Metabolism of D- and L-Tryptophan-7 a - 14 C in Rats and the Effects of Unlabeled Enantiomers 1

Abstract

Summary

DL-Tryptophan was chromatographically resolved into the D- and L-isomers on powdered paper columns. Both isomers of tryptophan were metabolized to ¹⁴CO₂ equally well by the rat. A load of either isomer increased the ¹⁴CO₂ production from the opposite labeled isomer almost twofold, but did not affect the urinary ¹⁴C levels. The kynurenic acid levels were slightly elevated when the D-isomer was given, but nothing like the 30% concentrations seen when labeled D-kynurenine was given to rats in previous studies. The loading doses of either isomer slightly increased the urinary kynurenic acid, quinolinic acid, and picolinic acid levels and the similarities in urinary levels of these components showed that comparable quantities of either isomer were metabolized via similar pathways. The concentration of activity from either isomer in the tissues decreased in the order of liver, plasma, kidney, testes, heart, spleen, and brain. The level of ¹⁴C in the tissues was depressed when a load of either unlabeled isomer was given in the presence of the opposite labeled isomer. No detectable activity appeared in urinary 3-hydroxykynurenine when L-tryptophan was given. Urinary levels of 5-hydroxyindoleacetic acid indicated that this pathway was of minor importance quantitatively.