Abstract
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 amphibia3,4.
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