Transport and metabolism of vitamin B6 in rabbit brain and choroid plexus

Pyridox(am)ine 5'-phosphate oxidase (PNPO) deficiency in zebrafish results in fatal seizures and metabolic aberrations

Pyridox(am)ine 5'-phosphate oxidase (PNPO) catalyzes oxidation of pyridoxine 5'-phosphate (PNP) and pyridoxamine 5'-phosphate (PMP) to pyridoxal 5'-phosphate (PLP), the active form of vitamin B₆. PNPO deficiency results in neonatal/infantile seizures and neurodevelopmental delay. To gain insight into this disorder we generated Pnpo deficient (pnpo^-/-) zebrafish (CRISPR/Cas9 gene editing). Locomotion analysis showed that pnpo^-/- zebrafish develop seizures resulting in only 38% of pnpo^-/- zebrafish surviving beyond 20 days post fertilization (dpf). The age of seizure onset varied and survival after the onset was brief. Biochemical profiling at 20 dpf revealed a reduction of PLP and pyridoxal (PL) and accumulation of PMP and pyridoxamine (PM). Amino acids involved in neurotransmission including glutamate, γ-aminobutyric acid (GABA) and glycine were decreased. Concentrations of several, mostly essential, amino acids were increased in pnpo^-/- zebrafish suggesting impaired activity of PLP-dependent transaminases involved in their degradation. PLP treatment increased survival at 20 dpf and led to complete normalization of PLP, PL, glutamate, GABA and glycine. However, amino acid profiles only partially normalized and accumulation of PMP and PM persisted. Taken together, our data indicate that not only decreased PLP but also accumulation of PMP may play a role in the clinical phenotype of PNPO deficiency.

The Role of Brain Barriers in Maintaining Brain Vitamin Levels

It is increasingly recognized that tissue-specific nutrient deficiencies can exist in the absence of whole-body deficiency and that these deficiencies may result from disease or disease-related physiological processes. Brain and central nervous system tissues require adequate nutrient levels to function. Many nutrients are concentrated in the cerebrospinal fluid relative to the serum in healthy individuals, and other nutrients resist depletion in the presence of whole-body nutrient depletion. The endothelial, epithelial, and arachnoid brain barriers work in concert to selectively transport, concentrate, and maintain levels of the specific nutrients required by the brain while also blocking the passage of blood-borne toxins and pathogens to brain and central nervous system tissues. These barriers preserve nutrient levels within the brain and actively concentrate nutrients within the cerebrospinal fluid and brain. The roles of physical and energetic barriers, including the blood–brain and blood–nerve barriers, in maintaining brain nutrient levels in health and disease are discussed.

Functional Nutrients for Epilepsy

Epilepsy is a common neurological disorder of which seizures are a core symptom. Approximately one third of epileptic patients are resistant to antiepileptic drugs and therefore require alternative therapeutic options. Dietary and nutritional supplements can in some cases replace drugs, but with the exception of ketogenic diets, there are no officially recommended dietary considerations for patients with epilepsy. In this review we summarize a selection of nutritional suggestions that have proved beneficial in treating different types of epilepsy. We describe the types of seizures and epilepsy and follow this with an introduction to basic molecular mechanisms. We then examine several functional nutrients for which there is clinical evidence of therapeutic efficacy in reducing seizures or epilepsy-associated sudden death. We also discuss experimental results that demonstrate possible molecular mechanisms elicited by the administration of various nutrients. The availability of multiple dietary and nutritional candidates that show favorable outcomes in animals implies that assessing the clinical potential of these substances will improve translational medicine, ultimately benefitting epilepsy patients.

