Brain pyridoxal kinase. Purification, substrate specificities, and sensitized photodestruction of an essential histidine

The Bioinorganic Chemistry of Mammalian Metallothioneins

The functions, purposes, and roles of metallothioneins have been the subject of speculations since the discovery of the protein over 60 years ago. This article guides through the history of investigations and resolves multiple contentions by providing new interpretations of the structure-stability-function relationship. It challenges the dogma that the biologically relevant structure of the mammalian proteins is only the one determined by X-ray diffraction and NMR spectroscopy. The terms metallothionein and thionein are ambiguous and insufficient to understand biological function. The proteins need to be seen in their biological context, which limits and defines the chemistry possible. They exist in multiple forms with different degrees of metalation and types of metal ions. The homoleptic thiolate coordination of mammalian metallothioneins is important for their molecular mechanism. It endows the proteins with redox activity and a specific pH dependence of their metal affinities. The proteins, therefore, also exist in different redox states of the sulfur donor ligands. Their coordination dynamics allows a vast conformational landscape for interactions with other proteins and ligands. Many fundamental signal transduction pathways regulate the expression of the dozen of human metallothionein genes. Recent advances in understanding the control of cellular zinc and copper homeostasis are the foundation for suggesting that mammalian metallothioneins provide a highly dynamic, regulated, and uniquely biological metal buffer to control the availability, fluctuations, and signaling transients of the most competitive Zn(II) and Cu(I) ions in cellular space and time.

Inborn errors in the vitamin B6 salvage enzymes associated with neonatal epileptic encephalopathy and other pathologies

Pyridoxal 5'-phosphate (PLP), the active cofactor form of vitamin B6 is required by over 160 PLP-dependent (vitamin B6) enzymes serving diverse biological roles, such as carbohydrates, amino acids, hemes, and neurotransmitters metabolism. Three key enzymes, pyridoxal kinase (PL kinase), pyridoxine 5'-phosphate oxidase (PNPO), and phosphatases metabolize and supply PLP to PLP-dependent enzymes through the salvage pathway. In born errors in the salvage enzymes are known to cause inadequate levels of PLP in the cell, particularly in neuronal cells. The resulting PLP deficiency is known to cause or implicated in several pathologies, most notably seizures. One such disorder, PNPO-dependent neonatal epileptic encephalopathy (NEE) results from natural mutations in PNPO and leads to null or reduced enzymatic activity. NEE does not respond to conventional antiepileptic drugs but may respond to treatment with the B6 vitamers PLP and/or pyridoxine (PN). In born errors that lead to PLP deficiency in cells have also been reported in PL kinase, however, to date none has been associated with epilepsy or seizure. One such pathology is polyneuropathy that responds to PLP therapy. Phosphatase deficiency or hypophosphatasia disorder due to pathogenic mutations in alkaline phosphatase is known to cause seizures that respond to PN therapy. In this article, we review the biochemical features of in born errors pertaining to the salvage enzyme's deficiency that leads to NEE and other pathologies. We also present perspective on vitamin B6 treatment for these disorders, along with attempts to develop zebrafish model to study the NEE syndrome in vivo.

Pyridoxal kinase inhibition by artemisinins down-regulates inhibitory neurotransmission

The antimalarial artemisinins have also been implicated in the regulation of various cellular pathways including immunomodulation of cancers and regulation of pancreatic cell signaling in mammals. Despite their widespread application, the cellular specificities and molecular mechanisms of target recognition by artemisinins remain poorly characterized. We recently demonstrated how these drugs modulate inhibitory postsynaptic signaling by direct binding to the postsynaptic scaffolding protein gephyrin. Here, we report the crystal structure of the central metabolic enzyme pyridoxal kinase (PDXK), which catalyzes the production of the active form of vitamin B6 (also known as pyridoxal 5'-phosphate [PLP]), in complex with artesunate at 2.4-Å resolution. Partially overlapping binding of artemisinins with the substrate pyridoxal inhibits PLP biosynthesis as demonstrated by kinetic measurements. Electrophysiological recordings from hippocampal slices and activity measurements of glutamic acid decarboxylase (GAD), a PLP-dependent enzyme synthesizing the neurotransmitter γ-aminobutyric acid (GABA), define how artemisinins also interfere presynaptically with GABAergic signaling. Our data provide a comprehensive picture of artemisinin-induced effects on inhibitory signaling in the brain.

