Isolation of Methionine from Protein Hydrolysates

In the preceding papers (1, 2) the preparation and properties of sulfonium salts of methionine were described. The ease of formation of the methylsulfonium salt and the solubility properties of its phosphotungstate suggest utilization for the characterization of methionine in amino acid mixtures, since the phosphotungstates of other a-amino acids are readily soluble in cold aqueous alcohol while ammonium phosphotungstate is insoluble in both aqueous alcohol and aqueous acetone (3). However, in protein hydrolysates the solubility relationships were found to be complex and the separation of phosphotungstates was not sharply delineated. The preliminary removal of ammonia particularly appeared to be desirable, and it was found that both ammonia and the basic amino acids could be precipitated by phosphotungstic acid from hydrochloric acid hydrolysates of protein with only minor losses of methionine. Methionine was then converted to the methylsulfonium salt and isolated without further difficulty. The procedure as finally evolved consists of the following steps: (1) hydrolysis of the protein with hydrochloric acid; (2) dilution and removal of ammonia and basic amino acids by phosphotungstic acid; (3) replacement of hydrochloric by sulfuric acid; (4) conversion of methionine to the methylsulfonium salt and precipitation with phosphotungstic acid; (5) conversion of the phosphotungstate to the bromide by means of tetraethylammonium bromide; (6) isolation of methionine methylsulfonium bromide from aqueous alcohol. Application of this procedure to casein resulted in isolation of 65 per cent of the methionine as L-methionine methylsulfonium bromide. With zein, recrystallization of the product was necessary and a yield of 45 per cent of the theory was obtained. The procedure may be modified, depending on the material under investigation and one’s needs. In many cases the phosphotungstate, which can be recrystallized from hot acidic