Vitamin B-6 Metabolism and Interactions with TNAP

Two observations stimulated the interest in vitamin B-6 and alkaline phosphatase in brain: the marked increase in plasma pyridoxal phosphate and the occurrence of pyridoxine responsive seizures in hypophosphatasia. The increase in plasma pyridoxal phosphate indicates the importance of tissue non-specific alkaline phosphatase (TNAP) in transferring vitamin B-6 into the tissues. Vitamin B-6 is involved in the biosynthesis of most of the neurotransmitters. Decreased gamma-aminobutyrate (GABA) appears to be most directly related to the development of seizures in vitamin B-6 deficiency. Cytosolic pyridoxal phosphatase/chronophin may interact with vitamin B-6 metabolism and neuronal development and function. Ethanolaminephosphate phospholyase interacts with phosphoethanolamine metabolism. Extracellular pyridoxal phosphate may interact with purinoceptors and calcium channels. In conclusion, TNAP clearly influences extracellular and intracellular metabolism of vitamin B-6 in brain, particularly during developmental stages. While effects on GABA metabolism appear to be the major contributor to seizures, multiple other intra- and extra-cellular metabolic systems may be affected directly and/or indirectly by altered vitamin B-6 hydrolysis and uptake resulting from variations in alkaline phosphatase activity.

Transendothelial Transport and Its Role in Therapeutics

Present review paper highlights role of BBB in endothelial transport of various substances into the brain. More specifically, permeability functions of BBB in transendothelial transport of various substances such as metabolic fuels, ethanol, amino acids, proteins, peptides, lipids, vitamins, neurotransmitters, monocarbxylic acids, gases, water, and minerals in the peripheral circulation and into the brain have been widely explained. In addition, roles of various receptors, ATP powered pumps, channels, and transporters in transport of vital molecules in maintenance of homeostasis and normal body functions have been described in detail. Major role of integral membrane proteins, carriers, or transporters in drug transport is highlighted. Both diffusion and carrier mediated transport mechanisms which facilitate molecular trafficking through transcellular route to maintain influx and outflux of important nutrients and metabolic substances are elucidated. Present review paper aims to emphasize role of important transport systems with their recent advancements in CNS protection mainly for providing a rapid clinical aid to patients. This review also suggests requirement of new well-designed therapeutic strategies mainly potential techniques, appropriate drug formulations, and new transport systems for quick, easy, and safe delivery of drugs across blood brain barrier to save the life of tumor and virus infected patients.

Vitamin B6 vitamer concentrations in cerebrospinal fluid differ between preterm and term newborn infants

BACKGROUND AND OBJECTIVE

Vitamin B(6) plays a pivotal role in brain development and functioning. Differences in vitamin B(6) homeostasis between preterm and term newborn infants have been reported. The authors sought to investigate whether B(6) vitamers in cerebrospinal fluid (CSF) of preterm and term newborn infants are different.

METHODS

B(6) vitamer concentrations were determined in 69 CSF samples of 36 newborn infants (26 born preterm and 10 born term) by ultra performance liquid chromatography-tandem mass spectrometry. CSF samples, taken from a subcutaneous intraventricular reservoir, were bedside frozen and protected from light.

RESULTS

Concentrations of pyridoxal (PL), pyridoxal phosphate (PLP), pyridoxic acid (PA), and pyridoxamine (PM) in preterm newborns (postmenstrual age 30-37 weeks) were at least twice as high as in older newborns (postmenstrual age ≥ 42 weeks). Pyridoxine and pyridoxamine phosphate concentrations were below limits of quantification in all newborns. In CSF of 2 very preterm newborns (postmenstrual age <30 weeks), significant amounts of pyridoxine were present besides high concentrations of PL, PA, and PM, whereas PLP concentrations were relatively low. B(6) vitamers in CSF were positively correlated, especially PA, PLP, and PL.

CONCLUSIONS

In CSF of newborn infants, PL, PLP, PA, and PM are present, and concentrations are strongly dependent on postmenstrual age. Our results indicate that vitamin B(6) homeostasis in brain differs between preterm and term newborns. These results should be taken into account for diagnosis and treatment of epilepsy and vitamin B(6) deficiency in newborn infants.

Cerebrospinal fluid and serum levels of vitamin B6 in status epilepticus children

