Willcock and Hopkins (11) made a study of the incomplete protein, zein, the principal protein of maize, which was comparable with Kauf-mann's studies with gelatin. Zein is entirely lacking in both lysin and tryptophan. These authors fed to young mice diets of purified food substances, in which zein was the sole source of protein (amino-acids), and observed that the animals died in less than 14 days in a state of torpor. When tryptophan was added to the food they lived about twice as long and remained lively until shortly before death. The addition of tyrosin did not improve the diet.

Willcock and Hopkins suggested that tryptophan may serve a two-fold function. It is an essential structural unit of the protein molecules of which the tissues are formed, and is perhaps a precursor of a substance which certain tissues elaborate. This, they suggested, is in the nature of a hormone or regulator of metabolism, without which death would supervene sooner than through specific starvation for protein alone.