JAMA. 1966;196(4):361. doi:10.1001/jama.1966.03100170103035.
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Observations associating mental retardation with alterations in the tryptophan metabolism have included decreased excretion of 5-hydroxyindolylacetic acid in patients with phenylketonuria. It has been shown that pyridoxal phosphate is essential as a coenzyme for the action of the enzyme 5-hydroxytryptophan decarboxylase, which can be inhibited in vitro by the influence of some phenylalanine metabolites. The in vivo inhibition of rat kidney decarboxylase could be prevented by adding pyridoxine to the phenylalanine-rich diet. One report states that when a phenylketonuric patient developed scleroderma during treatment with low-phenylalanine diet, scleroderma rapidly improved, and tryptophan metabolism became normal when phenylalanine was included in the diet. Pyridoxine and tryptophan metabolisms are closely related. The ratio of some tryptophan metabolites is usually inverted and abnormal in pyridoxine deficiency. In the study of this subject, Heeley1 examined tryptophan metabolism in children with phenylketonuria. Five out of eight untreated children and eight out of nine receiving


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