Cysteine (& cystine), methionine and tryptophan are destroyed during hydrolysis with 6N HCl. Cys and Met can be determined by oxidation with performic acid, yielding the acid stable forms cysteic acid and methionine sulfone, prior to the standard acid hydrolysis. The conversion of met is quantitative while that of cysteine is >90%. Tryptophan can quantitated but requires consultation with the MSF staff.
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