1A single oral L-tryptophan load (0.245 mol/kg: 50 mg/kg) was given to young women taking no drugs or taking an oral contraceptive. 2 Following tryptophan administration, plasma tryptophan concentrations rose approximately 5 fold reaching a peak 1-2 h after the load. Plasma kynurenine concentrations also rose reaching a peak 3 h after the load. Plasma tryptophan changes were the same in both groups but kynurenine concentrations were lower in the oral contraceptive group 1, 2 and 3 h after the load. 3 There was very little increase in urinary tryptophan or 5-hydroxyindoleacetic acid excretion in the 24 h following the load and just over a doubling of indoleacetic acid excretion. The excretion of these compounds was similar in both groups. 4 The group taking an oral contraceptive excreted less kynurenine in the urine than the control group both during the 24 h before the load and the 24 h following the load, although the percentage increase following the load was much greater in the oral contraceptive group.
SIn agreement with previous studies it was found that women on oral contraceptives excreted significantly greater quantities of 3-hydroxykynurenine and xanthurenic acid following the tryptophan load and 3-hydroxyanthranilic acid values also tended to be raised. 6 The volume of distribution, plasma clearance and plasma half-life of tryptophan was similar in both groups of subjects. 7 It is concluded that, in agreement with other studies, the excretion of certain tryptophan metabolites is raised in women on oral contraceptives. However, the data on plasma clearance indicates that, contrary to previous hypotheses this change is not attributable to an increase in the activity of liver tryptophan pyrrolase. Other possible reasons such as a relative deficiency of vitamin B6 are discussed.
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