Intellectual Disability Associated With Pyridoxine-Responsive Epilepsies: The Need to Protect Cognitive Development

Pyridoxine (vitamin B6)-responsive epilepsies are severe forms of epilepsy that manifest as seizures immediately after birth, sometimes in utero, sometimes months, or years after birth. Seizures may be treated efficiently by life-long supplementation with pyridoxine or its biologically active form, pyridoxal phosphate, but even so patients may become intellectually disabled, for which there currently is no effective treatment. The condition may be caused by mutations in several genes (TNSALP, PIGV, PIGL, PIGO, PNPO, PROSC, ALDH7A1, MOCS2, or ALDH4A1). Mutations in ALDH7A1, MOCS2, and ALDH4A1 entail build-up of reactive aldehydes (α-aminoadipic semialdehyde, γ-glutamic semialdehyde) that may react non-enzymatically with macromolecules of brain cells. Such reactions may alter the function of macromolecules, and they may produce "advanced glycation end products" (AGEs). AGEs trigger inflammation in the brain. This understanding points to aldehyde-quenching, anti-AGE, or anti-inflammatory therapies as possible strategies to protect cognitive development and prevent intellectual disability in affected children. Studies on how aldehydes traverse cell membranes and how they affect brain function could further the development of therapies for patients with pyridoxine-responsive epilepsies.

PPARβ/δ modulates ethanol-induced hepatic effects by decreasing pyridoxal kinase activity

Because of the significant morbidity and lethality caused by alcoholic liver disease (ALD), there remains a need to elucidate the regulatory mechanisms that can be targeted to prevent and treat ALD. Toward this goal, minimally invasive biomarker discovery represents an outstanding approach for these purposes. The mechanisms underlying ALD include hepatic lipid accumulation. As the peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) has been shown to inhibit steatosis, the present study examined the role of PPARβ/δ in ALD coupling metabolomic, biochemical and molecular biological analyses. Wild-type and Pparβ/δ-null mice were fed either a control or 4% ethanol diet and examined after 4-7 months of treatment. Ethanol fed Pparβ/δ-null mice exhibited steatosis after short-term treatment compared to controls, the latter effect appeared to be due to increased activity of sterol regulatory element binding protein 1c (SREBP1c). The wild-type and Pparβ/δ-null mice fed the control diet showed clear differences in their urinary metabolomic profiles. In particular, metabolites associated with arginine and proline metabolism, and glycerolipid metabolism, were markedly different between genotypes suggesting a constitutive role for PPARβ/δ in the metabolism of these amino acids. Interestingly, urinary excretion of taurine was present in ethanol-fed wild-type mice but markedly lower in similarly treated Pparβ/δ-null mice. Evidence suggests that PPARβ/δ modulates pyridoxal kinase activity by altering Km, consistent with the observed decreased in urinary taurine excretion. These data collectively suggest that PPARβ/δ prevents ethanol-induced hepatic effects by inhibiting hepatic lipogenesis, modulation of amino acid metabolism, and altering pyridoxal kinase activity.

Chemical, genetic and structural assessment of pyridoxal kinase as a drug target in the African trypanosome

Pyridoxal-5'-phosphate (vitamin B(6) ) is an essential cofactor for many important enzymatic reactions such as transamination and decarboxylation. African trypanosomes are unable to synthesise vitamin B(6) de novo and rely on uptake of B(6) vitamers such as pyridoxal and pyridoxamine from their hosts, which are subsequently phosphorylated by pyridoxal kinase (PdxK). A conditional null mutant of PdxK was generated in Trypanosoma brucei bloodstream forms showing that this enzyme is essential for growth of the parasite in vitro and for infectivity in mice. Activity of recombinant T. brucei PdxK was comparable to previously published work having a specific activity of 327 ± 13 mU mg(-1) and a K(m)(app) with respect to pyridoxal of 29.6 ± 3.9 µM. A coupled assay was developed demonstrating that the enzyme has equivalent catalytic efficiency with pyridoxal, pyridoxamine and pyridoxine, and that ginkgotoxin is an effective pseudo substrate. A high resolution structure of PdxK in complex with ATP revealed important structural differences with the human enzyme. These findings suggest that pyridoxal kinase is an essential and druggable target that could lead to much needed alternative treatments for this devastating disease.

Functional significance of some particular amino acid residues in Bombyx mori pyridoxal kinase

Pyridoxal kinase (PLK; EC 2.7.1.35) is a key enzyme for vitamin B(6) metabolism in animals. It catalyzes the ATP-dependent phosphorylation of pyridoxal, generating pyridoxal 5'-phosphate, an important cofactor for many enzymatic reactions. Bombyx mori PLK (BmPLK) is 10 or more residues shorter than mammalian PLKs, and some amino acid residues conserved in the PLKs from mammals are not maintained in the protein. Multiple sequence alignment suggested that amino acid residues Thr(47), Ile(54), Arg(88), Asn(121) and Glu(230) might play important roles in BmPLK. In this study, we used a site-directed specific mutagenesis approach to determine the functional significance of these particular amino acid residues in BmPLK. Our results demonstrated that the mutation of Asn(121) to Glu did not affect the catalytic function of BmPLK. The corresponding site-directed mutants of Thr(47) to Asn, Ile(54) to Phe, and Arg(88) to Ile displayed a decreased catalytic efficiency and an elevated Km value for substrate relative to the wild-type value, and no enzyme activity could be detected in mutant of Trp(230) to Glu. Circular dichroism analysis revealed that the mutation of Trp(230) to Glu resulted in mis-folding of the protein. Our results provided direct evidence that residue Trp(230) is crucial to maintain the structural and functional integrity of BmPLK. This study will add to the existing understanding of the characteristic of structure and function of BmPLK.

