1. Evidence of deficiency of, antagonism to, or abnormal dependency upon pyridoxine has been sought in four patients with primary hyperoxaluria. The urinary excretion of kynurenine, 3-hydroxykyurenine, 3-hydroxyanthranilic acid, kynurenic acid and xanthurenic acid, before and after a loading dose of l-tryptophan was used to assess pyridoxine nutrition.

2. Three of the four patients studied had abnormal l-tryptophan metabolite excretion patterns, but these were not of the type which is associated with abnormalities of pyridoxine nutrition.

3. The changes which were observed are compatible with impaired conversion of kynurenine to 3-hydroxykynurenine by the NADH2 dependent kynurenine 3-hydroxylase (EC enzyme system.

4. Large doses of pyridoxine hydrochloride reduced the urinary oxalate excretion by two of the patients to levels which were intermediate between the normal range and the pre-treatment values. This effect was maintained for 6 months, which was the longest period of observation.

5. It is concluded that this action of pyridoxine is mediated by a mechanism which does not involve the correction of an abnormality of pyridoxine metabolism.

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