The Effect of the Dispensible Amino Acids of Zein on the Utilization of L-Tryptophan for Pyridine Nucleotide Synthesis

(From the Introduction)

Pellagra, first described in 1730, was associated with the consumption of maize as early as 1789. Several theories have been suggested to explain the association between corn and pellagra. One is that there is some factor in corn which is toxic. There has been no evidence to substantiate or disprove this theory.

Since pellagra is a disease resulting from a deficiency in the diet of either niacin or its precursor tryptophan, a possible explanation offered for this association between diets high in corn and pellagra was the low tryptophan content of corn. However, Krehl et al. (1) found that good growth in rats was obtained by feeding non-corn, low-niacin diets containing only 108 mg. per cent tryptophan, whereas rats fed corn diets which contained more than 108 mg. per cent tryptophan showed poor growth.

In another study by Goldsmith et al. (2), human subjects fed corn diets developed clinical signs of a niacin deficiency within 50 days, while subjects fed a wheat diet which had the same niacin and tryptophan content as the corn diet did not show any niacin deficiency symptoms until the subjects had been on the diet 80 days or more.

The studies of these researchers suggested that some component of corn was interfering with the utilization of tryptophan and/or niacin. Both groups of researchers have postulated that the balance of the amino acids in corn may be responsible for this interference. Corn is defi­cient in tryptophan and has a high level of leucine. It has been suggested that this imbalance of amino acids in corn results in an increased requirement for tryptophan.

Much investigation has been done to determine whether or not an imbalance of amino acids in corn, resulting in an increased requirement for tryptophan in the diet, is responsible for the development of niacin deficiency symptoms in animals fed corn diets. Coulter (3) studied the ability of rats fed low-protein, niacin-free diets containing either corn, zein, mixtures of the indispensable amino acids in zein with and without leucine, or the dispensable amino acid mixture of zein to utilize tryptophan for the synthesis of pyridine nucleotides. The pyridine nucleotides are the metabolically-active form of niacin in the body. She found that leucine and one or more of the dispensable amino acids of zein interfere with the utilization of tryptophan.

The study reported in this thesis is a continuation of the work by Coulter and was designed to determine which of the dispensable amino acids in zein inhibit(s) the conversion of tryptophan to liver pyridine nucleotides in rats.