Presence Of Pyridoxal 5'-phosphate Research Articles

Abstract 125: Modified Angiotensin II Has Lower Vasoconstrictive Effect Than Angiotensin II

Introduction: The renin-angiotensin-aldosterone system (RAAS) is involved in the regulation of the blood pressure, water- and electrolyte balance. Pathophysiologically, this system is essential for the development and pathogenesis of both cardiovascular and renal diseases. Angiotensin II, a component of the RAAS, is the best-known vasoconstrictive and hypertensive peptide in the human organism. This peptide consists of 8 amino acids (asp-arg-val-tyr-ile-his-pro-phe). The first amino acid, aspartic acid, is converted to pyruvamide in the presence of pyridoxal-5’-phosphate (PLP). The influence of pyruvamide-angiotensin II on the blood pressure is not yet clarified. Hypothesis: We assessed the hypothesis that the transformation of angiotensin II to pyruvamide-angiotensin II by PLP leads to lowering the blood pressure. Methods: To clarify the influence of pyruvamide-angiotensin II on blood pressure ex vivo experiments were performed. Angiotensin II as well as pyruvamide-angiotensin II was tested in isolated perfused rat kidney. Further, in vivo experiments were performed. Wistar Kyoto rats (WKY) and Spontaneously Hypertensive Rats (SHR) were treated with angiotensin II in the absence and presence of PLP. The blood pressure was measured at different time. Results: Pressure in isolated perfused rat kidney increases by 50.11 ± 4.57 mmHg after injecting angiotensin II, whereas pressure increases by 38.40 ± 4.31 mmHg after injecting pyruvamide-angiotensin II. Data are shown as mean ± SEM of triplicate measurements from three independent experiments. Further, blood pressure of SHR treated with PLP decreases from 171/139 ± 5 mmHg to 129/98 ± 2 mmHg after three days. Blood pressure of WKY treated with angiotensin II increases to 167/133 ± 3 mmHg, whereas blood pressure of WKY treated with angiotensin II and PLP decreases to 129/99 ± 2 mmHg. Data are shown as mean ± SEM from five independent experiments. Conclusions: In conclusion, pyruvamide-angiotensin II has a lower vasoconstrictive effect than angiotensin II. Furthermore, PLP decreases the blood pressure in SHR and WKY.

MO093CLARIFICATION OF BIOSYNTHESIS OF ANGIOPROTECTIN

Abstract Background and Aims The renin-angiotensin-aldosterone system (RAAS) is involved in the regulation of the blood pressure, water- and electrolyte balance. Pathophysiologically, this system is essential for the development and pathogenesis of both cardiovascular and renal diseases. Recently, the angiotensin peptide ´angioprotectin´ was identified as an antagonist of the contractile effect of angiotensin II. The amino acid sequence of angioprotectin (pro-glu-val-tyr-ile-his-pro-phe) compared to the amino acid sequence of angiotensin II (asp-arg-val-tyr-ile-his-pro-phe) differs in the n-terminal amino acids asp1 and arg2, which are transformed to pro1 and glu2 by endothelial cells (Jankowski et al., 2011). The aim of the study is the identification of the underlying mechanism of the transformation of angiotensin II to angioprotectin. Method To clarify the transformation of angioprotectin diverse angiotensin peptides were incubated with enzymes like glutamic oxaloacetic transaminase (GOT), cofactors like pyridoxal-5’-phosphate (PLP) and other substances like vitamin B6-derivates. Aliquots were collected time-dependently and analyzed by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF). To investigate the impact of angioprotectin on the organism, rats were treated with angiotensin II in the absence and presence of PLP. The blood pressure was used as read-out of the effect of the treatment. Results The incubation of angiotensin II with pyridoxal-5’-phosphate in vitro caused in increased amount of angioprotectin. Since the first two amino acids aspartic acid and arginine of further peptides like angiotensin I, angiotensin (1-6) and cortistatin-17 were also metabolized to proline and glutamic acid, the underlying mechanism is not specific for angiotensin II, but for aspartic acid and arginine. In accordance with this, the blood pressure of spontaneously hypertensive rats (SHR) treated with PLP decreases after three days. The blood pressure of Wistar Kyoto rats (WKY) treated with angiotensin II increases, whereas the blood pressure of WKY rats treated with angiotensin II and PLP decreases to normal level. Conclusion Angiotensin II is obviously metabolized by pyridoxal-5’-phosphate (PLP) to angioprotectin. PLP decreases the blood pressure in spontaneously hypertensive rats and in Wistar Kyoto rats. The first two amino acids aspartic acid and arginine are also metabolized to proline and glutamic acid in other peptides.

