Determination of thiamin and its phosphate esters in cultured neurons and astrocytes using an ion-pair reversed-phase high-performance liquid chromatographic method

Brain levels of thiamine and its phosphate esters in Friedreich's ataxia and spinocerebellar ataxia type 1

Decreased blood and cerebrospinal fluid levels of thiamine have been reported in patients with spinocerebellar ataxia disorders. To determine whether a thiamine deficiency is present in the brain, we measured levels of thiamine and its phosphate esters thiamine monophosphate (TMP) and thiamine diphosphate (TDP), in postmortem cerebellar and cerebral cortices of patients with Friedreich's ataxia (FA) and spinocerebellar ataxia type 1 (SCA1). Brain levels of free (nonphosphorylated) thiamine, TMP, TDP, and total thiamine in FA and SCA1 were, on average, not significantly different from control values. However, a nonsignificant trend was observed for slightly reduced levels of TDP and total thiamine in cerebellar cortex of the SCA1 patients, a finding that might be related to the severe neuronal damage in this brain area. We conclude that in FA, brain thiamine concentrations are normal, whereas in SCA1 the levels are, at most, only slightly reduced.

Brain thiamine, its phosphate esters, and its metabolizing enzymes in Alzheimer's disease

Clinical data suggest that high-dose thiamine (vitamin B1) may have a mild beneficial effect in some patients with Alzheimer's disease (AD). Since this action could be related to a brain thiamine deficiency, we measured directly levels of free (nonphosphorylated) thiamine and its phosphate esters, thiamine monophosphate and thiamine diphosphate (TDP), and activities of three TDP-metabolizing enzymes (thiamine pyrophosphokinase, thiamine diphosphatase, and thiamine triphosphatase) in autopsied cerebral cortex of 18 patients with AD and 20 matched controls. In the AD group, mean levels of free thiamine and its monophosphate ester were normal, whereas levels of TDP were significantly reduced by 18 to 21% in all three cortical brain areas examined. Activities of the TDP-metabolizing enzymes were normal in the AD group, suggesting that decreased TDP is not due to altered levels of these enzymes. The TDP decrease could be explained by a cerebral cortical deficiency in AD of ATP, which is needed for TDP synthesis. Although the magnitude of the TDP reduction is slight, a chronic subclinical TDP deficiency could contribute to impaired brain function in AD and might provide the basis for the modest improvement by thiamine in cognitive status of some patients with AD.

The effects of thiamine and oxythiamine on the survival of cultured brain neurons

The effects of treatment with thiamine (Vitamin B1) alone or together with its antagonist oxythiamine on the survival of brain neurons in primary culture were investigated. Treatment with thiamine significantly promoted the survival of hippocampal neurons in high cell density culture, but had no effects on the neuronal survival in low cell density culture. In addition, the survival-promoting activity exerted by thiamine was remarkably decreased by the co-application of oxythiamine, although oxythiamine used alone revealed neither a trophic nor toxic effect on the neurons of examined brain regions. The neurotrophic function of thiamine may be due to its coenzymatic role in a biochemical reaction and/or its specific function on neurotransmission and nerve conduction.

Thiamine homeostasis in neuroblastoma cells

We recently showed that thiamine uptake by neuroblastoma cells is mediated by two saturable transport system: the first with high affinity for thiamine (Km = 35 nM) is blocked by veratridine; the other, with low affinity is blocked by Ca2+. The driving force for thiamine uptake is its phosphorylation to thiamine diphosphate (TDP) by thiamine pyrophosphokinase and subsequent binding of this cofactor to apoenzymes. Our results suggest that cells of neuronal origin possess mechanisms regulating the intracellular concentration of thiamine. At low external thiamine, the vitamin is taken up by a high-affinity transporter and pyrophosphorylated in thiamine diphosphate (TDP): this is the TDP pool of slow turnover. An intraover extracellular concentration gradient of free thiamine is observed at low external concentration of the vitamin. At higher external thiamine concentration, TDP accumulation is limited by the binding capacity to the apoenzymes and unbound TDP (i.e. a small pool of fast turnover) is quickly hydrolyzed. Thiamine is slowly released by the cells by at least two different mechanisms. The first, accounting for a maximum of 50% of total thiamine release, is stimulated by external thiamine and is blocked by veratridine, suggesting that it is a self-exchange mechanism catalyzed by the high affinity thiamine transporter. The remaining thiamine efflux is neither sensitive to veratridine nor to Ca2+ and its mechanism is unknown. About 25% of intracellular thiamine is not released, even after treatment of the cells with digitonin, thus maintaining an apparent gradient. This suggests a binding or sequestration in intracellular compartments.(ABSTRACT TRUNCATED AT 250 WORDS)