Vitamin B6 (B6) is an essential cofactor of glutamate decarboxylase and catalyzes the decarboxylation of the excitatory neurotransmitter glutamate to the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Although immediate administration of B6 to patients with acute encephalopathy with febrile convulsive status epilepticus (AEFCSE) is effective, it is not known whether prolonged seizures in status epilepticus (SE) children prevent the transport of B6 to the central nervous system (CNS) and/or induce the consumption of B6 inside the CNS. We evaluated the B6 concentration in cerebrospinal fluid (CSF) and serum in SE children. Further, we performed a sequential serum B6 analysis on days 1 and 2 after admission and on the day before discharge. Among the several vitamers of B6, we used pyridoxal (PL) as a representative of B6 in this study. We enrolled 15 SE children (8 boys and 7 girls; age range,1-11years; average age, 3.3years) and 21 control children (3 boys and 18 girls; age range, 7months-14years; average age, 3.0years) and each group was divided into 2 subgroups according to age (4months-1year and 2-14years). We found no significant differences in the CSF PL levels, CSF/serum PL ratios, and serum PL levels in the SE and control subgroups. Our results suggest that prolonged seizures do not result in B6 deficiency in CSF and serum in SE children. Whenever necessary, B6 should be administered to SE children with caution to prevent possible adverse effects.

Enhanced pyridoxal 5'-phosphate synthetic enzyme immunoreactivities do not contribute to GABAergic inhibition in the rat hippocampus following pilocarpine-induced status epilepticus

To comprehend the role of pyridoxal 5'-phosphate (PLP) in epilepsy or seizure, we investigated whether the expressions of two PLP synthetic enzymes (pyridoxal kinase, PLK; pyridoxine-5'-phosphate oxidase, PNPO) are altered in the hippocampus and whether changes in paired-pulse responses in the hippocampus are associated with altered PLP synthetic enzyme expressions following status epilepticus (SE). PLK and PNPO immunoreactivities were significantly increased in the rat hippocampus accompanied by reductions in paired-pulse inhibition at 1 day and 1 week after SE. Four weeks after SE, PLK and PNPO immunoreactivities in dentate granule cells were similar to those in control animals, while their immunoreactivities were markedly reduced in Cornu Ammonis 1 (CA1) pyramidal cells due to neuronal loss. Linear regression analysis identified a direct proportional relationship between PLK/PNPO immunoreactivity and normalized population spike amplitude ratio in the dentate gyrus and the CA1 region as excluded the data obtained from 4 weeks after SE. These findings indicate that the upregulation of PLK and PNPO immunoreactivities in principal neurons may not be involved in gamma-aminobutyric acid (GABA)ergic inhibition, but rather in enhanced excitability during epileptogenic periods.

REVIEW: Vitamin transport and homeostasis in mammalian brain: focus on Vitamins B and E

With the application of genetic and molecular biology techniques, there has been substantial progress in understanding how vitamins are transferred across the mammalian blood-brain barrier and choroid plexus into brain and CSF and how vitamin homeostasis in brain is achieved. In most cases (with the exception of the sodium-dependent multivitamin transporter for biotin, pantothenic acid, and lipoic acid), the vitamins are transported by separate carriers through the blood-brain barrier or choroid plexus. Then the vitamins are accumulated by brain cells by separate, specialized systems. This review focuses on six vitamins (B(1), B(3), B(6), pantothenic acid, biotin, and E) and the newer genetic information including relevant 'knockdown' or 'knockout' models in mice and humans. The overall objective is to integrate this newer information with previous physiological and biochemical observations to achieve a better understanding of vitamin transport and homeostasis in brain. This is especially important in view of the newly described non-cofactor vitamin roles in brain (e.g. of B(1), B(3), B(6), and E) and the potential roles of vitamins in the therapy of brain disorders.

Inborn errors affecting vitamin B6 metabolism

Vitamin B6 is an important vitamin for normal brain function. The metabolism of dietary vitamin B6 to its active cofactor pyridoxal 5´-phosphate is described. The mechanism of action of pyridoxal 5´-phosphate is described, as are some important functions in the brain. The clinical features and biochemistry of three inborn errors of metabolism affecting brain pyridoxal 5´-phosphate concentrations are described, each of which cause early-onset epilepsy of variable severity. These are pyridoxine phosphate oxidase deficiency, hyperprolinemia Type 2 and pyridoxine-dependent epilepsy caused by antiquitin deficiency. Hypophosphatasia is also discussed briefly, as the epilepsy that can complicate this disorder appears to be due to pyridoxal phosphate deficiency. Lastly, the antiepileptic properties of pyridoxine and pyridoxal phosphate are discussed.