Biochemical and Structural Analysis of a Dehydrogenase, KanD2, and an Aminotransferase, KanS2, That Are Responsible for the Construction of the Kanosamine Moiety in Kanamycin Biosynthesis.

Kanosamine (3-amino-3-deoxy-d-glucose) is a characteristic sugar unit found in kanamycins, a group of aminoglycoside antibiotics. The kanosamine moiety originates from d-glucose in kanamycin biosynthesis. However, the timing of the replacement of the 3-OH group of the d-glucose-derived biosynthetic intermediate with the amino group is elusive. Comparison of biosynthetic gene clusters for related aminoglycoside antibiotics suggests that the nicotinamide adenine dinucleotide (NAD+)-dependent dehydrogenase KanD2 and the pyridoxal 5'-phosphate (PLP)-dependent aminotransferase KanS2 are responsible for the introduction of the amino group at the C3 position of kanosamine. In this study, we demonstrated that KanD2 and KanS2 convert kanamycin A, B, and C to the corresponding 3″-deamino-3″-hydroxykanamycins (3″-hks) in the presence of PLP, 2-oxoglutarate, and NADH via a reverse reaction in the pathway. Furthermore, we observed that all of the 3″-hks are oxidized by KanD2 with NAD+, but d-glucose, UDP-d-glucose, d-glucose 6-phosphate, and d-glucose 1-phosphate are not. Crystal structure analysis of KanD2 complexed with 3″-hkB and NADH illustrated the selective recognition of pseudotrisaccharides, especially the d-glucose moiety with 2-deoxystreptamine, by a combination of hydrogen bonds and CH-π interactions. Overall, it was clarified that the kanosamine moiety of kanamycins is constructed after the glucosylation of the pseudodisaccharide biosynthetic intermediates in kanamycin biosynthesis.

Biochemical and biophysical studies of Helicobacter pylori arginine decarboxylase, an enzyme important for acid adaptation in host.

Despite importance of arginine decarboxylase (ADC: EC 4.1.1.19) of Helicobacter pylori (H. pylori) 26695 pathogenic strain for acid adaptation in host, the enzyme has not yet been studied at a molecular level. Using combined approaches that include kinetic assays, site-directed mutagenesis, circular dichroism, heat-induced denaturation, analytical gel-filtration, and homology modeling, we report here a detailed investigation of H. pylori ADC. The pyridoxal 5'-phosphate (PLP)-dependent enzyme exhibits higher catalytic activity in the presence of Mg2+ ions at pH ∼8.5. Unlike other bacterial ADCs, this homolog exists as a hexamer. The higher thermal stability (Tm ∼65.8 ± 0.2°C) of the enzyme observed from the heat-induced circular dichroism measurements indicates its secondary structural stabilization in the presence of PLP. The kinetic parameters Km and kcat of the enzyme are determined to be 3.4 ± 0.2 mM and 55.2 ± 1.0 min-1 , respectively. We elucidate that Cys487, a conserved residue located at the active-site, is involved in the catalysis, whose pKa value was estimated to be ∼7.2. The homology model of the protein show conserved α/β TIM barrel and β-sandwich domains, which are characteristic features of fold III decarboxylases. A lower sequence identity (∼21%) of this enzyme compared with its human counterpart has enabled us to screen putative inhibitors of H. pylori ADC. We found that α-difluoromethylarginine inhibits the activity of the H. pylori enzyme competitively with a Ki value ∼118 µM and thus it can serve as a basis to design inhibitors with higher efficacy against this ADC. © 2018 IUBMB Life, 70(7):658-669, 2018.

How pyridoxal 5'-phosphate differentially regulates human cytosolic and mitochondrial serine hydroxymethyltransferase oligomeric state.

