Metabolism of D- and L-Tryptophan in Dogs

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.