Bombyx mori Pyridoxal Kinase cDNA Cloning and Enzymatic Characterization

Pyridoxal kinase (PLK) (EC 2.7.1.35) catalyzes the ATP-dependent phosphorylation of pyridoxal, generating pyridoxal-5.-phosphate (PLP), an important cofactor for many enzymatic reactions. Bombyx mori, similar to mammals, relies on a nutritional source of vitamin B6 to synthesize PLP. This article describes how a cDNA encoding PLK was cloned from Bombyx mori using the PCR method (GenBank accession number: DQ452397). The cDNA has an 894 bp open reading frame and encodes a protein of 298 amino acid residues with a molecular mass of 33.1 kDa. The amino acid sequence shares 48.6% identity with that of human PLK, and it also contains signature conserved motifs of the PLK family. However, the protein is 10 or more amino acids shorter than the PLK from mammals and plants, and several amino acid residues conserved in the PLK from mammals and plants are changed in the protein. The cDNA cloned was expressed successfully in Escherichia coli using the T7 promoter/T7 RNA polymerase expression system, and the crude extracts containing the expressed product were found to have strong PLK enzymatic activity with a value of 30 nmol/min/mg, confirming that the cDNA encodes the functional PLK of Bombyx mori. This is the first identification of a gene encoding PLK in insects.

Tat-mediated protein transduction of human brain pyridoxal kinase into PC12 cells

Pyridoxal kinase (PK) catalyses the phosphorylation of vitamin B6 to pyridoxal-5'-phosphate (PLP). A human brain PK gene was fused with a gene fragment encoding the HIV-1 Tat protein transduction domain (RKKRRQRRR) in a bacterial expression vector to produce a genetic in-frame Tat-PK fusion protein. The expressed and purified Tat-PK fusion proteins transduced efficiently into PC12 cells in a time- and dose-dependent manner when added exogenously in culture media. Once inside the cells, the transduced Tat-PK proteins showed catalytic activity and are stable for 48 h. The intracellular concentration of PLP, which is known as a biologically active form of vitamin B6, was increased by pre-treatment of Tat-PK to the PC12 cells. Those results suggest that the transduction of Tat-PK fusion protein can be one of the ways to regulate the PLP level and to replenish this enzyme in the various neurological disorders related to vitamin B6.

Expression, purification, and kinetic constants for human and Escherichia coli pyridoxal kinases

Pyridoxal kinase is an ATP dependent enzyme that phosphorylates pyridoxal, pyridoxine, and pyridoxamine forming their respective 5'-phosphorylated esters. The kinase is a part of the salvage pathway for re-utilizing pyridoxal 5'-phosphate, which serves as a coenzyme for dozens of enzymes involved in amino acid and sugar metabolism. Clones of two pyridoxal kinases from Escherichia coli and one from human were inserted into a pET 22b plasmid and expressed in E. coli. All three enzymes were purified to near homogeneity and kinetic constants were determined for the three vitamin substrates. Previous studies had suggested that ZnATP was the preferred trinucleotide substrate, but our studies show that under physiological conditions MgATP is the preferred substrate. One of the two E. coli kinases has very low activity for pyridoxal, pyridoxine, and pyridoxamine. We conclude that in vivo this kinase may have an alternate substrate involved in another metabolic pathway and that pyridoxal has only a poor secondary activity for this kinase.

Analysis of the vitamin B6 biosynthesis pathway in the human malaria parasite Plasmodium falciparum

Vitamin B6 is an essential cofactor for more than 100 enzymatic reactions. Mammalian cells are unable to synthesize vitamin B6 de novo, whereas bacteria, plants, fungi, and as shown here Plasmodium falciparum possess a functional vitamin B6 synthesis pathway. P. falciparum expresses the proteins Pdx1 and Pdx2, corresponding to the yeast enzymes Snz1-p and Sno1-p, which are essential for the vitamin B6 biosynthesis. An involvement of PfPdx1 and PfPdx2 in the de novo synthesis of vitamin B6 was shown by complementation of pyridoxine auxotroph yeast cells. Both plasmodial proteins act together in the glutaminase activity with a specific activity of 209 nmol min(-1) mg(-1) and a K(m) value for glutamine of 1.3 mm. Incubation of the parasites with methylene blue revealed by Northern blot analysis an elevated transcriptional level of pdx1 and pdx2, suggesting a participation of these proteins in the defenses against singlet oxygen. To be an active cofactor, vitamin B6 has to be phosphorylated by the pyridoxine kinase (PdxK). The recombinant plasmodial PdxK revealed K(m) values for the B6 vitamers pyridoxine and pyridoxal and for ATP of 212, 70, and 82 microM, respectively. All three enzymes expose a stage-specific transcription pattern within the trophozoite stage that guarantees the concurrent expression of Pdx1, Pdx2, and PdxK for the indispensable provision of vitamin B6. The occurrence of the vitamin B6 de novo synthesis pathway displays a potential new drug target, which can be exploited for the development of new chemotherapeutics against the human malaria parasite P. falciparum.