Adaptive metabolic reprogramming gives cancer cells a proliferative advantage. Tumour cells extensively use glycolysis to sustain anabolism and produce serine, which not only refuels the one-carbon units necessary for the synthesis of nucleotide precursors and for DNA methylation, but also affects the cellular redox homeostasis. Given its central role in serine metabolism, serine hydroxymethyltransferase (SHMT), a pyridoxal 5'-phosphate (PLP)-dependent enzyme, is an attractive target for tumour chemotherapy. In humans, the cytosolic isoform (SHMT1) and the mitochondrial isoform (SHMT2) have distinct cellular roles, but high sequence identity and comparable catalytic properties, which may complicate development of successful therapeutic strategies. Here, we investigated how binding of the cofactor PLP controls the oligomeric state of the human isoforms. The fact that eukaryotic SHMTs are tetrameric proteins while bacterial SHMTs function as dimers may suggest that the quaternary assembly in eukaryotes provides an advantage to fine-tune SHMT function and differentially regulate intertwined metabolic fluxes, and may provide a tool to address the specificity problem. We determined the crystal structure of SHMT2, and compared it to the apo-enzyme structure, showing that PLP binding triggers a disorder-to-order transition accompanied by a large rigid-body movement of the two cofactor-binding domains. Moreover, we demonstrated that SHMT1 exists in solution as a tetramer, both in the absence and presence of PLP, while SHMT2 undergoes a dimer-to-tetramer transition upon PLP binding. These findings indicate an unexpected structural difference between the two human SHMT isoforms, which opens new perspectives for understanding their differing behaviours, roles or regulation mechanisms in response to PLP availability invivo.

Role of the aminotransferase domain in Bacillus subtilis GabR, a pyridoxal 5'-phosphate-dependent transcriptional regulator.

MocR/GabR family proteins are widely distributed prokaryotic transcriptional regulators containing pyridoxal 5'-phosphate (PLP), a coenzyme form of vitamin B6. The Bacillus subtilis GabR, probably the most extensively studied MocR/GabR family protein, consists of an N-terminal DNA-binding domain and a PLP-binding C-terminal domain that has a structure homologous to aminotransferases. GabR suppresses transcription of gabR and activates transcription of gabT and gabD, which encode γ-aminobutyrate (GΑΒΑ) aminotransferase and succinate semialdehyde dehydrogenase, respectively, in the presence of PLP and GABA. In this study, we examined the mechanism underlying GabR-mediated gabTD transcription with spectroscopic, crystallographic and thermodynamic studies, focusing on the function of the aminotransferase domain. Spectroscopic studies revealed that GABA forms an external aldimine with the PLP in the aminotransferase domain. Isothermal calorimetry demonstrated that two GabR molecules bind to the 51-bp DNA fragment that contains the GabR-binding region. GABA minimally affected ΔG(binding) upon binding of GabR to the DNA fragment but greatly affected the contributions of ΔH and ΔS to ΔG(binding). GABA forms an external aldimine with PLP and causes a conformational change in the aminotransferase domain, and this change likely rearranges GabR binding to the promoter and thus activates gabTD transcription.

Identification of the incednine biosynthetic gene cluster: characterization of novel β-glutamate-β-decarboxylase IdnL3

A biosynthetic gene cluster for the 24-membered macrolactam antibiotic incednine was identified from the producer strain, Streptomyces sp. ML694-90F3. Among the putative incednine biosynthetic enzymes, a novel pyridoxal 5'-phosphate (PLP)-dependent β-glutamate-β-decarboxylase, IdnL3, was functionally characterized in vitro by demonstrating its (S)-3-aminobutyrate-forming activity with β-glutamate in the presence of PLP. Because (S)-3-aminobutyrate is known for the direct precursor of incednine, this enzyme supplies the unique β-amino acid starter unit. The identified gene cluster encodes five characteristic β-amino acid carrying enzymes, consisting of a pathway-specific ATP-dependent ligase, a discrete acyl carrier protein (ACP), β-aminoacyl-ACP β-amino group-protecting ATP-dependent ligase, dipeptidyl-ACP:PKS-loading ACP dipeptidyltransferase and a terminal amino acid peptidase, which are completely conserved in β-amino acid-containing macrolactam biosynthetic gene clusters. Overall, a plausible biosynthetic pathway for incednine was proposed.

