John A. Blair

Serum Aluminum Levels and Erythrocyte Dihydropteridine Reductase Activity in Patients on Hemodialysis

Aluminum intoxication due to aluminum-containing antacids or dialysate can cause encephalopathy in patients undergoing hemodialysis, but the biochemical mechanism has not been defined. The enzyme dihydropteridine reductase (DHPR) is essential for the maintenance of normal brain concentrations of tetrahydrobiopterin, which is itself required for the synthesis of specific neurotransmitters. This enzyme is also present in erythrocytes. We measured erythrocyte DHPR activity and concentrations of the biopterin derivatives of its substrate and of aluminum in 38 patients on hemodialysis who had no clinical evidence of encephalopathy. Serum aluminum levels ranged from 15 to 190 micrograms per liter (mean, 67.6 +/- 7.7) as compared with 4.9 +/- 0.99 micrograms per liter in normal subjects. DHPR activity was inversely related to the serum aluminum concentration (r = -0.61, P less than 0.001) and was less than the activity predicted from the hemoglobin concentration in these patients. Serum concentrations of biopterin derivatives were markedly elevated. Eighteen patients were given the aluminum-chelating agent deferoxamine in a single dose, after which DHPR activity doubled. These studies suggest that aluminum inhibits DHPR activity in erythrocytes and that aluminum chelation reverses this effect. Although we did not directly measure DHPR activity in the brains of dialysis patients without encephalopathy, we propose that the reduction in activity in erythrocytes may reflect a similar reduction in the brain. Our findings could help to explain the encephalopathy associated with aluminum intoxication.

The influence of buffer pH, glucose and sodium ion concentration on the acid microclimate in rat proximal jejunum in vitro

Surface pH of rat proximal jejunum as measured in vitro by pH-electrode was found to be significantly more acid than bulk bathing media of neutral pH. Below a bulk pH value between pH 4 and 5, surface pH was isohydric with bulk pH but with increasing alkalinity of the bulk pH, surface pH remained consistently more acidic. The maintenance of a low surface pH was sensitive to external medium glucose concentration, particularly between 2 and 10 mM concentration. A Lineweaver-Burk plot of the glucose-dependent decrement in surface pH demonstrated an apparentKmof 2 mM. Anoxia caused a significant elevation in the measured surface pH at 10 mM glucose concentration. Surface pH was found to be sensitive to buffer sodium ion concentration in the jejunum, an effect which was less marked in the distal ileum.The measured low pH at the mucosal surface of rat jejunum experimentally confirms the ‘acid-microclimate’ hypothesis for drug absorption as originally proposed by Schanker and colleagues. The existence of a low pH has important consequences for the intestinal absorption of dissociable compounds. The implication for the experimental verification of active transport of solutes capable of dissociation is discussed.

Pterin metabolism in depression : an extension of the amine hypothesis and possible marker of response to ECT

Urinary excretion of neopterins and biopterins was measured in 23 patients with severe depression before and after receiving electroconvulsive therapy (ECT) and 26 healthy control subjects. Patients with psychotic depression and those responding to ECT had neopterin:biopterin (N:B) ratio significantly higher than controls before commencing ECT and positive therapeutic response was associated with reduction of N:B ratio towards control values. As a raised N:B ratio implies failure to convert neopterin to biopterin it is possible that reduced availability of tetrahydrobiopterin, the essential cofactor for the formation of noradrenaline, serotonin and dopamine, may exert rate limiting control over the synthesis of monoamines implicated in the pathogenesis of depressive disorders. The N:B ratio may be a marker for certain depressive subtypes and response to ECT.

Tetrahydrobiopterin metabolism in the streptozotocin induced diabetic state in rats.

