakens the interactions between dimers in a tetramer. These alterations prevent the natural cofactor, a tetrahydrobiopterin, from exerting a negative effect on activity.     

Interaction of pyridoxal 5'-phosphate with tryptophan-139 at the subunit interface of dimeric D-amino acid transaminase

The crystal structure of dimeric bacterial D-amino acid transaminase shows that the indole rings of the two Trp-139 side chains face each other in the subunit interface about 10 angstroms from the coenzyme, pyridoxal 5'-phosphate. To determine whether it has a role in the catalytic efficiency of the enzyme or interacts with the coenzyme, Trp-139 has been substituted by several different types of amino acids, and the properties of these recombinant mutant enzymes have been compared to the wild-type enzyme. In the native wild-type holoenzyme, the fluorescence of one of the three Trp residues per monomer is almost completely quenched, probably due to its interaction with PLP since in the native wild-type apoenzyme devoid of PLP, tryptophan fluorescence is not quenched. Upon reconstitution of this apoenzyme with PLP, the tryptophan fluorescence is quenched to about the same extent as it is in the native wild-type enzyme. The site of fluorescence quenching is Trp-139 since the W139F mutant in which Trp-139 is replaced by Phe has about the same amount of fluorescence as the wild-type enzyme. The circular dichroism spectra of the holo and the apo forms of both the wild-type and the W139F enzymes in the far-ultraviolet show about the same degree of ellipticity, consistent with the absence of extensive global changes in protein structure. Furthermore, comparison of the circular dichroism spectrum of the W139F enzyme at 280 nm with the corresponding spectral region of the wild-type enzyme suggests a restricted microenvironment for Trp-139 in the latter enzyme. The functional importance of Trp-139 is also demonstrated by the finding that its replacement by Phe, His, Pro, or Ala gives mutant enzymes that are optimally active at temperatures below that of the wild-type enzyme and undergo the E-PLP --> E-PMP transition as a function of D-Ala concentration with reduced efficiency. The results suggest that a fully functional dimeric interface with the two juxtaposed indole rings of Trp-139 is important for optimal catalytic function and maximum thermostability of the enzyme and, furthermore, that there might be energy transfer between Trp-139 and coenzyme PLP.     

Effects of the E177K mutation in D-amino acid transaminase. Studies on an essential coenzyme anchoring group that contributes to stereochemical fidelity

D-Amino acid transaminase is a bacterial enzyme that uses pyridoxal phosphate (PLP) as a cofactor to catalyze the conversion of D-amino acids into their corresponding alpha-keto acids. This enzyme has already been established as a target for novel antibacterial agents through suicide inactivation by a number of compounds. To improve their potency and specificity, the detailed enzyme mechanism, especially the role of its PLP cofactor, is under investigation. Many PLP-dependent transaminases have a negatively charged amino acid residue forming a salt-bridge with the pyridine nitrogen of its cofactor that promotes its protonation to stabilize the formation of a ketimine intermediate, which is subsequently hydrolyzed in the normal transaminase reaction pathway. However, alanine racemase has a positively charged arginine held rigidly in place by an extensive hydrogen bond network that may destabilize the ketimine intermediate, and make it too short-lived for a transaminase type of hydrolysis to occur. To test this hypothesis, we changed Glu-177 into a titratable, positively charged lysine (E177K). The crystal structure of this mutant shows that the positive charge of the newly introduced lysine side chain points away from the nitrogen of the cofactor, which may be due to electrostatic repulsions not being overcome by a hydrogen bond network such as found in alanine racemase. This mutation makes the active site more accessible, as exemplified by both biochemical and crystallographic data: CD measurements indicated a change in the microenvironment of the protein, some SH groups become more easily titratable, and at pH 9.0 the PMP peak appeared around 315 nm rather than at 330 nm. The ability of this mutant to convert L-alanine into D-alanine increased about 10-fold compared to wild-type and to about the same extent as found with other active site mutants. On the other hand, the specific activity of the E177K mutant decreased more than 1000-fold compared to wild-type. Furthermore, titration with L-alanine resulted in the appearance of an enzyme-substrate quinonoid intermediate absorbing around 500 nm, which is not observed with usual substrates or with the wild-type enzyme in the presence of L-alanine. The results overall indicate the importance of charged amino acid side chains relative to the coenzyme to maintain high catalytic efficiency.     

An apyrase from Mimosa pudica contains N5,N10-methenyl tetrahydrofolate and is stimulated by light

An apyrase (NTP/NDPase) implicated in the response of Mimosa pudica to stimuli, such as touch, has been cloned, sequenced and expressed in Escherichia coli. While purifying and characterizing this enzyme, it was observed that a chromophore is associated with it, having absorption in the ultraviolet-A/blue region of the spectrum. The absorbance maximum of the chromophore, purified from the enzyme complex by gel filtration and HPLC, is around 350 nm. The chromophore has been identified as N5,N10-methenyl tetrahydrofolate (MTHF) by comparing the excitation and emission spectra of synthetic MTHF and the isolated cofactor, and by reconstitution of the enzyme complex with synthetic MTHF. Upon excitation with light (350 nm), an increase of apyrase activity was observed in the purified or reconstituted holoenzyme but not in the apoenzyme. The wavelength dependence of the light stimulation matched well with the fluorescence excitation spectra of the cofactor, MTHF. Possible implications of the results for signal transduction in M. pudica have been discussed.     