Crystal structures of the Chromobacterium violaceumω‐transaminase reveal major structural rearrangements upon binding of coenzyme PLP

The bacterial ω-transaminase from Chromobacterium violaceum (Cv-ωTA, EC2.6.1.18) catalyses industrially important transamination reactions by use of the coenzyme pyridoxal 5'-phosphate (PLP). Here, we present four crystal structures of Cv-ωTA: two in the apo form, one in the holo form and one in an intermediate state, at resolutions between 1.35 and 2.4 Å. The enzyme is a homodimer with a molecular mass of ∼ 100 kDa. Each monomer has an active site at the dimeric interface that involves amino acid residues from both subunits. The apo-Cv-ωTA structure reveals unique 'relaxed' conformations of three critical loops involved in structuring the active site that have not previously been seen in a transaminase. Analysis of the four crystal structures reveals major structural rearrangements involving elements of the large and small domains of both monomers that reorganize the active site in the presence of PLP. The conformational change appears to be triggered by binding of the phosphate group of PLP. Furthermore, one of the apo structures shows a disordered 'roof ' over the PLP-binding site, whereas in the other apo form and the holo form the 'roof' is ordered. Comparison with other known transaminase crystal structures suggests that ordering of the 'roof' structure may be associated with substrate binding in Cv-ωTA and some other transaminases. The atomic coordinates and structure factors for the Chromobacterium violaceumω-transaminase crystal structures can be found in the RCSB Protein Data Bank (http://www.rcsb.org) under the accession codes 4A6U for the holoenzyme, 4A6R for the apo1 form, 4A6T for the apo2 form and 4A72 for the mixed form • -transaminases and -transaminases bind by dynamic light scattering (View interaction) • -transaminase and -transaminase bind by x-ray crystallography (View interaction) • -transaminase and -transaminase bind by x-ray crystallography (View interaction).

Semi-rational chemical modification of endoinulinase by pyridoxal 5′-phosphate and ascorbic acid

It is important to improve the quality of the enzyme inulinase used in industrial applications without allowing the treatment to have any adverse effects on enzyme activity. We achieved preferential chemical modification of the non-catalytic domain of endoinulinase (EC 3.2.1.7) to enhance the thermostability of the enzyme. We used pyridoxal 5′-phosphate (PLP) to modify the more accessible lysine residues at the surface of endoinulinase and then performed a necessary step of reduction with ascorbate. Endoinulinase was incubated in the presence of PLP at various concentrations; this step was followed by reduction of the resulting Schiff base and dialysis. The effects of different PLP concentrations and incubation times on enzyme modification were evaluated. Enzyme deactivation was observed immediately after treatment, even at low PLP concentrations, while reactivation was observed for samples treated with low PLP concentrations after a period of time. Structural analysis revealed that the α-helix content increased from 13.60% to 17.60% after applying the modification strategy; consequently, enzyme stabilization was achieved. The melting temperature ( T m ) of the modified enzyme increased from 64.1 °C to 72.2 °C, and a comparative study of thermal stability at 25 °C, 45 °C, and 50 °C for 150 min confirmed that the enzyme was stabilized because of increase in its half-life ( t 1/2) after PLP modification/ascorbate reduction. The modification process was optimized to achieve the optimum mole ratio for the PLP/endoinulinase (1.37). Excess moles of the modifier are thought to be responsible for enzyme deactivation through unwanted/nonspecific and noncovalent interactions, and the optimization ensured that there was no excess modifier after the desired covalent reaction was complete.

