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  • Title: Metabolism of an oral tryptophan load by women and evidence against the induction of tryptophan pyrrolase by oral contraceptives.
    Author: Green AR, Bloomfield MR, Woods HF, Seed M.
    Journal: Br J Clin Pharmacol; 1978 Mar; 5(3):233-41. PubMed ID: 656268.
    Abstract:
    Rates of tryptophan and kynurenine appearance and disappearance in plasma following an oral tryptophan load have been studied in female volunteers, aged 19-39 years, taking no drugs or taking oral contraceptives (OCs). Following a single oral L-tryptophan load (50 mg/kg), plasma tryptophan concentrations rose 5 fold reaching a peak after 1-2 hours. Plasma kynurenine concentrations also rose reaching a peak after 3 hours. Tryptophan changes were similar in both groups but kynurenine concentrations were lower in the OC group 1 hour (p .01), 2 hours (p .05), and 3 hours (p .05) after the load. In both groups there was little increase in urinary tryptophan or 5-hydroxyindoleacetic acid excretion in the 24 hours after the load and just over a doubling of indoleacetic acid excretion. The OC group excreted less kynurenine than the controls both during the 24 hours before the load and the 24 hours after, although the percentage increase following the load was much greater in the OC group. The OC group also excreted more 3-hydroxykynurenine, xanthurenic acid, and 3-hydroxyanthranilic acid following the tryptophan load. Both groups revealed similar values for volume of distribution, plasma clearance, and plasma 1/2-life of tryptophan. It is concluded that the excretion of certain tryptophan metabolites is raised in women in OCs. This change does not appear to be attributable to an increase in the activity of liver tryptophan pyrrolase; 1 possible reason may be a relative deficiency of vitamin B6.
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