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S U M M A R Y1. A vitamin &-deficient diet was fed to an adult male subject to confirm previously described changes in tryptophan metabolism and urinary 4-pyridoxic acid excretion, and erythrocyte alanine and aspartate aminotransferase activities.2. The results were compared with those obtained in women taking oestrogencontaining oral contraceptives.3. The development of dietary vitamin B, deficiency was indicated by decreased 4-pyridoxic acid excretion, increased urinary concentrations of xanthurenic acid, kynurenine and 3-hydroxykynurenine, an elevated 3-hydroxykynurenine/3-hydroxyanthranilic acid ratio and impaired erythrocyte aminotransferase activities.4. Tryptophan metabolites and 4-pyridoxic acid excretion were determined in thirty-one women when they had been taking an oral contraceptive for 6-36 months. Of these, twenty-six had abnormal tryptophan metabolism, but the 4-pyridoxic acid was decreased in only seven. In six of these seven a raised 3-hydroxykynurenine/3-hydroxyanthranilic acid ratio supported a diagnosis of subclinical vitamin B, deficiency; erythrocyte alanine aminotransferase activity was determined in five of the six, and was decreased in three. 5. Erythrocyte aminotransferases were determined in sixteen women when they had been taking an oral contraceptive for 3-6 months, and in thirty-four women after 6-36 months treatment. Neither group showed any change in alanine aminotransferase activity, but the aspartate aminotransferase was elevated in the group treated for 6 months or longer.The metabolic pathway by which L-tryptophan is converted into nicotinic acid ribonucleotide, and several of the side reactions along the main pathway, involve pyridoxal phosphate dependent enzyme systems (Fig. I), and in consequence a disturbance of tryptophan metabol-

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