Cloning and characterization of the NAD-linked glycerol-3-phosphate dehydrogenases of Trypanosoma brucei brucei and Leishmania mexicana mexicana and expression of the trypanosome enzyme in Escherichia coli

A polyclonal antiserum raised against the purified glycosomal glycerol-3-phosphate dehydrogenase of Trypanosoma brucei brucei has been used to identify the corresponding cDNA clone in a T.b. brucei expression library. This cDNA was subsequently used to obtain genomic clones containing glycerol-3-phosphate dehydrogenase genes. Two tandemly arranged genes were detected in these clones. Characterization of one of the genes showed that it codes for a polypeptide of 353 amino acids, with a molecular mass of 37,651 Da and a calculated net charge of +8. Using the T.b. brucei gene as a probe, a corresponding glycerol-3-phosphate dehydrogenase gene was also identified in a genomic library of Leishmania mexicana mexicana. The L.m. mexicana gene codes for a polypeptide of 365 amino acids, with a molecular mass of 39,140 Da and a calculated net charge of +8. The amino-acid sequences of both polypeptides are 63% identical and carry a type-1 peroxisomal targeting signal (PTS1) SKM and -SKL at their respective C-termini. Moreover, the L.m. mexicana polypeptide also carries a short N-terminal extension reminiscent of a mitochondrial transit sequence. Subcellular localisation analysis showed that in L.m. mexicana the glycerol-3-phosphate dehydrogenase activity co-fractionated both with mitochondria and with glycosomes. This is not the case in T. brucei, where the enzyme is predominantly glycosomal. The two trypanosomatid sequences resemble their prokaryotic homologues (32-36%) more than their eukaryotic counterparts (25-31%) and carry typical prokaryotic signatures. The possible reason for this prokaryotic nature of a trypanosomatid glycerol-3-phosphate dehydrogenase is discussed.     

Characterization of Euphorbia characias latex amine oxidase

A copper-containing amine oxidase from the latex of Euphorbia characias was purified to homogeneity and the copper-free enzyme obtained by a ligand-exchange procedure. The interactions of highly purified apo- and holoenzyme with several substrates, carbonyl reagents, and copper ligands were investigated by optical spectroscopy under both aerobic and anaerobic conditions. The extinction coefficients at 278 and 490 nm were determined as 3.78 x 10(5) M-1 cm-1 and 6000 M-1 cm-1, respectively. Active-site titration of highly purified enzyme with substrates and carbonyl reagents showed the presence of one cofactor at each enzyme subunit. In anaerobiosis the native enzyme oxidized one equivalent substrate and released one equivalent aldehyde per enzyme subunit. The apoenzyme gave exactly the same 1:1:1 stoichiometry in anaerobiosis and in aerobiosis. These findings demonstrate unequivocally that copper-free amine oxidase can oxidize substrates with a single half-catalytic cycle. The DNA-derived protein sequence shows a characteristic hexapeptide present in most 6-hydroxydopa quinone-containing amine oxidases. This hexapeptide contains the tyrosinyl residue that can be modified into the cofactor 6-hydroxydopa quinone.     

Ornithine decarboxylase activity during development of cerebellar granule neurons

Ornithine decarboxylase (ODC), the key enzyme for polyamine biosynthesis, dramatically decreases in activity during normal cerebellar development, in parallel with the progressive differentiation of granule neurons. We have studied whether a similar pattern is displayed by cerebellar granule neurons during survival and differentiation in culture. We report that when granule cells were kept in vitro under trophic conditions (high K+ concentration), ODC activity progressively decreased in parallel with neuronal differentiation. Under nontrophic conditions (cultures kept in low K+ concentration), the enzymatic activity dropped quickly in parallel with an increased apoptotic elimination of cells. Cultures kept in high K+ but chronically exposed to 10 mM lithium showed both an increased rate of apoptotic cell death at 2 and 4 days in vitro and a quicker drop of ODC activity and immunocytochemical staining. A short chronic treatment of rat pups with lithium also resulted in transient decrease of cerebellar ODC activity and increased programmed cell death, as revealed by in situ detection of apoptotic granule neurons. The present data indicate that a sustained ODC activity is associated with the phase of survival and differentiation of granule neurons and that, conversely, conditions that favor their apoptotic elimination are accompanied by a down-regulation of the enzymatic activity.     

Reaction of alanine racemase with 1-aminoethylphosphonic acid forms a stable external aldimine

(R)-1-Aminoethylphosphonic acid (L-Ala-P), a synthetic L-alanine analogue, has antibacterial activity and is a time-dependent inactivator of all purified Gram-positive bacterial alanine racemases that have been tested. L-Ala-P forms an external aldimine with the bound pyridoxal 5'-phosphate (PLP) cofactor, but is neither racemized nor efficiently hydrolyzed. To understand the structural basis of the inactivation of the enzyme by L-Ala-P, we determined the crystal structure of the complex between L-Ala-P and alanine racemase at 1.6 A resolution. The cofactor derivative in the inhibited structure tilts outward from the protein approximately 20 degrees relative to the internal aldimine. The phosphonate oxygens are within hydrogen bonding distance of four amino acid residues and two water molecules in the active site of the enzyme. L-Ala-P is an effective inhibitor of alanine racemase because, upon formation of the external aldimine, the phosphonate group interacts with putative catalytic residues, thereby rendering them unavailable for catalysis. Furthermore, this aldimine appears to be inappropriately aligned for efficient Calpha proton abstraction. The combination of these effects leads to a stable aldimine derivative and potent inactivation of alanine racemase by this compound.