Studies on Digestibility of Proteins in Vitro v. Rate of Liberation of Cystine on Hydrolysis of Casein. Some Observations on Colorimetric Tests for Cystine When Applied to Peptic and Acid Digests of Casein* by D. Breese Jones

This paper is a continuation of a series of studies carried on in this laboratory some time ago on the digestibility of proteins in vitro (l-3). The data now presented are the results of some preliminary experiments carried out in connection with proposed studies on the rate of liberation of amino acids from different proteins on digestion. It is known that on enzymic hydrolysis different amino acids are liberated at different rates. According to Abderhalden (4) gastric digestion does not liberate from proteins and peptones free amino acids. So called a-biuret products are formed which contain high percentages of proline and phenylalanine. On the other hand, certain amino acids are readily liberated from proteins on pancreatic digestion, others are liberated more slowly, and still others are liberated from their combination with great difficulty. Tyrosine, tryptophane, and cystine are liberated very early in tryptic digestion. Proline and phenylalanine were found to be difficultly liberated, if at all, and glutamic acid was liberated fairly readily, but not so rapidly as tyrosine. Fiirth and Lieben (5), on the other hand, claim that tryptophane is not set free during the first stages of tryptic digestion. Ragins (6) found that after 1 hour’s digestion with pancreatin, casein