A new sulfur-containing amino-acid isolated from the hydrolytic products of protein(Mueller, J. H. (1923) J. Biol. Chem. 56, 157-169).

During an investigation of the cultural requirements of certain bacteria, the writer isolated from the hydrolytic products of casein, a fraction of material which was apparently required by hemolytic streptococci for satisfactory growth, and which was found on examination to contain a considerable amount of sulfur (1). The sulfur was apparently not in the form of cystine since it did not give the customary reactions (lead blackening and sodium nitroprusside) which are given by the sulfur in this compound. Since the participation, as growth factors under the experimental conditions, of any of the known amino-acids had been ruled out, it was thought possible that this new sulfur compound might be the factor which was being sought from the casein. An attempt was, therefore, made to isolate the material in sufficient quantity and purity to determine its chemical nature; and although the final separation in pure form proved exceedingly difficult, it was found that the sulfur compound lost its activity as a growthinducing substance when it became even relatively pure. It is not yet clear whether this is due to some kind of chemical change in the sulfur compound, or simply to the elimination of a separate active substance during the purification. In view, however, of the general impression that there was present in protein one or more other sulfur-containing amino-acids, besides cystine, it seemed to be important to carry through the isolation of the