Chloride permeability of rat brain membrane vesicles correlates with thiamine triphosphate content

Incubation of rat brain homogenates with thiamine or thiamine diphosphate (TDP) leads to a synthesis of thiamine triphosphate (TTP). In membrane vesicles subsequently prepared from the homogenates, increased TTP content correlates with increased 36Cl- uptake. A hyperbolic relationship was obtained with a K0.5 of 0.27 nmol TTP/mg protein. In crude mitochondrial fractions from the brains of animals previously treated with thiamine or sulbutiamine, a positive correlation between 36Cl- uptake and TTP content was found. These results, together with other results previously obtained with the patch-clamp technique, suggest that TTP is an activator of chloride channels having a large unit conductance.

Subcellular localization and compartmentation of thiamine derivatives in rat brain

The subcellular distribution of thiamine derivatives in rat brain was studied. Thiamine diphosphate content was highest in the mitochondrial and synaptosomal fractions, and lowest in microsomal, myelin and cytosolic fractions. Only 3-5% of total thiamine diphosphate was bound to transketolase, a cytosolic enzyme. Thiamine triphosphate was barely detectable in the microsomal and cytosolic fraction, but synaptosomes were slightly enriched in this compound compared to the crude homogenate. Both myelin and mitochondrial fractions contained significant amounts of thiamine triphosphate. In order to estimate the relative turnover rates of these compounds, the animals received an intraperitoneal injection of either [14C]thiamine or [14C]sulbutiamine (isobutyrylthiamine disulfide) 1 h before decapitation. The specific radioactivities of thiamine compounds found in the brain decreased in the order: thiamine > thiamine triphosphate > thiamine monophosphate > thiamine diphosphate. Incorporation of radioactivity into thiamine triphosphate was more marked with [14C]sulbutiamine than with [14C]thiamine. The highest specific radioactivity of thiamine diphosphate was found in the cytosolic fraction of the brain, though this pool represents less than 10% of total thiamine diphosphate. Cytosolic thiamine diphosphate had a twice higher specific radioactivity when [14C]sulbutiamine was used as precursor compared with thiamine though no significant differences were found in the other cellular compartments. Our results suggest the existence of two thiamine diphosphate pools: the bound cofactor pool is essentially mitochondrial and has a low turnover; a much smaller cytosolic pool (6-7% of total TDP) of high turnover is the likely precursor of thiamine triphosphate.

The compartmentation of phosphorylated thiamine derivatives in cultured neuroblastoma cells

Thiamine transport in cultured neuroblastoma cells is mediated by a high-affinity carrier (KM = 40 nM). In contrast, the uptake of the more hydrophobic sulbutiamine (isobutyrylthiamine disulfide) is unsaturable and its initial transport rate is 20-times faster than for thiamine. In the cytoplasm, sulbutiamine is rapidly hydrolyzed and reduced to free thiamine, the overall process resulting in a rapid and concentrative thiamine accumulation. Incorporation of radioactivity from [14C]thiamine or [14C]sulbutiamine into intracellular thiamine diphosphate is slow in both cases. Despite the fact that the diphosphate is probably the direct precursor for both thiamine monophosphate and triphosphate, the specific radioactivity increased much faster for the latter two compounds than for thiamine diphosphate. This suggests the existence of two pools of thiamine diphosphate, the larger one having a very slow turnover (about 17 h); a much smaller, rapidly turning over pool would be the precursor of thiamine mono- and triphosphate. The turnover time for thiamine triphosphate could be estimated to be 1-2 h. When preloading the cells with [14C]sulbutiamine was followed by a chase with the same concentration of the unlabeled compound, the specific radioactivities of thiamine and thiamine monophosphate decreased exponentially as expected, but labeling of the diphosphate continued to increase slowly. Specific radioactivity of thiamine triphosphate increased first, but after 30 min it began to slowly decrease. These results show for the first time the existence of distinct thiamine diphosphate pools in the same homogeneous cell population. They also suggest a complex compartmentation of thiamine metabolism.