Pyridoxine-dependent seizures: new genetic and biochemical clues to help with diagnosis and treatment

PURPOSE OF REVIEW

Pyridoxine dependency is an uncommon but important cause of intractable seizures presenting in infancy and early childhood. This paper discusses recent clinical, biochemical and genetic studies and how the findings should change our approach in evaluating young patients with antiepileptic drug-resistant seizures.

RECENT FINDINGS

Originally thought to be due to abnormal binding of pyridoxal phosphate to glutamic acid decarboxylase resulting in decreased gamma-aminobutyric acid, mutations in the gene encoding this enzyme have been ruled out. While linkage to 5q31 has been demonstrated, a disease-causing gene in that region has not been identified. Further haplotype analysis of six affected kindreds has demonstrated genetic heterogeneity for this rare disorder. Other studies demonstrate that some children with intractable seizures respond to pyridoxal phosphate rather than pyridoxine, including a rare form of neonatal epileptic encephalopathy shown to be due to mutations in the PNPO gene for pyridox(am)ine 5'-phosphate oxidase. While the biochemical explanation for this finding is not clear, elevated pipecolic acid levels may serve as a diagnostic marker for patients with pyridoxine-dependent seizures.

SUMMARY

The results of these studies should prompt clinicians to adopt new strategies for diagnosis and therapy for young patients with intractable seizures. Levels of both pipecolic acid and certain metabolites shown to be elevated in patients with PNPO mutations should be measured, and therapeutic trials of pyridoxal phosphate as well as pyridoxine should be considered early in the course of the management of infants and young children with intractable seizures.

Age-dependent changes of pyridoxal phosphate synthesizing enzymes immunoreactivities and activities in the gerbil hippocampal CA1 region

In the present study, age-related changes of pyridoxal 5'-phosphate (PLP) synthesizing enzymes, pyridoxal kinase (PLK) and pyridoxine 5'-phosphate oxidase (PNPO), their protein contents and activities were examined in the gerbil hippocampus proper. Significant age-dependent changes in PLK and PNPO immunoreactivities were found in the CA1 region, but not in the CA2/3 region. In the postnatal month 1 (PM 1) group, PLK and PNPO immunoreactivities were detected mainly in the stratum pyramidale of the CA1 region. PLK and PNPO immunoreactivities and their protein contents were highest in the PM 6 group, showing that many CA1 pyramidal cells had strong PLK and PNPO immunoreactivities. Thereafter, PLK and PNPO immunoreactivities started to decrease and were very low at PM 24. Alterations in the change patterns in protein contents and total activities of PLK and PNPO corresponded to the immunohistochemical data, but their specific activities were not altered in any experimental group. Based on double immunofluorescence study, PLK and PNPO immunoreactive cells in the strata oriens and radiatum were identified as GABAergic cells. Therefore, decreases of PLK and PNPO in the hippocampal CA1 region of aged brains may be involved in aging processes related with gamma-aminobutyric acid (GABA) function.

Age-related changes of γ-aminobutyric acid transaminase immunoreactivity in the hippocampus and dentate gyrus of the Mongolian gerbil

We investigated the age-related changes of gamma-aminobutyric acid transaminase (GABA-T, a GABA degradation enzyme) in the hippocampus and dentate gyrus of the gerbil at postnatal month 1 (PM 1), PM 3, PM 6, PM 12, and PM 24. Age-related changes of GABA-T immunoreactivity were distinct in the hippocampal CA1 region and in the dentate gyrus. GABA-T immunoreactivity was weak at PM 1, but at PM 3, it had increased significantly, and then increased further. Between PM 6 and PM 12, strong GABA-T immunoreactivity was found in nonpyramidal cells (GABAergic) in the stratum pyramidale of the CA1 region, and at PM 6, strong GABA-T immunoreactivity was found in neurons of the dentate gyrus subgranular zone. At PM 24, CA1 pyramidal cells showed strong GABA-T immunoreactivity. Western blot analysis showed a pattern of GABA-T expression similar to that shown by immunohistochemistry at various ages. In conclusion, our results suggest that the age-related changes of GABA-T provide important information about the aged brain with GABA dysfunction.

