Tryptophan Induction of the Serotonin and Kynurenine Pathways in the Early Amphibian Embryo

THE amino-acid tryptophan can be metabolized along two different pathways (Fig. 1), each of which is characteristic for a particular type of tissue. As the substrate of L-tryptophan oxygenase, it is a source of pyridine nucleotide. This metabolic pathway occurs only in the liver¹. Alternatively, tryptophan can be hydroxylated in the 5-position and further metabolized to yield serotonin and melatonin. Serotonin is synthesized mainly by cells derived from the embryonic neuroectoderm²; melatonin production, which in mammals is restricted to the pineal gland, also occurs in the eye and brain of amphibia^3,4.