DETERMINATION OF TRYPTOPHAN AND METHIONINE BY MERCAPTOETHANESULFONIC ACID-HYDROLYSIS AT ELEVATED-TEMPERATURE

Three mol l-(1) mercaptoethanesulfonic acid (MES-OH) was used for the hydrolysis of different samples (pure proteins, free tryptophan and mi lk powder with high milk sugar content). Different temperatures (160, 170 and 180 degrees C) and time periods (15-90 min) were compared unde r standard conditions to minimise side reactions during 3 mol l(-1) me rcaptoethanesulfonic acid hydrolysis for the best recovery of the amin o acids (especially tryptophan and methionine). The following material s were used for testing the hydrolysis methods: Bovine ribonuclease, l ysozyme, citochrom C, free tryptophan and mare's milk powder. The hydr olysis at high temperature was successfully applied for the amino acid analysis of milk powder with high contents of carbohydrate and pure p roteins. It is stated that the higher temperature and the shorter time give results which are very similar to the original method (125 degre es C/24 h). In some cases, such as tryptophan and methionine determina tion at 160-170 degrees C for 15-30 min, the results were better than those obtained by the original method. A disadvantage of MES-OH hydrol ysis method is that it reduces cystine to cysteine, which coelutes wit h proline from the ion exchange column and it may interfere with the d etermination of proline in high-cystine proteins.