CONVERSION OF D‐ AND L‐TRYPTOPHAN TO BRAIN SEROTONIN AND 5‐HYDROXYINDOLEACETIC ACID AND TO BLOOD SEROTONIN 1

Abstract— Intraperitoneal administration of both D‐ or L‐tryptophan elevated the levels of serotonin and 5‐hydroxyindoleacetic acid in the brains of hypophysectomized and intact rats. In intact rats, the increase in brain 5‐hydroxyindoles was slower after D‐tryptophan than after L‐tryptophan. Similarly, brain tryptophan rose more slowly after administration of D‐tryptophan. The uptake of L‐tryptophan from blood into brain was at a rate about one‐third that of 3H2O. D‐tryptophan uptake was at 1/25 that of 3H2O. Brain and liver tryptophan aminotransferase activities were stereospecific for the L‐isomer and no evidence could be found for a tryptophan racemase in brain. Evisceration prevented the increase in brain 5‐hydroxyindoles following peripheral administration of D‐tryptophan administration but not that after L‐tryptophan. The serotonin ratios between the two brain regions examined remained constant following administration of either D‐ or L‐tryptophan. On the basis of these results we concluded that the increase in brain 5‐hydroxyindoles following administration of L‐tryptophan was not dependent upon stress‐induced changes in pituitary hormones and that the elevations after D‐tryptophan were dependent upon its prior conversion to L‐tryptophan via peripheral deamination and subsequent transamination.