Glutamate export at the choroid plexus in health, thiamin deficiency, and ethanol intoxication: review and hypothesis.

INTRODUCTION: The earliest observed effect in the pathogenesis of experimental Wernicke's encephalopathy and of ethanol intoxication in rats is impairment of the blood cerebrospinal fluid (CSF) barrier at the choroid plexus (CP). For an explanation, these observations direct attention to the role of the CP in maintaining glutamate homeostasis in the CSF. METHODS: Characteristics of the CP epithelium (CPE) are reviewed, focusing on its role in removal of glutamate from the CSF and its potential for impairment by ethanol oxidation or by thiamin-deficient glucose oxidation. RESULTS: The export of glutamate from CSF to blood at the CP is energy dependent, saturable, and stereospecific. However, the incapacity of the CP to convert glutamate to other metabolites makes it vulnerable to glutamate accumulation should alpha-ketoglutarate dehydrogenase activity be decreased. Elsewhere ethanol metabolism and thiamin-deficiency independently decrease the activity of this mitochondrial enzyme. We argue that they have the same effect within the mitochondria-rich CPE, thereby decreasing energy production necessary for export of glutamate from CSF to blood; diverting its energy metabolism to further glutamate production; and impairing its blood CSF barrier function. This impairment appears to be mediated by glutamate and is attenuated by MK801 but whether it involves one of the CPE glutamate receptors is yet uncertain. This impairment exposes the CSF and hence the paraventricular brain extracellular fluid to neuroactive substances from the blood, including further glutamate, explaining the paraventricular location of neuropathology in Wernicke's encephalopathy. Other organs normally protected from blood by a barrier are affected also by ethanol abuse and by thiamin deficiency, namely the eye, peripheral nerves, and the testis. Much less is known regarding the function of these barriers. CONCLUSIONS: Impairment of the CP by ethanol intoxication and by thiamin-deficient carbohydrate metabolism has a common, rational explanation that can guide future research.

Choroid plexus dysfunction: the initial event in the pathogenesis of Wernicke's encephalopathy and ethanol intoxication.

BACKGROUND: In both acute ethanol intoxication and in thiamin deficient glucose metabolism, previous studies have detected blood-brain barrier (BBB) and/or blood-CSF-barrier (BCSFB) impairment but were unable to assess their significance in relation to other changes in the brain. METHODS: Contrast-enhanced, magnetic resonance imaging (MRI) was used to detect and time any impairment of the BBB or BCSFB in rats given an acute ethanol load or in rats made thiamin deficient to the point of mild ataxia and then given an acute glucose load. RESULTS: The BCSFB at the choroid plexus (CP) was impaired within 10 minutes by either (i) a single i.p. dose of glucose in thiamin-deficiency, an effect that was attenuated by prior MK801 and preceded the published onset of exacerbation of motor incoordination and elevation of brain glutamate derivatives; or (ii) a single i.p. dose of ethanol in thiamin-sufficiency, an effect that was proportional to the blood alcohol concentration and preceded the published onset of signs of intoxication. In contrast to the BCSFB, the BBB remained intact throughout the 90 minutes period of these experiments. CONCLUSIONS: In both ethanol intoxication and thiamin-deficient glucose metabolism, BCSFB impairment exposes the CSF and hence the brain extracellular fluid to neuroactive substances from the blood. CP impairment is the earliest detected event in both these animal models; and explains the paraventricular location of WE neuropathology and why WE is associated with, but not dependent on, alcoholism.

Thiamin nutrition and catalysis-induced instability of thiamin diphosphate.

Thiamin (vitamin B1) is required in animal diets because it is the precursor of the enzyme cofactor, thiamin diphosphate. Unlike other B vitamins, the dietary thiamin requirement is proportional to non-fat energy intake but there is no obvious biochemical reason for this relationship. In the present communication we show for two enzymes that the cofactor undergoes a slow destruction during catalysis, which may explain the interdependence of thiamin and energy intakes.

Quantitative description of the interaction between folate and the folate-binding protein from cow's milk.

A detailed study has been carried out on the dependence of folate binding on the concentration of FBP (folate-binding protein) at pH 5.0, conditions selected to prevent complications arising from the pre-existing self-association of the acceptor. In contrast with the mandatory requirement that reversible interaction of ligand with a single acceptor site should exhibit a unique, rectangular hyperbolic binding curve, results obtained by ultrafiltration for the FBP-folate system required description in terms of (i) a sigmoidal relationship between concentrations of bound and free folate and (ii) an inverse dependence of affinity on FBP concentration. These findings have been attributed to the difficulties in determining the free ligand concentration in the FBP-folate mixtures for which reaction is essentially stoichiometric. This explanation also accounts for the similar published behaviour of the FBP-folate system at neutral pH, which had been attributed erroneously to acceptor self-association, a phenomenon incompatible with the experimental findings because of its prediction of a greater affinity for folate with increasing FBP concentration.

Dietary interactions influence the effects of bovine folate-binding protein on the bioavailability of tetrahydrofolates in rats.

The newborns of mammals have a high folate demand, yet obtain adequate folate nutrition solely from their mothers' milk despite its low folate content. Milk folate is entirely bound by an excess of folate-binding protein (FBP), prompting speculation that FBP may affect the bioavailability of the limited folate supply. Previous research has shown that FBP-bound folic acid is more gradually absorbed, thereby reducing the peak plasma folate concentration and preventing loss into the urine. Natural folates are reduced derivatives of folic acid, with milk predominantly containing 5-methyltetrahydrofolate, yet little research has been carried out to determine the role of FBP in the bioavailability of reduced folates. We studied the effect of FBP on folate nutrition of rats in both single-dose and 4-wk feeding experiments. The effect of FBP was influenced by the presence of other milk components. FBP increased bioavailability of dietary folate when it was consumed with other whey proteins or with soluble casein. However, in the presence of acid-precipitated casein or a whey preparation enriched in lipids, bioavailability was decreased. These results highlight the difficulties of extrapolating from experimental results obtained using purified diets alone and of studying interactions among dietary components. They suggest that the addition of FBP-rich foods to folate-rich foods could enhance the bioavailability of natural folates, but that the outcome of such a combination would depend on interactions with other components of the diet.

Tetrahydrofolates are greatly stabilized by binding to bovine milk folate-binding protein.

The dietary supply of folates and their measurement are both affected, potentially, by the instability of some folates. Labile folates appear to be stabilized by binding to folate-binding protein (FBP); this paper reports measurements of that stabilization. The degradation rates of the very labile tetrahydrofolate (H(4)folate) and moderately labile 5-methyltetrahydrofolate (5-CH(3)H(4)folate) were measured with the compounds free or bound to either soluble or immobilized bovine milk FBP. Complexation increased stability from 2- to > 1000-fold, depending on buffer and temperature conditions. H(4)folate at 4 degrees C and pH 6.7 appeared to be quite stable for > 100 d when bound to soluble FBP but had a half-life of < 1 h when free. Stabilization of milk folates may be a role of FBP and would improve the bioavailability of milk folate to newborns and other consumers.