Crystal structure of brain pyridoxal kinase, a novel member of the ribokinase superfamily

The three-dimensional structures of brain pyridoxal kinase and its complex with the nucleotide ATP have been elucidated in the dimeric form at 2.1 and 2.6 A, respectively. Results have shown that pyridoxal kinase, as an enzyme obeying random sequential kinetics in catalysis, does not possess a lid shape structure common to all kinases in the ribokinase superfamily. This finding has been shown to be in line with the condition that pyridoxal kinase binds substrates with variable sizes of chemical groups at position 4 of vitamin B(6) and its derivatives. In addition, the enzyme contains a 12-residue peptide loop in the active site for the prevention of premature hydrolysis of ATP. Conserved amino acid residues Asp(118) and Tyr(127) in the peptide loop could be moved to a position covering the nucleotide after its binding so that its chance to hydrolyze in the aqueous environment of the active site was reduced. With respect to the evolutionary trend of kinase enzymes, the existence of this loop in pyridoxal kinase could be classified as an independent category in the ribokinase superfamily according to the structural feature found and mechanism followed in catalysis.

Study of substrate-enzyme interaction between immobilized pyridoxamine and recombinant porcine pyridoxal kinase using surface plasmon resonance biosensor

Pyridoxal kinase (PK) is an important enzyme involved in bioactivation of vitamin B(6). Binding of PK with its substrate is the prerequisite step for the subsequent catalytic phosphorylation of the substrate. In the present study, a surface plasmon resonance biosensor (BIAcore) was employed to characterize the binding interaction between wild-type porcine PK and an immobilized substrate, pyridoxamine. Pyridoxamine was modified with 11-mercaptoundecanic acid and immobilized on a sensor chip through the formation of a self-assembled monolayer. The binding of PK to the immobilized pyridoxamine was followed in real time and the kinetic parameters were derived from non-linear analysis of the sensorgram. The effects of buffer pH, monovalent cations (Na(+), K(+)) and divalent cations (Mn(2+), Zn(2+), Mg(2+)) on the binding kinetics were determined. Optimal pH for PK-pyridoxamine interaction in the absence of divalent ions is at around 7.4. While K(+) increased and Na(+) decreased the binding affinity (K(A)) of PK to immobilized pyridoxamine, all divalent cations increased the K(A) of PK for pyridoxamine. Solution phase affinity measurement based on a competitive binding assay was used to determine the affinities of PK for different vitamin B(6) analogues. The order of affinity of PK for different analogues is: pyridoxal-oxime>pyridoxine>pyridoxamine>pyridoxal>pyridoxal phosphate. This is the first study to demonstrate that buffer conditions such as pH and concentration of monovalent and/or divalent ions can directly alter the binding of PK for its substrates. The quantitative kinetic and thermodynamic parameters obtained by SPR measurement provide the insight information into the catalytic activity of this enzyme.

Production and characterization of monoclonal antibodies to porcine brain pyridoxal kinase

Six monoclonal antibodies that recognize porcine brain pyridoxal kinase have been selected and designated as PK67, PK86, PK91, PK144, PK252 and PK275. A total of six monoclonal antibodies recognizing different epitopes of the enzyme were obtained, of which four inhibited the enzyme activity. When total proteins of porcine brain homogenate separated by SDS-PAGE were subjected to monoclonal antibodies, a single reactive protein band of molecular weight 39 kDa which comigrated with purified porcine pyridoxal kinase was detected. Using the anti-pyridoxal kinase antibodies as probes, the cross reactivities of brain pyridoxal kinase from human and other mammalian tissues and from avian sources were also investigated. Among human and all animal tissues tested, immunoreactive bands on Western blots appeared to have the same molecular mass of 39 kDa. These results indicate that mammalian brains contain only one major type of immunologically similar pyridoxal kinase, although some properties of the enzymes reported previously differed from one another.

Human pyridoxal kinase. cDNA cloning, expression, and modulation by ligands of the benzodiazepine receptor

Peptide fragments of a porcine benzodiazepine-binding protein were used to isolate the cDNA of a related human protein. The cDNA encodes a polypeptide of 312 amino acid residues that is homologous to a bacterial pyridoxal kinase. Transient expression of the cDNA in human embryonic kidney cells confirmed that it encodes human pyridoxal kinase. The recombinant enzyme displayed a Km value of 3.3 microM for pyridoxal and was inhibited competitively by 4-deoxypyridoxine (Ki = 2.8 microM). Benzodiazepine receptor ligands that bound to the purified porcine protein also exerted a potent inhibitory effect on human pyridoxal kinase activity. Transcripts of the pyridoxal kinase gene were detectable in all human tissues examined, and were particularly abundant in the testes. The gene is localized on chromosome 21q22.3 and represents a candidate gene for at least one genetic disorder that has been mapped to this region (autoimmune polyglandular disease type 1).

