Metabolism of D- and L-Tryptophan in Dogs

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Abstract
The metabolism of D- and L-[benzene ring-U-14C]tryptophan by dogs was studied. The distribution of label from each isomer in urine, feces, CO2 and various tissues was determined. Thirteen different urinary tryptophan metabolites were isolated by ion exchange cellulose chromatography. D-[14C]Trcyptophan was poorly converted to 14CO2 relative to the L-isomer, while giving rise to nearly three times as much urinary 14C as did the L-isomer. The major urinary metabolites of D-tryptophan were unchanged D-tryptophan, D-kynurenine and kynurenic acid. The major urinary metabolite from L-tryptophan was kynurenic acid. Inversion of D-tryptophan to L-tryptophan via indolepyruvic acid appeared to be the major fate of ingested D-tryptophan, with renal excretion of the unchanged D-isomer the next most important fate. The dog apparently utilizes D-tryptophan more efficiently than does the human but much less efficiently than does the rat. The dog appears to be a reasonable animal model for the human in studies of D-tryptophan metabolism.