Molecular Insight into the Synergism between the Minor Allele of Human Liver Peroxisomal Alanine:Glyoxylate Aminotransferase and the F152I Mutation

Human liver peroxisomal alanine:glyoxylate aminotransferase (AGT) is a pyridoxal 5'-phosphate (PLP)-dependent enzyme that converts glyoxylate into glycine. AGT deficiency causes primary hyperoxaluria type 1 (PH1), a rare autosomal recessive disorder, due to a marked increase in hepatic oxalate production. Normal human AGT exists as two polymorphic variants: the major (AGT-Ma) and the minor (AGT-Mi) allele. AGT-Mi causes the PH1 disease only when combined with some mutations. In this study, the molecular basis of the synergism between AGT-Mi and F152I mutation has been investigated through a detailed biochemical characterization of AGT-Mi and the Phe(152) variants combined either with the major (F152I-Ma, F152A-Ma) or the minor allele (F152I-Mi). Although these species show spectral features, kinetic parameters, and PLP binding affinity similar to those of AGT-Ma, the Phe(152) variants exhibit the following differences with respect to AGT-Ma and AGT-Mi: (i) pyridoxamine 5'-phosphate (PMP) is released during the overall transamination leading to the conversion into apoenzymes, and (ii) the PMP binding affinity is at least 200-1400-fold lower. Thus, Phe(152) is not an essential residue for transaminase activity, but plays a role in selectively stabilizing the AGT-PMP complex, by a proper orientation of Trp(108), as suggested by bioinformatic analysis. These data, together with the finding that apoF152I-Mi is the only species that at physiological temperature undergoes a time-dependent inactivation and concomitant aggregation, shed light on the molecular defects resulting from the association of the F152I mutation with AGT-Mi, and allow to speculate on the responsiveness to pyridoxine therapy of PH1 patients carrying this mutation.

Kinetic and structural studies of the role of the active site residue Asp235 of human pyridoxal kinase

Pyridoxal kinase catalyzes the phosphorylation of pyridoxal (PL) to pyridoxal 5′-phosphate (PLP). A D235A variant shows 7-fold and 15-fold decreases in substrate affinity and activity, respectively. A D235N variant shows ∼2-fold decrease in both PL affinity and activity. The crystal structure of D235A (2.5 Å) shows bound ATP, PL and PLP, while D235N (2.3 Å) shows bound ATP and sulfate. These results document the role of Asp235 in PL kinase activity. The observation that the active site of PL kinase can accommodate both ATP and PLP suggests that formation of a ternary Enz·PLP·ATP complex could occur in the wild-type enzyme, consistent with severe MgATP substrate inhibition of PL kinase in the presence of PLP.

2-Deoxy-d-ribose-induced oxidative stress causes apoptosis in human monocytic cells: Prevention by pyridoxal-5′-phosphate

Hyperglycemia-induced oxidative stress plays a crucial role in the pathogenesis of diabetic complications. Although some clinical evidences suggest the use of pyridoxal-5'-phosphate (PLP) in diabetes with nephropathy, the exact mechanism of PLP has not been fully understood. In the present study, the effect of PLP on 2-deoxy-D-ribose (dRib)-induced oxidative damages and apoptosis on human monocytic cells (U937) was investigated. U937 cells exposed to dRib (30 mM) exhibited abnormal properties, including loss pf cell viability, overproduction of reactive oxygen species (ROS), glutathione depletion and some biochemical features of apoptosis. Treatment with PLP at two effective concentrations (100 and 250 microM) strongly inhibited ROS production and glutathione depletion in dRib-treated U937 cells. The extent of Lipid peroxidation and protein oxidation was also decreased in the presence of PLP. In addition, PLP suppressed dRib-induced apoptotic criteria such as sub-G1 apoptotic population and annexin-V staining. The use of N-acetylcysteine (NAC), a thiol antioxidant and GSH precursor, prevented the extent of apoptosis. The results demonstrate that dRib induces the generation of ROS leading to dRib-mediated apoptosis which can be attenuated by PLP through antioxidant mechanisms.

Interaction between pyridoxal kinase and pyridoxal-5-phosphate-dependent enzymes.

The interactions of two pyridoxal-5-phosphate (PLP)-dependent enzymes, alanine aminotransferase (ALT) and glutamate decarboxylase (GAD), with pyridoxal kinase (PK) were studied by fluorescence polarization as well as surface plasmon resonance techniques. The results demonstrated that PK can specifically bind to ALT and GAD. Moreover, binding profiles of both enzymes to immobilized PK were altered by excess amount of PLP. The equilibrium affinity constants for ALT in the absence and presence of PLP are 20.4 x 10(4) M(-1)and 6.7 x 10(4) M(-1), and for GAD are 37 x 10(4) M(-1)and 20.8 x 10(4) M(-1), respectively. It appears that specific interactions occur between PK and PLP-dependent enzymes, and the binding affinities of PK for PLP-dependent enzymes decrease in the presence of PLP. The results support our hypothesis that PLP transfer from PK to PLP-dependent enzymes requires a specific interaction between PK and the enzyme.

