ABSTRACT
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Our knowledge of tryptophan began some 100 years ago. In 1901 Hopkins and Cole
isolated tryptophan from a pancreatic digest of casein. Its structure was established in 1907 by Ellinger and Flamand,
who synthesized a substance that was identical to the tryptophan isolated by Hopkins and Cole. It is noteworthy that about 50 years prior to the discovery of tryptophan by Hopkins and Cole,
aspects of tryptophan metabolism began to appear in the research literature, when in 1853 Liebig discovered kynurenic acid in dog urine.