Neuronal development in vitamin B6 deficiency

The morphological changes observed in developing brain regions associated with maternal vitamin B6 deficits are summarized in Table 4. Brain development is a complex and orderly process consisting of cell division, proliferation, migration, and maturation. In the rat, vitamin B6 deficits imposed in utero and up to 30 days postnatal interfere with this orderly process. Deficits of the vitamin imposed in utero have been associated with reduced numbers of total and normal neurons in neocortex and with increased shrunken neurons (700-1500% of controls) in this region. These changes reflect the critical role of vitamin B6 in both neurogenesis and neuron longevity in neocortex. Postnatal cellular events in the neocortex, that is, neuron differentiation and synaptogenesis, were also altered by vitamin B6 deficits; higher order dendrites were reduced on stellate neurons in Layer II and on pyramidal neurons in Layer V. Synaptic density was less in the neutrophil of neocortex and in caudate/putamen, but structural integrity of the synapse was maintained. In cerebellum, both the molecular and granular areas were reduced, the monolayer organization of Purkinje cells was disrupted, and dendritic arborization of the cells was decreased. The number of myelinated axons, as determined by electron microscopy, was decreased in the mediodorsal portion of the pyramidal tract in the medulla oblongata as well as the specific activity of myelination of the total brain. Thus the functional consequences of vitamin B6 deficits during neuronal development may be through reduced connections among neurons and decreased myelination, which alter the rate and magnitude of transmission of nerve impulses.

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.

Accumulation of pantothenic acid by the isolated choroid plexus and brain slices in vitro

In vitro, the transport of [14C]pantothenic acid into and from the isolated rabbit choroid plexus, an anatomical locus of the blood-CSF barrier, and brain slices was studied. The choroid plexus accumulated [14C]pantothenic acid from the medium against a concentration gradient, although at low concentrations (less than 1 microM) there was substantial intracellular phosphorylation and binding of the [14C]pantothenic acid. The saturable accumulation process in choroid plexus was inhibited by probenecid and caproic acid but not by nicotinic acid or by weak bases. The accumulation process was markedly inhibited by N-ethylmaleimide, poly-L-lysine (which blocks sodium transport), and low temperatures. [14C]Pantothenic acid was readily released from choroid plexus by a temperature-dependent process. Brain slices also accumulated and, at low concentrations, phosphorylated [14C]pantothenic acid from the medium by a temperature-, probenecid-, and N-ethylmaleimide-sensitive saturable process. However, unlike choroid plexus, brain slices did not concentrate free pantothenic acid and [14C]pantothenic acid accumulation was not sensitive to poly-L-lysine. [14C]Pantothenic acid was readily released from brain slices by a temperature-sensitive process. These results are consistent with the view that [14C]pantothenic acid enters the isolated choroid plexus and brain slices by active transport and facilitated diffusion, respectively.

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.

A fluorimetric method for the measurement of pyridoxal and pyridoxal phosphate in human plasma and leucocytes, and its application to patients with sideroblastic marrows

A highly sensitive fluorimetric assay for the measurement of pyridoxal and pyridoxal phosphate in biological tissues is described. The method involves the enzymic hydrolysis of pyridoxal phosphate to pyridoxal. The pyridoxal (free or total) is separated on an anion-exchange column, concentrated by cation-exchange chromatography and reacted with potassium cyanide under slightly alkaline conditions to form 4-pyridoxolactone, a highly fluorescent compound. The method is applied to the measurement of pyridoxal, pyridoxal phosphate and total pyridoxal in plasma and neutrophils from control subjects and patients with sideroblastic marrow and identified the patient with pyridoxine-responsive sideroblastic anaemia.

Levels and subcellular localisation of pyridoxal and pyridoxal phosphate in human polymorphonuclear leukocytes and their relationship to alkaline phosphatase activity

Neutrophil leukocytes, isolated from normal subjects and subjected to analytical subcellular fractionation by sucrose density gradient centrifugation, showed very similar cytosol distributions of pyridoxal and pyridoxal phosphate and lactate dehydrogenase. The small amounts of pyridoxal and pyridoxal phosphate associated with the dense granule fractions were not associated with the alkaline phosphatase containing granules. The levels of pyridoxal and pyridoxal phosphate were determined in neutrophils from control subjects, women in the third trimester of pregnancy and patients with chronic granulocytic leukaemia. Neutrophil pyridoxal phosphate was increased in women in the third trimester of pregnancy compared to controls, but there was little variation in the level of pyridoxal between the groups. There was no consistent correlation between the pyridoxal phosphate and the neutrophil alkaline phosphatase activity in the patient groups. Although in vitro neutrophil alkaline phosphatase rapidly hydrolyses pyridoxal phosphate, it is suggested that in vivo this is unlikely to be the principal function of the enzyme.

