S u m m a r yThe method for the synthesis of L-cysteine hydrochloride labelled with 35S is described. L -C~s t i n e -~~S obtained from baker's yeast was reduced with tin in hydrochloric acid and radiochemically pure L-cysteine-35S hydrochloride was isolated by ion-exchange chromatography on a column.The obtained L -~y s t e i n e -~~S hydrochloride was stable in aqueous solution, pH 2, for six months and had specific activity of 20-125 mCi/mmol and radiochemical purity better than 9 5 % .
S u m m a r y The modified method f o r b i o s y n t h e s i s o f L -m e t h i~n i n e -~~Sand L -~y s t i n e -~~S from baker's y e a s t (Saccharomyces c e r e v i s i a e ) is d e s c r i b e d . The y e a s t was c u l t i v a t e d i n a sulphur-deplated medium c o n t a i n i n g c a r r i e r -f r e e N a 35S04, under a i r bubbling dur i n g 24 hours. Yeast p r o t e i n s were hydrolyzed enzymatically i nt o f r e e amino a c i d s , a n d L -m e t h i~n i n e -~~S and L -c y~t i n e -~* S wer e i s o l a t e d by ion-exchange chromatography on a column. Radioa c t i v e y i e l d s f o r L -m e t h i~n i n e -~~S and L -~y s t i n e -~~S were 30% and lo%, r e s p e c t i v e l y . The obtained products have h i g h s p e c i f i c a c t i v i t y , l -l O C i / m o l , a n d radiochemical p u r i t y b e t t e r t h a n 95%.
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