Streptozotocin induced diabetes has been shown to have associated changes in the metabolism of tetrahydrobiopterin. De novo biosynthesis of tetrahydrobiopterin in the brain was reduced as was dihydropteridine reductase activity. Consequently there is a shift from fully reduced biopterins to more oxidised species. Both can cause reduced levels in the cofactor pool and may cause reduced levels of the associated neurotransmitters and neurological deficits.

Acidification in the rat proximal jejunum.

1. Production of hydrogen ions by the rat proximal jejunum was investigated using the everted sac technique. 2. Acidification occurred in the absence of glucose, increasing on addition of glucose to reach a maximal value. An apparent Km of 1‐78 mM was derived for the glucose‐dependent process. 3. Acidification in the presence of glucose was inhibited by 10 mM‐2:4‐dinitrophenol, 10 mM phlorrhizin, 10 mM aminophylline and anaerobiosis. 4. Histamine, ethylenediamine tetraacetic acid (EDTA), ouabain and acetazolamide, compounds known to alter acid production in gastric mucosa had no effect on jejunal acidification. 5. Galactose and 3‐O‐methylglucose failed to increase acidification; in contrast, mannose and fructose did lead to increases, indicating metabolic origin of the hydrogen ions. 6. Serosal and mucosal lactate production were measured and the calculated percentage of hydrogen ions possibly derived from this source was shown to account for only a small proportion of acidification. 7. The greatest increase in acidification with minimal simultaneous production of lactate occurred with ATP which was shown not to enter intestinal tissues. 8. A hypothesis for acidification, that of the break‐down at the mucosal surface of ATP from intracellular metabolic sources, is proposed and its relevance to the postulated microclimate is discussed.

The role of pterins in depression and the effects of antidepressive therapy.

Urinary excretion of neopterins (N) and biopterins (B) was measured in 48 patients with depression before and after treatment with placebo, antidepressants, or electroconvulsive therapy (ECT), and in 26 healthy control subjects. Patients prior to and after treatment had a significantly greater neopterin/biopterin (N:B) ratio than control subjects. There was a significant correlation between N:B ratios and the severity of depression and plasma cortisol. As a raised N:B ratio implies failure to convert neopterin to biopterin, it is possible that reduced availability of tetrahydrobiopterin, the essential cofactor for the formation of noradrenaline, serotonin and dopamine, may exert rate-limiting control over the synthesis of monoamines implicated in the pathogenesis of depressive illness.

Recovery from depression after electroconvulsive therapy is accompanied by evidence of increased tetrahydrobiopterin‐dependent hydroxylation

Serum phenylalanine and tyrosine levels were measured in 26 patients with severe depression before and after receiving electroconvulsive therapy. The phenylalanine:tyrosine [P:T] ratio declined significantly for those responding to treatment but not for nonresponders. These findings are discussed in relation to tetrahydrobiopterin, the essential cofactor for the formation of noradrenaline, dopamine and serotonin and the hydroxylation of phenylalanine to tyrosine.

The effects of pathological and normal physiological processes on biopterin derivative levels in man

Mean serum biopterin derivative levels in adult males and females were 1.75 microgram/l and 1.53 microgram/l respectively, increased with age and followed a cyclic pattern during menstruation. Foetal brain and liver levels at 16 weeks approximated adult concentrations. Wide distribution in adult tissues and variation in concentration independent of serum levels demonstrated local synthesis. Serum levels were significantly decreased in senile dementia, lead poisoning, coeliac disease and malignant carcinoid disease. Abnormal metabolism of phenylalanine in senile dementia suggest lowered levels of tetrahydrobiopterin in tissue. Methotrexate increased serum biopterin levels by inhibition of dihydropteridine reductase; a saturation effect was noted.

The transport of the lead cation across the intestinal membrane

1. The transport of the lead cation has been investigated using the everted sac preparation of Wilson & Wiseman (1954).

Absorption of folic acid by everted segments of rat jejunum

1. Everted rings of rat intestine were used to study the initial uptake rate of folic acid at various concentrations and incubation temperatures in vitro.