Overexpression and characterization of dimeric and tetrameric forms of recombinant serine hydroxymethyltransferase from Bacillus stearothermophilus.

Serine hydroxymethyltransferase (SHMT), a pyridoxal-5' -phosphate (PLP) dependent enzyme catalyzes the interconversion of L-Ser and Gly using tetrahydrofolate as a substrate. The gene encoding for SHMT was amplified by PCR from genomic DNA of Bacillus stearothermophilus and the PCR product was cloned and overexpressed in Escherichia coli. The purified recombinant enzyme was isolated as a mixture of dimer (90%) and tetramer (10%). This is the first report demonstrating the existence of SHMT as a dimer and tetramer in the same organism. The specific activities at 37 C of the dimeric and tetrameric forms were 6 7 U/mg and 4 1 U/mg, respectively. The purified dimer was extremely thermostable with a T(m) of 85 degrees C in the presence of PLP and L-Ser. The temperature optimum of the dimer was 80 degrees C with a specific activity of 32 4 U/mg at this temperature. The enzyme catalyzed tetrahydrofolate-independent reactions at a slower rate compared to the tetrahydrofolate-dependent retro-aldol cleavage of L-Ser. The interaction with substrates and their analogues indicated that the orientation of PLP ring of B. stearothermophilus SHMT was probably different from sheep liver cytosolic recombinant SHMT (scSHMT).

High catalytic activity of alanine racemase from psychrophilic Bacillus psychrosaccharolyticus at high temperatures in the presence of pyridoxal 5′-phosphate

We examined the effect of the pyridoxal 5′-phosphate (PLP) cofactor on the activity and stability of the psychrophilic alanine racemase, having a high catalytic activity at low temperature, from Bacillus psychrosaccharolyticus at high temperatures. The decrease in the enzyme activity at incubation temperatures over 40°C was consistent with the decrease in the amount of bound PLP. Unfolding of the enzyme at temperatures above 40°C was suppressed in the presence of PLP. In the presence of 0.125 mM PLP, the specific activity of the psychrophilic enzyme was higher than that of a thermophilic alanine racemase, having a high catalytic activity at high temperature, from Bacillus stearothermophilus even at 60°C.

High catalytic activity of alanine racemase from psychrophilic Bacillus psychrosaccharolyticus at high temperatures in the presence of pyridoxal 5'-phosphate.

We examined the effect of the pyridoxal 5'-phosphate (PLP) cofactor on the activity and stability of the psychrophilic alanine racemase, having a high catalytic activity at low temperature, from Bacillus psychrosaccharolyticus at high temperatures. The decrease in the enzyme activity at incubation temperatures over 40 degrees C was consistent with the decrease in the amount of bound PLP. Unfolding of the enzyme at temperatures above 40 degrees C was suppressed in the presence of PLP. In the presence of 0.125 mM PLP, the specific activity of the psychrophilic enzyme was higher than that of a thermophilic alanine racemase, having a high catalytic activity at high temperature, from Bacillus stearothermophilus even at 60 degrees C.

Guanidine hydrochloride-induced reversible unfolding of sheep liver serine hydroxymethyltransferase

