The effect of lead on tetrahydrobiopterin metabolism. A possible mechanism for neurotoxicity.

The use of low levels of lead in vivo in rats has been found to inhibit dihydropteridine reductase and cause an apparent increase in tetrahydrobiopterin biosynthesis. At higher dose levels inhibition of tetrahydrobiopterin biosynthesis has been observed. At low levels the disruption of tetrahydrobiopterin metabolism has been found to give an increase in the total level of biopterin derivatives but a movement away from the fully reduced form to the oxidised species in a manner consistent with dihydropteridine reductase inhibition. Lead has been found to inhibit dihydropteridine reductase in man.

The effect of tetrahydrofolate on tetrahydrobiopterin metabolism.

5-Methyltetrahydrofolate and vitamin B12 appear to be required for the biosynthesis of tetrahydrobiopterin. A deficiency of either could be sufficient to bring about neurological change which can be corrected by reversing the deficiency. Patients with senile dementia could possibly be benefited by the administration of 5-methyltetrahydrofolate.

Tetrahydrobiopterin metabolism in the temporal lobe of patients dying with senile dementia of Alzheimer type.

There is a defect in tetrahydrobiopterin metabolism in brains from subjects with senile dementia of Alzheimer type compared to age-matched controls. This defect results in lowered total biopterin concentrations in brain. Brains from subjects with senile dementia of Alzheimer type retain their ability to synthesis neopterin and have normal dihydropteridine reductase activity, indicating a specific loss of ability to convert dihydroneopterin triphosphate to tetrahydrobiopterin.

Blood spots on Guthrie cards can be used for inherited tetrahydrobiopterin deficiency screening in hyperphenylalaninaemic infants.

We describe a method of screening for dihydropteridine reductase deficiency and dihydrobiopterin synthesis deficiency--the two inherited defects that cause tetrahydrobiopterin deficiency--using blood spots on Guthrie cards. Dihydropteridine reductase deficiency may be identified positively, and a biopterin value of less than 6.0 micrograms/l in the presence of hyperphenylalaninaemia indicates further investigation for dihydrobiopterin synthesis deficiency.

Inhibition of dihydropteridine reductase by dopamine.

Dihydropteridine reductase has been purified 900-fold from rat liver. Dopamine inhibited the enzyme up to 50% at a concentration of 0.11mm. In the presence of dopamine the enzyme gave non-hyperbolic v-against-[S] plots. This enzyme may have a role in control of dopamine biosynthesis.

The effect of methotrexate on folate metabolism in the rat.

The metabolism of 2-[14C]-folic acid, 2-[14C]-5-methyltetrahydrofolate 5-[14C]-methyltetrahydrofolate, and a mixture of 2-[14C]-folic acid and 3',5',7,9-[3H]-folic acid has been studied in rats that were dosed with methotrexate (MTX) 24 h before receiving the radioactive folate. Methotrexate increases urinary excretion of radioactivity in rats given 2-[14C]-folic acid, but there was no significant increase in urinary radioactivity in animals given 5-methyltetrahydrofolate. Animals dosed with MTX had less of the dose in the liver, and excreted more of the dose via the faeces. These results are consistent with the known biochemical effects of methotrexate. Experiments with a mixture of 2-[14C]-folic acid and 3',5',7,9-[3H]-folic acid indicate that there is an increase in scission of the folate molecule following a dose of MTX.

The metabolic fate of (2-14C)folic acid and a mixture of (2-14C)- and (3',5',9-3h)-folic acid in the rat.

The metabolism of [2-14C]folic acid over 13 days and a mixture of [2-14C]- and [3',5',9-3h]-folic acid in rats over a 6-day period is described. Both 14C and 3H are excreted in urine over the 6-day period, but 3H and 14C are only detectable in faeces for 2 days. A breakdown product of folic acid labelled with 3H only was found in some urine samples, but no metabolite corresponding to the part of the molecule containing 14C was detected. These experiments show that in the whole animal a substantial portion of orally administered folic acid undergoes scission shortly after administration [Blair Biochem. J. (1957) 68, 385-387] and that the retained folates are a shortage form for folate monoglutamates.

Effect of an implanted Walker tumour on metabolism of folic acid in the rat.

The metabolism of 2-[14C] folic acid has been studied in rats with an implanted Walker 256 tumour and in a closely matched group of controls. In animals with tumours, more of the labelled folic acid is converted to 10-formyltetrahydrofolate and 10-formylfolate than in normal animals. No 5-methyltetrahydrofolate could be detected in tumour tissue, or in the livers of tumour-bearing animals. When a mixture of 2-[14C]- and 3',5',9-[3H]-folic acid is given to tumour-bearing rats a similar pattern of metabolites is found. There is apparenly less scission of the folate molecule in tumour-bearing rats than in normal rats.

Biliary excretion of some anionic derivatives of diethylstilboestrol and phenolphthalein in the guinea pig.

The metabolic fates and modes of excretion of diethylstilboestrol mono[35S]sulphate and diethylstilboestrol di[35S]sulphate were studied in the guinea pig. Comparative studies were also made with [G-3H]diethylstilboestrol and phenolphthalein di[35S]sulphate. Diethylstiboesterol di[35S]sulphate was extensively eliminated in the bile unchanged. After administration of diethylstilboestrol mono[35S]sulphate, extensive biliary elimination of radioactivity was also recorded. Radioactive components were identified as diethylstilboestrol disulphate, diethylstilboestrol monosulphate monoglucuronide and unchanged diethylstilboestrol monosulphate. When [G-3H]diethylstilboestrol was administered, 3H-labelled diethylstilboestrol monoglucuronide, diethylstilboestrol monosulphate monoglucuronide and diethylstilboestrol disulphate appeared in the bile. Phenolphthalein di[35S]sulphate was excreted unchanged in bile. These findings are discussed in relation to studies carried out in the rat [Barford, Olavesen, Curtis & Powell (1977) Biochem. J. 164, 423--430] and species differences are related to differences in enzyme activities in rat and guinea-pig liver.

Retained folates in the rat.

The retention of radioactivity after doses of 14C- and 3H-labelled folic acid is described. Radioactivity was retained in liver, kidney and gut of rats for some time after administration of the dose. The retained radioactivity could not be displaced by large doses of unlabelled folic acid or unlabelled 5-methyltetrahydrofolate. 14C- and 3H-labbelled folates showed similar chromatographic behaviour onion-exchange chromatography to 5-methyltetrahydrofolate, and on ion-exchange and gel-permeation chromatography to synthetic pteroylhepta-gamma-glutamate.

Metabolic fates of diethylstilboestrol sulphates in the rat.

The metabolic fates and modes of excretion of diethylstilboestrol mono[35S]sulphate and diethylstilboestrol di[35S]sulphate were studied in the rat. Both of the esters were desulphated to some extent in vivo. In addition, significant amounts of radioactivity appeared in the bile as diethylstilboestrol mono[35S]sulphate monoglucuronide. The percentage of the dose appearing in bile as the diconjugate was substantially greater in experiments with diethylstilboestrol mono[35S]sulphate than with diethylstilboestrol di[35S]sulphate. Whole-body radioautography and studies with isolated perfused liver confirmed the liver as the major metabolic organ for both esters. When the metabolite diethylstilboestrol mono[35S]sulphate monoglucuronide isolated from the bile was reinjected, it was excreted in the bile unchanged. Studies in vitro demonstrated that both esters were substrates for arylsulphatase C with Km values in the range 52-76 micrometer. The metabolic fates and modes of excretion of the esters are discussed in relation to the enzyme complement of rat liver.