Kinetic limitation and cellular amount of pyridoxine (pyridoxamine) 5'-phosphate oxidase of Escherichia coli K-12

We report the purification and enzymological characterization of Escherichia coli K-12 pyridoxine (pyridoxamine) 5'-phosphate (PNP/PMP) oxidase, which is a key committed enzyme in the biosynthesis of the essential coenzyme pyridoxal 5'-phosphate (PLP). The enzyme encoded by pdxH was overexpressed and purified to electrophoretic homogeneity by four steps of column chromatography. The purified PdxH enzyme is a thermally stable 51-kDa homodimer containing one molecule of flavin mononucleotide (FMN). In the presence of molecular oxygen, the PdxH enzyme uses PNP or PMP as a substrate (Km = 2 and 105 microM and kcat = 0.76 and 1.72 s-1 for PNP and PMP, respectively) and produces hydrogen peroxide. Thus, under aerobic conditions, the PdxH enzyme acts as a classical monofunctional flavoprotein oxidase with an extremely low kcat turnover number. Comparison of kcat/Km values suggests that PNP rather than PMP is the in vivo substrate of E. coli PdxH oxidase. In contrast, the eukaryotic enzyme has similar kcat/Km values for PNP and PMP and seems to act as a scavenger. E. coli PNP/PMP oxidase activities were competitively inhibited by the pathway end product, PLP, and by the analog, 4-deoxy-PNP, with Ki values of 8 and 105 microM, respectively. Immunoinhibition studies suggested that the catalytic domain of the enzyme may be composed of discontinuous residues on the polypeptide sequence. Two independent quantitation methods showed that PNP/PMP oxidase was present in about 700 to 1,200 dimer enzyme molecules per cell in E. coli growing exponentially in minimal medium plus glucose at 37 degrees C. Thus, E. coli PNP/PMP oxidase is an example of a relatively abundant, but catalytically sluggish, enzyme committed to PLP coenzyme biosynthesis.

Purification and properties of pyridoxal kinase from bovine brain

A 27,000-fold purification of pyridoxal kinase from bovine brain tissue has been achieved by a combination of ammonium sulfate fractionation, DEAE-cellulose chromatography, hydroxyapatite chromatography, Sephadex G-150 gel filtration, Blue Sepharose CL-6B chromatography, and Phenyl-Superose chromatography. The final chromatography step yields a homogeneous preparation of high specific activity (2105 nmol/min/mg protein). The molecular mass of the native enzyme was estimated to be approximately 80,000 on gel filtration. The subunit molecular mass was determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis to be approximately 39,500. This indicates that pyridoxal kinase is a dimeric enzyme.

Purification and characterization of metallothionein and its activation of pyridoxal phosphokinase in trout (Salmo gairdneri) brain

1. Brain metallothionein was isolated and purified for the first time from rainbow trout. 2. Brain metallothionein exhibited an elution volume (Ve/Vo) of 2.0, had a molecular weight of 6762 Daltons, and contained a zinc content of 9 micrograms/mg protein. 3. Brain pyridoxal phosphokinase was isolated and assayed for the first time in rainbow trout. 4. Zinc (0.20 microM) or zinc metallothionein (6-30 microM) stimulated the activity of brain pyridoxal kinase in a linear fashion. 5. The results of these studies are interpreted to suggest that in trout brain zinc metallothionein may participate in metabolism of vitamin B6 and formation of pyridoxal phosphate, the active coenzyme.

Cleavage of pyridoxal kinase into two structural domains: kinetics of proteolysis monitored by emission anisotropy

Two sulfhydryl residues/dimer of pyridoxal kinase react with iodoacetamide fluorescence (IAF) to yield catalytically active species. Limited chymotryptic digestion of IAF pyridoxal kinase resulted in the release of two fragments of 24 and 16 KDA. One of the fragments (16 KDA) is labeled with IAF. After complete tryptic digestion of IAF-pyridoxal kinase, only one peptide labeled with IAF was separated by reverse-phase HPLC and its amino acid sequence determined by automated Edman degradation. The kinetics of chymotryptic cleavage of IAF-pyridoxal kinase was monitored by steady-state emission anisotropy measurements. Analysis of the kinetic results revealed that the rate of proteolysis is significantly reduced by the substrate pyridoxal (0.2 mM). ATP (1 mM) does not influence the rate of proteolysis. The technique of emission anisotropy was also applied to monitor the effect of viscosity on the rate of proteolysis. A kinetic model is proposed to explain the mechanism of limited proteolysis. The model is based on the assumption that unfolding of the native conformation of the protein-substrate complex plays a dominant role in proteolysis.

Zinc and the regulation of vitamin B6 metabolism

Pyridoxal kinase, a key enzyme in the formation of vitamin B6 coenzymes, requires a zinc-ATP complex as a substrate. Recent findings show that zinc-metallothionein facilitates the formation of the zinc-ATP complex. Thus, the concentration of zinc-metallothionein in tissues may serve in the regulation of vitamin B6 metabolism.

