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.