Thiamine triphosphate activates an anion channel of large unit conductance in neuroblastoma cells

In neuroblastoma cells, the intracellular thiamine triphosphate (TTP) concentration was found to be about 0.5 microM, which is several times above the amount of cultured neurons or glial cells. In inside-out patches, addition of TTP (1 or 10 microM) to the bath activated an anion channel of large unit conductance (350-400 pS) in symmetrical 150 mM NaCl solution. The activation occurred after a delay of about 4 min and was not reversed when TTP was washed out. A possible explanation is that the channel has been irreversibly phosphorylated by TTP. The channel open probability (Po) shows a bell-shaped behavior as a function of pipette potential (Vp). Po is maximal for -25 mV < Vp < 10 mV and steeply decreases outside this potential range. From reversal potentials, permeability ratios of PCl/PNa = 20 and PCl/Pgluconate = 3 were estimated. ATP (5 mM) at the cytoplasmic side of the channel decreased the mean single channel conductance by about 50%, but thiamine derivatives did not affect unit conductance; 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (0.1 mM) increased the flickering of the channel between the open and closed state, finally leading to its closure. Addition of oxythiamine (1 mM), a thiamine antimetabolite, to the pipette filling solution potentiates the time-dependent inactivation of the channel at Vp = -20 mV but had the opposite effect at +30 mV. This finding corresponds to a shift of Po towards more negative resting membrane potentials. These observations agree with our previous results showing a modulation of chloride permeability by thiamine derivatives in membrane vesicles from rat brain.

Metabolism of thiamine triphosphate in rat brain: correlation with chloride permeability

Our results show that a net synthesis of thiamine triphosphate (TTP) can be demonstrated in vitro using rat brain extracts. The total homogenate was preincubated with thiamine or its diphosphate derivative (TDP), centrifuged, and washed twice. With TDP (1 mM) as substrate, a 10-fold increase in TTP content was observed in this fraction (nuclear fraction, membrane vesicles). A smaller, but significant, increase was observed in the P2 fraction (mitochondrial/synaptosomal fraction). In view of the low TTP content of our fractions, it was carefully assessed that authentic TTP was being formed. Incorporation of radioactivity from [beta-32P]TDP and [gamma-32P]ATP in TTP suggests that these two compounds are its precursors. Furthermore, TTP synthesis was inhibited by ADP and relatively low concentrations of Zn2+. These results suggest that TTP synthesis is catalyzed by an ATP:TDP transphosphorylase rather than by the cytoplasmic adenylate kinase that may be present in the vesicles. After osmotic lysis of the vesicles at alkaline pH, TTP was recovered in protein-bound form. Concomitantly, a soluble thiamine triphosphatase, with alkaline pH optimum, was also released from the vesicles. No net synthesis could be obtained in the cytosolic fraction or in detergent-solubilized systems. Like TTP synthesis, chloride permeability of the vesicles was increased when the homogenate had been incubated with thiamine and particularly with TDP. Our results suggest a regulatory role of TTP on chloride permeability, but the target remains to be characterized.

Thiamin and derivatives as modulators of rat brain chloride channels

Several membrane fractions were prepared from rat brain by differential and sucrose density gradient centrifugation. Most fractions took up 36Cl- rapidly at a rate linear with time during the first 30-60 s, then the rate progressively slowed down. The lowest rate of uptake was found in the mitochondrial fraction. Oxythiamin partially inhibited 36Cl- uptake in all fractions. In P2 (crude synaptosomal fraction), oxythiamin decreased the initial rate of uptake by 32%, the apparent Ki being 1.5 mM. Thiamin and amprolium were less effective as inhibitors. 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid (0.1-1 mM) inhibited 36Cl- uptake by 40-50%. In the presence of this compound at a concentration > or = 5 x 10(-4) M, oxythiamin became ineffective. 36Cl- uptake was increased by GABA (0.1 mM) and this effect was antagonized by picrotoxin as expected, but not by oxythiamin. The rate of 36Cl- uptake did not appreciably depend on the external chloride concentration and was unaffected by bumetanide or by replacement of external Na+ by choline. Taken together, these data suggest that the oxythiamin-sensitive 36Cl- influx is essentially diffusional and is not related to the GABA receptor or the Na:K:2Cl co-transport. Partial replacement of external Na+ by K+ or treatment with 0.1 mM veratridine (which should both result in membrane depolarization) increased 36Cl- uptake by 50 and 30% respectively; the inhibitory effect of oxythiamin was enhanced to the same proportion.(ABSTRACT TRUNCATED AT 250 WORDS)