Pyridoxal kinase. Structure and function

Chymotryptic digestion of sheep brain pyridoxal kinase, a dimer of identical subunits each of 40 kDa, yields two fragments of 24 and 16 kDa with concomitant loss of catalytic activity. These fragments were separated by HPLC and used for binding studies with ATP and pyridoxal analogues. The spectroscopic properties of trinitrophenyl-ATP bound to the 24-kDa fragment are indistinguishable from those of TNP-ATP bound to the native kinase. The small 16-kDa fragment, generated by proteolytic cleavage of the kinase, does not bind any of the analogues. The same pattern of digestion was observed when IAF pyridoxal kinase, carrying a fluorescent probe covalently bound to a specific SH residue, was preincubated with chymotrypsin. The kinetics of proteolysis of IAF-pyridoxal kinase was monitored by emission anisotropy, and the analysis of the initial rate of proteolysis at various concentrations of chymotrypsin reveals that the rate of unfolding of native pyridoxal kinase plays a dominant role in the proteolytic process.

Activation of pyridoxal kinase by metallothionein

Brain pyridoxal kinase, which uses ATP complexed to either Zn(II) or Co(II) as substrates, displays high catalytic activity in the presence of Zn-thionein and Co-thionein. Several steps intervene in the process of pyridoxal kinase activation, i.e., binding of Zn ions to ATP and interaction between Zn-ATP and the enzyme. Equilibrium binding studies show that ATP mediates the release of Zn ions from the metal-thiolate clusters of the thioneins, whereas spectroscopic measurements conducted on Co-thionein reveal that the absorption transitions corresponding to the metal-thiolate of the protein are perturbed by ATP. The binding Zn-ATP to the kinase proceeds with a delta G = -6.3 kcal/mol as demonstrated by fluorometric titrations. Direct interaction between the kinase and derivatized-metallothionein could not be detected by emission anisotropy measurements, indicating that juxtaposition of the proteins does not influence the exchange of metal ions. Since the concentration of free Zn in several mammalian tissues is lower than 1 nM, it is postulated that under in vivo conditions the concentration of metallothionein regulates the catalytic activity of pyridoxal kinase.

Kinetic studies of the pyridoxal kinase from pig liver: slow-binding inhibition by adenosine tetraphosphopyridoxal

Pyridoxal kinase from pig liver has been purified 10,000-fold to apparent homogeneity. The enzyme is a dimer of subunits of Mr 32,000. The enzyme is strongly inhibited by the product pyridoxal 5'-phosphate. Liver pyridoxamine phosphate oxidase, another enzyme involved in the biosynthesis of pyridoxal 5'-phosphate, is also strongly inhibited by this compound [Wada, H., & Snell, E. E. (1961) J. Biol. Chem. 236, 2089-2095]. Thus, the biosynthesis of pyridoxal 5'-phosphate in the liver might be regulated by the product inhibition of both pyridoxamine phosphate oxidase and pyridoxal kinase. Kinetic studies revealed that the catalytic reaction of liver pyridoxal kinase follows an ordered mechanism in which pyridoxal and ATP bind to the enzyme and ADP and pyridoxal 5'-phosphate are released from the enzyme, in this order. Adenosine tetraphosphopyridoxal was found to be a slow-binding inhibitor of pyridoxal kinase. Pre-steady-state kinetics of the inhibition revealed that the inhibitor and the enzyme form an initial weak complex prior to the formation of a tighter and slowly reversing complex. The overall inhibition constant was 2.4 microM. ATP markedly protects the enzyme against time-dependent inhibition by the inhibitor, whereas another substrate pyridoxal affords no protection. By contrast, adenosine triphosphopyridoxal is not a slow-binding inhibitor of this enzyme.

Proteolytic cleavage of pyridoxal kinase into two structural domains

Chymotryptic digestion of sheep brain pyridoxal kinase, a dimer of identical subunits each of 40 kDa, yields 2 fragments of 24 and 16 kDa with concomitant loss of catalytic activity. These fragments were separated by chromatographic techniques and analyzed for interaction with the ATP analogue, trinitrophenyl-ATP, using fluorescence spectroscopy. The absorption and fluorescence properties of trinitrophenyl-ATP bound to the fragment of 24 kDa (emission maximum, 540 nm, emission anisotropy at 460 nm, 0.30, and fluorescence lifetime, gamma = lns) are indistinguishable from those of the ATP analogue bound to the native enzyme. The fragment of 16 kDa does not bind trinitrophenyl-ATP. The results are consistent with the hypothesis that monomeric pyridoxal kinase is folded into 2 domains connected by a single polypeptide chain sensitive to proteolytic cleavage.

