ABSTRACT
The existence of tryptophan was recognized some 100 years ago. First, it was discovered to be a vital foodstuff, an essential building block for proteins in animals. Then, it was chemically identified. Its presence in proteins was quantified, and its degradative pathways were investigated. Gradually, its major metabolic pathway, as well as its minor pathways, were unraveled. The enzymes involved in these pathways and the hormones that affected the enzymes became the subject and interest of many investigations. Early on, it became apparent that tryptophan was the precursor of a number of important compounds, particularly serotonin. Interest in serotonin and its biologic importance rapidly overshadowed the interest in tryptophan itself. More recently, melatonin, another metabolite of tryptophan, has gained much attention and prominence for its biologic actions. Other less known metabolites have been investigated, such as quinolinic and kynurenic acid, but their actions or functions are still somewhat speculative at the present time. Perhaps there are still other jewels among the many metabolites with important biologic significance that need to be uncovered and defined.
