Amino-Acids in Staple Foods. The Rôle of Cystine in Proteolytic Digestion

Abstract

The author presented his views on the synthesis of amino-acids in the animal body at the Washington annual meeting of the Society of Biological Chemists in 1936 (unpublished) and concluded that cystine, contrary to the view held at that time, is a dispensable amino-acid. His working hypothesis was that sugar forming amino-acids are dispensable and non-sugar formers are indispensable, and also suggested that valine being a non-sugar former would prove to be an indispensable amino-acid. There are so many examples in the literature where the diet under investigation was supplemented with cystine which promptly converted it to a normal growth-producing one that a claim of cystine deficiency was readily granted. Johns and Finks 1 fed cooked navy bean meal supplemented with cystine to rats and obtained normal growth so that they concluded that cystine deficiency was the controlling factor in the bean diet. Mitchell and Smuts' 2 feeding experiments demonstrated clearly that by supplementing raw soybean meal with cystine the rats not only stayed alive but gained in weight. Csonka and Jones 3 stated, however, that there cannot be a quantitative deficiency in soybean meal when the defatted meal contains an average of 0.4% cystine. Regardless of the cystine content of the experimental diets of soybean (high) and navy bean (low) the addition of cystine produced growth. Since the dispensability of cystine to growth has been now demonstrated by Rose and coworkers 4 experimentally, we are forced to assume that supplementary cystine performs some physiological function other than correcting a nutritional deficiency. There is some justification for considering first that free -SH has an activating effect upon proteolytic enzymes and, second, that the oxidation-reduction properties of this cystine (or cysteine) may affect digestion favorably.