Brain pyridoxal kinase dissociation of the dimeric structure and catalytic activity of the monomeric species

Reversible dissociation of the dimeric structure of brain pyridoxal kinase into subunits was attained by addition of guanidinium HCl (2 M). The molecular mass of the subunits (40 kDa) was determined by HPLC chromatography. Separation of the processes of refolding and association of the monomeric species was achieved by attaching the protein subunits to a rigid matrix (Affi-gel 15). The matrix-bound monomer is catalytically competent. The reaction of the crosslinking reagent 4,4'-dimaleimidestilbene 2,2'-disulfonate (DMDS), a derivatized stilbene, with the dimeric structure of pyridoxal kinase resulted in the formation of an oligomeric species of 80 kDa detectable by SDS-PAGE. The crosslinked subunits exhibit the same catalytic parameters as the native enzyme. The presence of two nucleotide-binding sites per dimer was determined by fluorimetric titrations using pyridoxyl-ATP, a strong competitive inhibitor with respect to ATP. The ATP analog binds with a Kd = 5 microM to each nucleotide site of the dimeric enzyme. The mode of binding pyridoxyl-ATP to the kinase is discussed in reference to a model which assumes the presence of two binding domains per subunit.

Biosynthesis of 4-aminobutyrate aminotransferase

Mitochondrial 4-aminobutyrate aminotransferase was synthesized in a cell-free reticulocyte lysate using polysomal RNA isolated from pig brain. Its primary translation product has a higher molecular mass than the mature enzyme. The difference in relative molecular mass is approximately 2000 as revealed by SDS/polyacrylamide gel electrophoresis. The precursor of 4-aminobutyrate aminotransferase recognizes polyvalent antibodies raised against the mature enzyme. The precursor of 4-aminobutyrate aminotransferase binds pyridoxal-5-P and displays catalytic activity. Enzymatic activity was detected using a sensitive fluorimetric method, which is based on the formation of condensation products between succinic semialdehyde and cyclohexane-1,3-dione. It is concluded that removal of an extra peptide from the precursor is not an obligatory first step in the production of biological active species.

Sheep brain pyridoxal kinase: fluorescence spectroscopy of the dimeric enzyme

Pyridoxal kinase (ATP:pyridoxal 5-phosphotransferase, EC 2.7.1.35) has been purified 9000-fold from sheep brain by affinity chromatography. The enzyme of 80,000 molecular weight is made up of two identical-size subunits. The interaction of the inhibitor N-dansyl-1,8-diaminooctane with the nucleotide site of the kinase was examined by means of steady and nanosecond fluorescence spectroscopy. N-Dansyl-1,8-diaminooctane is a competitive inhibitor with respect to ATP at saturating concentrations of pyridoxal. It binds to the nucleotide site of the enzyme with Kd = 2.2 microM. Bound N-dansyl-1,8-diaminooctane is shielded from collisional encounters with the external quencher acrylamide. The collisional rate constant for bound N-dansyl-1,8-diaminooctane (Kq = 1.4 X 10(8) M-1 X s-1) is 10-times lower than the value obtained for the free chromophore. Nanosecond emission anisotropy measurements yield a rotational correlation time of 42 ns for the inhibitor complexes to the kinase. Both steady and nanosecond fluorescence results are consistent with a model in which the inhibitor bound to the nucleotide site is immobilized by amino acids located at the catalytic site.

Glutamate decarboxylase side reactions catalyzed by the enzyme

A homogeneous glutamate decarboxylase isolated from pig brain contains 0.8 mol of tightly bound pyridoxal 5-phosphate/enzyme dimer. Upon addition of exogenous pyridoxal 5-phosphate (pyridoxal-5-P), the enzyme acquires maximum catalytic activity. Preincubation of the enzyme with L-glutamate (10 mM) brings about changes in the absorption spectrum of bound pyridoxal-5-P with the concomitant formation of succinic semialdehyde. However, the rate of this slow secondary reaction, i.e. decarboxylative transamination, is 10(-4) times the rate of normal decarboxylation. It is postulated that under physiological conditions enzymatically inactive species of glutamate decarboxylase, generated by the process of decarboxylative transamination, are reconstituted by pyridoxal-5-P produced by the cytosolic enzymes pyridoxal kinase and pyridoxine-5-P oxidase. The catalytic activity of resolved glutamate decarboxylase is recovered by preincubation with phospho-pyridoxyl-ethanolamine phosphate. The experimental evidence is consistent with the interpretation that the resolved enzyme binds the P-pyridoxyl analog, reduces the stability of the covalent bond of the phospho-pyridoxyl moiety, and catalyzes the formation of pyridoxal-5-P.