Equilibrium unfolding studies of sheep liver tetrameric serine hydroxymethyltransferase (SHMT, EC 2.1.2.1) revealed that the enzyme assumed apparent random coil structure above 3 M guanidine hydrochloride (GdnHCl). In the presence of non-ionic detergent Brij-35 and polyethylene glycol, the 6 M GdnHCI unfolded enzyme could be completely (> 95%) refolded by a 40-fold dilution. The refolded enzyme was fully active and had kinetic constants similar to the native enzyme. The midpoint of inactivation (0.12 M GdnHCl) was well below the midpoint of unfolding (1.6±0.1 M GdnHCl) as monitored by far UV CD at 222 nm. In the presence of PLP, the midpoint of inactivation shifted to a higher concentration of GdnHCl (0.6 M) showing that PLP stabilizes the quaternary structure of the enzyme. However, 50% release of pyridoxal-5′-phosphate (PLP) from the active site occurred at a concentration (0.6 M) higher than the midpoint of inactivation suggesting that GdnHCl may also act as a competitive inhibitor of the enzyme at low concentrations which was confirmed by activity measurements. PLP was not required for the initiation of refolding and inactive tetramers were the end products of refolding which could be converted to active tetramers upon the addition of PLP. Size exclusion chromatography of the apoenzyme showed that the tetramer unfolds via the intermediate formation of dimers. Low concentrations (0.3–0.6 M) of GdnHCl stabilized at least one intermediate which was in slow equilibrium with the dimer. The binding of ANS was maximum at 0.4–0.6 M GdnHCl suggesting that the unfolding intermediate that accumulates at this concentration is less compact than the native enzyme.

New Enzymatic Method with Tryptophanase for Determining Potassium in Serum

We established a simple and rapid enzymatic method for measuring potassium ion in serum by using tryptophanase (EC 4.1.99.1) purified from Escherichia coli K12 strain (E. coli K12 IFO 3301). The presence of pyridoxal 5-phosphate promotes this enzymatic reaction, and potassium and (or) ammonium ions further accelerate it, with ammonium and potassium ions providing equivalent acceleration. We eliminated endogenous ammonium ion by using glutamate dehydrogenase (GLDH; EC 1.4.1.4), then produced ammonium ion in the presence of tryptophanase, tryptophan, and pyridoxal 5-phosphate. The concentration of formed ammonium ion, which was proportional to that of potassium ion in sample, was determined by adding GLDH to produce NADP+ in the presence of 2-oxoglutarate and NADPH; we then read the change of absorbance at 340 nm. The standard curve was linear for potassium ion concentrations up to 7.00 mmol/L. The within-assay variation (CV) was 0.89% at 5.51 mmol/L and 1.32% at 3.37 mmol/L. The day-to-day CVs were 0.99% at 6.85 mmol/L and 1.71% at 3.52 mmol/L. Analytical recoveries ranged from 98.7% to 108.9%. The correlation coefficient between values obtained with this enzymatic assay (y) and by flame photometry (x) was 0.995: y = 0.984x + 0.091 mmol/L (Sy.x = 0.105, n = 100). The presence of hemoglobin, bilirubin, or other cations little affects this system.

Pyridoxine‐Responsive Primary Acquired Sideroblastic Anaemia

The activity of 5-aminolaevulinate (ALA) synthase, the first and rate-limiting of haem synthesis, was markedly reduced (13% of controls) in erythroblasts of a patient with acquired, primary sideroblastic anaemia (PASA). The reduced activity of ALA synthase could not be restored in vitro with 1 mmol/l pyridoxal-5-phosphate (PLP). Treatment of the patient with pyridoxine for several months increased the ALA synthase activity from 13% to 50% of controls in the absence and to 100% in the presence of PLP in the incubation medium. These studies suggest that both increased degradation of apo-ALA synthase and decreased affinity of ALA synthase for PLP may be involved in pyridoxine-responsive PASA.

Decrease in cystathionase activity during sporulation of Bacillus subtilis.

Cystathionase decreased in activity during sporulation of Bacillus subtilis. The experimental data obtained from the extract of vegetative cells and sporulating cells suggest that the decrease in cystathionase activity was caused by a heat-labile protease in cells. The extract from sporulating cells was applied to a DEAE-cellulose column and a Sephadex G-75 column. In both chromatograms Fraction I having azocasein-hydrolyzing activity (protease B) showed cystathionase-inactivating activity. Cystathionase was inactivated by protease B in the absence of pyridoxal 5′-phosphate (PLP) but its activity was maintained in the presence of PLP in vitro condition. During sporulation cystathionase level in cells decreased after t1.5 stage, and the intracellular level of phosphorylated vitamin B6 decreased after t2 stage. On the other hand, the production of intracellular protease B commenced at t0 stage. From these data it is suggested that cystathionase activity in vegetative cells of B. subtilis is maintained by PL...