Intact methionine residues in proteins were rapidly and precisely determined by measuring methyl thiocyanate released during the reaction with CNBr and separated by g.l.c. Conditions for the reaction and for chromatography on columns of Porapak P-S are described. The recovery of methyl thiocyanate from several methionine derivatives and analogues were examined. Carbamoylmethionine was adopted as a stable primary standard and ethyl thiocyanate as internal standard. The measured methionine content of several isolated proteins was close to the theoretical value indicated by previous work and the results for these and a range of food proteins agreed well with results obtained by ion-exchange chromatography after performic acid oxidation. Since CNBr does not react with methionine sulphoxide and a preliminary hydrolysis is not required, the method discriminates between methionine and any methionine sulphoxide that may be present. It could be useful in studies on the nutritional availability of methionine in processed foods.