Purification and characterization of pyridoxal kinase from human erythrocytes

Pyridoxal kinase has been purified 50,000-fold from human erythrocytes. The purification procedure included dextran-induced aggregation of red blood cells, ammonium sulphate fractionation of the haemolysate, DEAE-cellulose chromatography, hydroxyapatite chromatography. Sephadex G-100 gel filtration and omega-aminooctyl agarose chromatography. The enzyme preparation migrated as a single protein and activity band on analytical gel electrophoresis. Determination of the Michaelis constants for pyridoxal, pyridoxine and pyridoxamine using a new assay gave comparable values of 33 microM, 16 microM and 6.2 microM respectively. Various amines were shown as competitive inhibitors of pyridoxal kinase with respect to ATP. The inhibition order was: N-dansyl-1,8-diaminooctane greater than 1,8-diaminooctane greater than 1,6-diaminohexane greater than 1,4-diaminobutane greater than gamma-aminobutyric acid, whereas octane, hexane and butane were not inhibitors. Results suggest that the amino groups on the above inhibitors are essential for competitive inhibition at saturating concentrations of pyridoxal. It was also observed that increasing the chain length of the hydrophobic backbone of these competitive inhibitors can facilitate its action.

Brain pyridoxal kinase. Purification and characterization

Pyridoxal kinase has been purified 9000-fold from sheep brain. The purification procedure involves ammonium sulphate fractionation, DEAE-cellulose chromatography, affinity chromatography and Sephadex G-100 gel filtration. The final chromatography step yields a homogeneous preparation of high specific activity with a pI of 5. The molecular mass of the native enzyme was estimated to be approximately 80 kDa by 10-25% gradient polyacrylamide gel electrophoresis and Sephadex G-200 gel filtration. The subunit molecular mass was determined by sodium dodecyl sulphate (SDS)/polyacrylamide gel electrophoresis to be 40 kDa compared with a series of molecular mass standards. This indicates that pyridoxal kinase is a dimeric enzyme. Further results obtained from electron microscopy, using a negative staining technique, provide evidence that pyridoxal kinase exists as a dispherical subunit structure.

Binding of new PLP analogs to the catalytic domain of GABA transaminase

The binding site of Pyridoxal-5-P in 4-aminobutyrate aminotransferase was studied by using analogs of the cofactor. A phosphorothioate analog (PLP(S] recognizes the catalytic site; it forms a stable complex with the apoenzyme (KD = 1nM) and serves as cofactor during catalysis. Replacement of a non-bridged oxygen by sulfur in the phosphate side chain renders a compound which preserves the negative charges needed for correct alignment of the cofactor at the catalytic site. This phosphorothioate analog of PLP can be used to investigate the catalytic site of vitamin B6 dependent enzymes by means of 31P NMR spectroscopy. A bulky P-pyridoxamine derivative, ie, N-4-azido-2-nitrophenyl pyridoxyl-5-P (NANP) competes with natural cofactor for its binding site. Upon illumination, the arylazide of P-pyridoxamine acts as an efficient photolabeling reagent of the protein. A characteristic feature of this photolabeling reagent, ie, its ability to recognize the cofactor binding site, can be exploited to ascertain the chemical nature of amino acid residues at the catalytic site.

Mechanism of pyridoxine uptake by isolated rat liver cells

Uptake of [3H]pyridoxine by isolated rat hepatocytes was detected at substrate concentrations as low as 0.5 microM. At this concentration, an initial uptake rate was 1.87 +/- 0.17 pmol/10(6) cells X min at 37 degrees C. Both the initial and a subsequent much slower pyridoxine accumulation were strongly inhibited by low temperature as well as by 10 mM ethionine, which competes for available ATP, or 1 microM carbonylcyanide-p-trifluoromethoxyphenylhydrazone, which inhibits oxidative phosphorylation and the supply of ATP. The uptake process is apparently insensitive to 1 mM ouabain and is Na+ independent. The initial uptake rate was saturable at higher concentrations of [3H]pyridoxine with an apparent Km of 28 +/- 8 microM and Vmax of 106 +/- 27 pmol/10(6) cells X min. The Km value corresponds to that reported for pyridoxine as a substrate of pyridoxal kinase. Moreover, the transport process was inhibited by structural analogs of [3H]pyridoxine even at concentrations equimolar to the normal substrate. The established order of inhibitory effectiveness, 4'-deoxypyridoxine greater than unlabeled pyridoxine greater than 5'-deoxypyridoxine, is in agreement with known properties of the kinase. It is concluded that pyridoxine uptake probably occurs by diffusion, simple or facilitated, followed by metabolic trapping due to pyridoxal kinase-catalyzed phosphorylation. Pyridoxine deficiency had no significant effect on uptake of this B6 vitamer by hepatocytes.

Activity of pyridoxamine as a substrate for brain pyridoxal kinase

The substrate activity of pyridoxamine (PM) for brain pyridoxal (PL) kinase was examined in view of a recent report which indicated that PM was a poor substrate for this enzyme. Bovine brain PL kinase was shown by liquid chromatography to catalyze the phosphorylation of PM (Km = 65 microM). The identity of the reaction product, pyridoxamine 5'-phosphate, was confirmed by is ability to act as a substrate for liver pyridoxine (pyridoxamine) 5'-phosphate oxidase. The results, which indicate that PM is a good substrate for brain PL kinase, are consistent with the proposed role of intracellular phosphorylation in the uptake of vitamin B-6 brain tissue.