Tetrahydrobiopterin metabolism, neurotransmitters and behaviour in the rat

Various neurotoxins were investigated to assess their suitability for developing an animal model to study partial brain BH4 deficiency, neurotransmitters and behavioural alterations. Acute dosing with lead, diethylstilboestrol (DES), amphetamine and scopolamine produced no significant changes in rat brain BH4 metabolism though total biopterins in the liver were significantly reduced by lead and DES. Acute starvation of adult rats decreased brain biopterins. This loss of biopterins may be due to enhanced oxidative catabolism of the active cofactor caused by glutathione depletion. Dietary administration of a BH4 biosynthesis inhibitor, DAHP, consistently decreased brain total biopterins in weaner rats but did not alter the levels of DA, NA, 5-HT or metabolites. However the DAHP diet also induced a marked reduction in food intake. Rats subjected to an equivalent degree of food restriction without inhibitor showed significant but less severe reductions in brain biopterins and again no effect on transmitter levels. DAHP produced a significant decrease in locomotor activity and rearing. This could not be ascribed to reduction in food intake as animals subjected to just dietary restriction showed an increase in these activities. As gross brain levels of DA, NA and 5-HT were unaltered by DAHP the behavioural changes associated with the induced deficiency in brain total biopterins might not have been mediated through the action of these compounds. Although localised changes in neurotransmitter levels may have been obscured by gross analysis it is also possible that the behaviour changes were mediated by a role of BH4 not yet elucidated. Long-term administration of a high aluminium low calcium diet to mice produced no effect on gross brain total biopterins, catecholamines, serotonin or choline acetyltransferase activity though significant behavioural changes were observed.

Oxidation effects on tetrahydropterin metabolism

The susceptibility of tetrahydropterins to oxidation was investigated in vitro and related to in vivo metabolism. At physiological pH, tetrahydrobiopterin (BH4) was oxidized, with considerable loss of the biopterin skeleton, by molecular oxygen. The hydroxyl radical (.OH) was found to increase this oxidation and degradation, whilst physiological concentrations of glutathione (GSH) retarded both the dioxygen and .OH mediated oxidation. Nitrite, at acid pH, oxidized BH4 to biopterin and tetrahydrofolates to products devoid of folate structure. Loss of dietary folates, from the stomach, due to nitrite mediated catabolism is suggested. The in vivo response of BH4 metabolism to oxidising conditions was examined in the rat brain and liver. Acute starvation depressed brain biopterins and transiently BH4 biosynthetic and salvage (dihydropteridine reductase, DHPR) pathways. Loss of biopterins, in starvation, is suggested to arise primarily from catabolism, due to oxygen radical formation and GSH depletion. L-cysteine administration to starving rats was found to elevate tissue biopterins, whilst depletion of GSH in feeding rats, by L-buthionine sulfoximine, decreased biopterins. An in vivo role for GSH to protect tetrahydropterins from oxidation is suggested. The in vivo effect of phenelzine dosing was investigated. Administration lowered brain biopterins, in the presence of dietary tyrosine. This loss is considered to arise from p-tyramine generation and subsequent DHPR inhibition. Observed elevations in plasma biopterins were in line with this mechanism. In conditions other than gross inhibition of DHPR or BH4 biosynthesis, plasma total biopterins were seen to be poor indicators of tissue BH4 metabolism. Evidence is presented indicating that the pterin formed in tissue samples by acid iodine oxidation originates from the tetrahydrofolate pool and 7,8-dihydropterin derived from BH4 oxidation. The observed reduction in this pterin by prior in vivo nitrous oxide exposure and elevation by starvation and phenelzine administration is discussed in this light. The biochemical importance of the changes in tetrahydropterin metabolism observed in this thesis are discussed with extrapolation to the situation in man, where appropriate. An additional role for BH4 as a tissue antioxidant and reductant is also considered.

