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Tryptophan and Methionine Metabolism of Adult Females as Affected by Vitamin B-6 Deficiency¹

Department of Nutritional Sciences, College of Agricultural and Life Sciences, University of Wisconsin, Madison, Wisconsin 53706

The effect of a vitamin B-6 deficiency on the metabolism of tryptophan and of methionine was determined with five female subjects who were not pregnant or taking oral contraceptives. When subjects consumed the experimental diet and daily supplements of 2.0 mg of pyridoxine, they excreted low basal and posttryptophan amounts of hydroxykynurenine, kynurenine, kynurenic acid and xanthurenic acid. After 13 days of vitamin B-6 deficiency the subjects excreted 38.4% of an oral 2-g loading dose of L-tryptophan in the form of the four metabolites with 14.5% being excreted as hydroxykynurenine, 11.7 as kynurenine, 9.7 as xanthurenic acid and 1.9 as kynurenic acid. Both basal and postmethionine loading excretions of cystathionine were affected by the deficiency, the mean basal predepletion value and the value after 12 days of vitamin B-6 deficiency being 122 and 517 µmoles, respectively. After oral loading with 3 g L-methionine, the values for urinary cystathionine were 226 µmoles before depletion and 495 and 1,877 µmoles after 7 and 14 days of deficiency. Basal excretion of cysteine sulfinic acid was not affected by vitamin B-6 deficiency, but it did increase following methionine loading after 14 days of deficiency. Homocystine was not detected in the urine of the subjects until after 14 days of vitamin B-6 deficiency and then only following methionine loading. Repletion with 2.0 mg pyridoxine daily for 1 week caused the above urinary metabolites to return to predepletion values.

KEY WORDS: • methionine metabolism • tryptophan metabolism • vitamin B-6 deficiency

¹ Research supported by the College of Agricultural and Life Sciences and by the Graduate School, University of Wisconsin, Madison, Wisc. 53706.

Manuscript received 15 May 1974.

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