Influence of adrenalectomy on vitamin B6 status

Adrenalectomy results in depletion of vitamin B6 stores. This depletion is rapid and significant in liver, as compared to brain. When the adrenalectomized rats are fed with excess vitamin B6 or injected with corticosterone, the normal vitamin levels are maintained. Thus, it is possible that adrenocortical hormones help in the retention of vitamin B6 in the body.

Drug-pyridoxal phosphate interactions

In this review it has been pointed out that vitamin B6 and its vitamers can be involved in many interactions with a number of drugs, as well as with the actions of various endocrines and neurotransmitters. Nutritional deficiencies, especially of vitamins and proteins, can affect the manner in which drugs undergo biotransformation, and thereby may also modify the therapeutic efficacy of certain drugs. The differences between nutritional vitamin B6 deficiency and the hereditary disorder producing pyridoxine dependency are discussed. In addition to a pyridoxine deficiency being able to adversely affect drug actions, the improper supplementation with vitamin B6 can in some instances also adversely affect drug efficacy. A decrease by pyridoxine in the efficacy of levodopa used in the treatment of Parkinsonism is an example. The interrelationships and enzymatic interconversions among pyridoxine vitamers, both phosphorylated and non-phosphorylated, are briefly discussed, particularly regarding their pharmacokinetic properties. The ways in which the normal biochemical functions of vitamin B6 may be interfered with by various drugs are reviewed. (1) The chronic administration of isoniazid for the prevention or treatment of tuberculosis can produce peripheral neuropathy which can be prevented by the concurrent administration of pyridoxine. An acute toxic overdose of isoniazid causes generalized convulsions, and the intravenous administration of pyridoxine hydrochloride will prevent or stop these seizures. (2) The acute ingestion of excessive monosodium glutamate will, in some individuals, cause a group of symptoms including among others headache, weakness, stiffness, and heartburn, collectively known as the 'Chinese Restaurant Syndrome.' These symptoms can be prevented by prior supplementation with vitamin B6. The beneficial effect is ascribed to the correction of a deficiency in the activity of glutamic oxaloacetic transaminase, an enzyme that is dependent on pyridoxal phosphate. Some interesting relationships are pointed out between vitamin B6, picolinic acid, and zinc. It is postulated that the intestinal absorption of zinc is facilitated by picolinic acid, a metabolite of tryptophan. The derivation of picolinic acid from tryptophan depends on the action of the enzyme kynureninase, which is dependent on pyridoxal phosphate; therefore, the adequate absorption of zinc is indirectly dependent on an adequate supply of vitamin B6. The formation of pyridoxal phosphate, on the other hand, appears to be indirectly dependent on Zn2++ which activates pyridoxal kinase.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural lesions of pyridoxine toxicity in beagle dogs

Three adult Beagle dogs given pyridoxine hydrochloride orally at a dose of 150 mg/kg body weight/day for about 100 days developed ataxia and had spastic, dysmetric leg movements. Ultrastructural alterations in the dorsal funiculus of the spinal cord were degeneration and loss of axons and myelin, and secondary changes of the myelin sheaths. Possible pathogenic mechanisms of pyridoxine neurotoxicity are discussed.

Subcellular localization and properties of pyridoxal phosphate phosphatases of human polymorphonuclear leukocytes and their relationship to acid and alkaline phosphatase