Tetrahydrobiopterin metabolism in dementia

The aim of this study was to establish levels of the enzymes involved in tetrahydrobiopterin (BH4) metabolism in human and rat brain preparations; to determine whether BH4 metabolism is altered in dementia, particularly in relation to senile dementia of the Alzheimer type (SDAT); and to examine the effect of aluminium on BH4 metabolism. Overall BH4 synthesis and dihydropteridine reductase (DHPR) activity were greater in the locus coeruleus than in the neocortex of elderly subjects. Sepiapterin reductase and DHPR activity showed a linear correlation with age in the temporal cortex. DHPR activity in the frontal cortex was relatively constant until the mid 60s and then fell with age. Overall BH4 synthesis showed a non-significant decline in temporal cortex and was significantly reduced in locus coeruleus preparations from SDAT subjects compared to control subjects. As DHPR, sepiapterin reductase and GTP cyclohydrolase activity were unaltered in SDAT we suggested that there is a lesion on the biosynthetic pathway between dihydroneopterin in triphosphate and BH4 in SDAT, possibly at the level of 6-pyruvoyl tetrahydropterin synthase. DHPR activity and BH4 synthesis capacity were unaltered in temporal cortex preparations from Huntingdon's disease subjects indicating that the defect in BH4 metabolism in SDAT is specific to the disease process and not a secondary consequence of dementia. The implications of altered BH4 metabolism in ageing and dementia are discussed. BH4 metabolism was examined in temporal and frontal cortex preparations from 4 subjects who had received peritoneal dialysis treatment. All patients had elevated serum aluminium levels. The data suggests that aluminium may inhibit DHPR activity in the frontal cortex resulting in diminished BH4 levels in the cells which leads to a compensatory increase in the activity of the biosynthetic pathway. Aluminium reversibly inhibited sepiapterin reductase activity in rat brain preparations but did not alter sepiapterin reductase activity in vivo. Overall BH4 synthesis and OTP cyclohydrolase activity were not affected by aluminium in vitro. The biosynthetic pathway was unaltered in rat brain preparations from animals receiving aluminium orally compared to control animals. DHPR activity was unaltered or increased in rat brain preparations from aluminium treated rats compared to the control group.

Structural Characterization of Metal Hydrides for Energy Applications

Hydrogen can be an unlimited source of clean energy for future because of its very high energy density compared to the conventional fuels like gasoline. An efficient and safer way of storing hydrogen is in metals and alloys as hydrides. Light metal hydrides, alanates and borohydrides have very good hydrogen storage capacity, but high operation temperatures hinder their application. Improvement of thermodynamic properties of these hydrides is important for their commercial use as a source of energy. Application of pressure on materials can have influence on their properties favoring hydrogen storage. Hydrogen desorption in many complex hydrides occurs above the transition temperature. Therefore, it is important to study the physical properties of the hydride compounds at ambient and high pressure and/or high temperature conditions, which can assist in the design of suitable storage materials with desired thermodynamic properties. ^ The high pressure-temperature phase diagram, thermal expansion and compressibility have only been evaluated for a limited number of hydrides so far. This situation serves as a main motivation for studying such properties of a number of technologically important hydrides. Focus of this dissertation was on X-ray diffraction and Raman spectroscopy studies of Mg2FeH6, Ca(BH4) 2, Mg(BH4)2, NaBH4, NaAlH4, LiAlH4, LiNH2BH3 and mixture of MgH 2 with AlH3 or Si, at different conditions of pressure and temperature, to obtain their bulk modulus and thermal expansion coefficient. These data are potential source of information regarding inter-atomic forces and also serve as a basis for developing theoretical models. Some high pressure phases were identified for the complex hydrides in this study which may have better hydrogen storage properties than the ambient phase. The results showed that the highly compressible B-H or Al-H bonds and the associated bond disordering under pressure is responsible for phase transitions observed in brorohydrides or alanates. Complex hydrides exhibited very high compressibility suggesting possibility to destabilize them with pressure. With high capacity and favorable thermodynamics, complex hydrides are suitable for reversible storage. Further studies are required to overcome the kinetic barriers in complex hydrides by catalytic addition. A comparative study of the hydride properties with that of the constituting metal, and their inter relationships were carried out with many interesting features.^