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1/3. Combined deficiency of xanthine oxidase and sulphite oxidase: a defect of molybdenum metabolism or transport?

    A child is described who presented in the neonatal period with feeding difficulties, severe neurological abnormalities, lens dislocation of the eyes and dysmorphic symptoms of the head. Routine laboratory investigations revealed a decreased serum urate and a positive sulphite reaction of the urine. Subsequent chromatographic examinations showed xanthinuria and increased excretion of S-sulphocysteine and taurine to be present. In addition, high thiosulphate and low sulphate excretions in the urine were observed. xanthine oxidase deficiency was demonstrated in a jejunal biopsy specimen, whereas the excretion of sulphur containing substances was considered to be characteristic of sulphite oxidase deficiency. This new combination of defects may be the result of malfunctioning of both enzymes, possibly caused by alterations in the essential molybdenum containing active centre of the enzymes, which they share in common.
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keywords = transport
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2/3. Studies in primary hypomagnesaemia: evidence for defective carrier-mediated small intestinal transport of magnesium.

    A 4 year old male with primary hypomagnesaemia was studied using balance and steady-state perfusion techniques. magnesium balance was negative and could be accounted for by increased faecal losses, renal conservation being normal; calcium balance was normal. After oral magnesium therapy magnesium balance became positive. The perfusion studies demonstrated net loss of magnesium into the intestinal lumen when low concentrations (1 and 2 mmol/l) of magnesium were perfused in contrast with control subjects; whereas at high concentrations (10 mmol/l a net absorption of a magnitude similar to control values was observed. In the control subjects sequential perfusion of increasing concentrations of magnesium demonstrated a curvilinear relationship between rates of absorption and the lower concentrations (1, 2, and 4 mmol/l) with an apparent Km and Vmax of 4.5 mmol/l and 91 nmol/min/cm respectively. At the higher concentrations (6 and 10 mmol/l) the relationship was linear. These data suggest that two separate transport systems participate in the absorption of magnesium from the proximal small intestine; a carrier-mediated system which saturates at low intraluminal concentrations, and a simple diffusional process. The possibility of the second transport system being a carrier-mediated process with a very much higher Km cannot be excluded. In primary hypomagnaesaemia the results suggest that the primary abnormality is a defect in carrier-mediated transport of magnesium from low intraluminal concentrations of magnesium.
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ranking = 1.75
keywords = transport
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3/3. An unusual neurological disorder of copper metabolism clinically resembling Wilson's disease but biochemically a distinct entity.

    A patient with progressive neurological disease resembling Wilson's disease but in whom Kayser-Fleischer rings were absent, was given 67Cu and 64Cu, orally and intravenously, to measure the rate of absorption of copper using a convolution integral. The data show an abnormal distribution of body copper resulting in low copper concentrations in plasma, urine and liver but with an accumulation in the lower bowel probably due to a defect in mucosal transport. The importance of differentiating this condition from Wilson's disease is stressed.
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keywords = transport
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