Using a novel fluorimetric assay for pyridoxal phosphate phosphatase, human polymorphonuclear leucocytes were found to exhibit both acid an alkaline activities. The neutrophils were homogenised in isotonic sucrose and subjected to analytical subcellular fractionation by sucrose density gradient centrigfugation. The alkaline pyridoxal phosphate phosphatase showed a very similar distribution to alkaline phosphatase an was located solely to the phosphasome granules. Fractionation experiments on neutrophils treated with isotonic sucrose containing digitonin and inhibitor studies with diazotised sulphanilic acid and levamisole further confirmed that both enzyme activities had similar locations and properties. Acid pyridoxal phosphate phosphatase activity was located primarily to the tertiary granule with a partial azurophil distribution. Fractionation studies on neutrophils homogenised in isotonic sucrose containing digitonin and specific inhibitor studies showed that acid pyridoxal phosphate phosphatase and acid phosphatase were not the result of a single enzyme activity, Neutrophils were isolated from control subjects, patients with chronic granulocytic leukaemia and patients in the third trimester of pregnancy. The specific activities (munits/mg protein) of alkaline pyridoxal phosphate phosphatase an alkaline phosphatase varied widely in the three groups and the alterations occurred in a parallel manner. The specific activities of acid pyridoxal phosphate phosphatase and of acid phosphatase were similar in the three groups. These results, together with the fractionation experiments and inhibition studies strongly suggest that pyridoxal phosphate is a physiological substrate for neutrophil alkaline phosphatase.

A highly sensitive fluorimetric assay for pyridoxal phosphate phosphatase

A highly sensitive fluorimetric assay for pyridoxal phosphate phosphatase is described. The assay involves separation of the substrate and product by ion-exchange chromatography followed by treatment of pyridoxal with potassium cyanide under slightly alkaline conditions to form 4-pyridoxolactone, a highly fluorescent compound. Certain kinetic properties of the enzyme activities in human neutrophils are described.

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.

Transport and metabolism of pyridoxine in rat liver

Evidence, obtained with in situ perfused rat liver, indicated that pyridoxine is taken up from the perfusate by a non-concentrative process, followed by metabolic trapping. These conclusions were reached on the basis of the fact that at low concentrations (0.125 microM), the 3H of [3H]pyridoxine accumulated against a concentration gradient, but high concentrations (333 microM) of pyridoxine or 4-deoxypyridoxine prevented this apparent concentrative uptake. Under no conditions did the tissue water:perfusate concentration ratio of [3H]pyridoxine exceed unity. The perfused liver rapidly converted the labeled pyridoxine to pyridoxine phosphate, pyridoxal phosphate and pyridoxamine phosphate and released a substantial amount of pyridoxal and some pyridoxal phosphate into the perfusate. Since muscle and erythrocytes failed to oxidize pyridoxine phosphate to pyridoxal phosphate, it is suggested that the liver plays a major role in oxidizing dietary pyridoxine and pyridoxamine as their phosphate esters to supply pyridoxal phosphate which then reaches to other organs chiefly as circulating pyridoxal.

Methionine recycling in brain: a role for folates and vitamin B-12

The recycling of methionine via homocysteine was measured in vivo in brain. After constant intravenous infusions (5 h) of both [3H-methyl]methionine and [35S]methionine into rats, the ratios of [3H-methyl]methionine to [35S]methionine in liver, brain, and plasma were determined. Similar experiments were performed in rabbits, except that the [3H-methyl]- and [35S]methionine were injected intraventricularly. If the methyl group of methionine was removed with the formation of homocysteine and then replaced by another (unlabeled) methyl group, the specific activity of the [3H-methyl]methionine would decrease more than that of [35S]methionine; i.e., the ratio of [3H-methyl]- to [35S]methionine in the tissue would decline. The results showed that the ratios of [3H-methyl]- to [35S]methionine in liver and brain were less than the same ratio in plasma in the rats. The comparable ratios in the brain and CSF of rabbits were less than the ratio in the injectate. Since brain contains only one enzyme capable of remethylating homocysteine to methionine, the vitamin B-12-dependent methyltetrahydrofolate-homocysteine methyltransferase (EC 2.1.1.13), our results for methionine recycling via homocysteine in brain strongly support the activity of this enzyme in brain in vivo.

Is idiopathic dementia a regional vitamin deficiency state?

We hypothesize that some cases of idiopathic dementia are due to a gradual undernourishment of the brain with water-soluble vitamins. This occurs because the choroid plexus and possibly other transport loci in the central nervous system become unable to transport water-soluble vitamins from blood into the central nervous system in adequate amounts. If this testable hypothesis is correct, direct injections of vitamins into the ventricular cerebrospinal fluid should ameliorate the development of dementia.