Lead and tetrahydrobiopterin metabolism: possible effects on IQ

Effects of the metals on dihydropteridine reductase activity

Metals are the oldest toxins known to human. Particularly, occupational and environmental exposure to aluminium, lead, mercury, cadmium, and manganese cause serious health problems by interaction with biological systems. Cellular targets of these metals are mostly specific biochemical processes (enzymes) and/or membranes of cells and organelles. To prevent and/or reduce the untoward or irreversible toxic effects of the metals by using biomarkers are as important as to know and to understand of their toxicity mechanisms. Dihydropteridine reductase (DHPR), which possessed essential thiol groups at the activity site, plays a crucial role in the maintenance of tetrahydrobiopterin (BH4). BH4 is the cofactor in the synthesis and regulation of neurotransmitters. A limited number of the evidences have shown that DHPR may be a target for the metals. Therefore, the present study was designed to assess possible in vitro effects of the commonly exposed metals on the enzyme activity. It was found that aluminium, cadmium, mercury, di-phenyl mercury, lead, diethyl lead, in chloride forms, and manganese, in sulphate form, led to statistically significant decreases in DHPR activity, in a concentration-dependent manner, in vitro.

Vitiligo, psoralen, and melanogenesis: some observations and understanding

Since the etiology of vitiligo is still unknown, we searched for some abnormal biochemical parameters, if any, in subjects with vitiligo. Higher urinary excretion of indole metabolites in vitiliginous patients have been noted, in association with higher dioxygenase, superoxide dismutase, and tyrosine aminotransferase activity in their serum. Similar results have also been found in an animal model, Bufo melanostictus, during induced tyrosinase inhibition. Treatment with psoralen can reverse the parameters, except tyrosine aminotransferase, to a normal level. Although psoralens are not the magic bullet for the therapy of vitiligo, they are still being used as a chemotherapeutic agent against vitiligo on a major scale to date. Tryptophan was found to participate in the pathway of melanogenesis, as a precursor as well as a positive regulator of tyrosinase. Its behavior in this regard is much more similar to the conventional substrates tyrosine and dopa (dihydroxyphenylalanine). In consideration of combined participation of tyrosine and tryptophan in the synthesis of melanin and its breakdown, the possible influence of different enzymatic reactions, like mono-oxygenase, dioxygenase, and deamination, has been suggested.

Lead intoxication--new insights into an old problem

The epidemiology, clinical symptomatology, diagnosis and management of acute and chronic lead intoxication are reviewed. While acute lead intoxication has become rare, the elevated environmental lead burden is thought to play a causal role in hypertension. Lead may also be linked with neuropsychological disorders of children and possibly even chronic renal failure. The epidemiological and experimental evidence for this hypothesis is critically discussed.

Action of lead on neurotransmission in rats

1. The neurotoxic effect of lead on the catecholaminergic and cholinergic nervous systems has been investigated using a rat model of lead exposure. 2. This model of lead exposure resulted in significant quantities of lead accumulating in the blood, brain and femur of the lead-exposed animals. 3. The biochemical effect of lead on brain neurochemistry was dependent on the degree and duration of lead exposure. However, the data points to a selective action of lead, with the midbrain and diencephalon being prime targets while very few lead-related alterations were observed in the cerebellum or the telencephalon. 4. Within the catecholaminergic nervous system, lead exposure resulted in alterations in the concentrations of the transmitters, noradrenaline and dopamine, in addition to changes in the activities of the enzymes tyrosine hydroxylase and phenylethanolamine-N-methyl transferase. The activity of the cholinergic biosynthetic enzyme, choline acetyltransferase was also noted to be altered by lead exposure.

Analysis and clinical significance of pterins

This review briefly describes the biochemistry of pterins, their involvement in pathological processes and the use of pterin measurement in diagnosis and monitoring of disease. Chromatographic and other methods of pterin analysis are detailed with particular emphasis being placed on the need for correct sample collection and handling.

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.

Neurotoxic action of lead: effect on tetrahydrobiopterin metabolism in the rat

The effects of lead, a known neurotoxin on the metabolism of a vital tetrahydrobiopterin cofactor for the hydroxylation enzymes tyrosine hydroxylase, phenylalanine hydroxylase and tryptophan hydroxylase, have been investigated. Reduced availability of this pteridine has the potential to reduce the level of the neurotransmitters noradrenaline, adrenaline and 5-hydroxytryptophan in the brain. Using the rat as a model, increases in tetrahydrobiopterin concentration and in the activity of dihydropteridine reductase, an enzyme involved in tetrahydrobiopterin metabolism, were observed after exposure to lead via the drinking water. Possible explanations for this increased level of tetrahydrobiopterin relate to alterations in the balance between synthesis and salvage of this co-factor.