T. Liu, A. Inglis
1972
Citations
0
Influential Citations
13
Citations
Journal
Methods in enzymology
Abstract
Publisher Summary This chapter describes a procedure for the determination, of both cysteine and cystine in acid hydrolyzates as S-sulfocysteine by reduction of cystine with dithiothreitol followed by treatment of the reaction solution with excess sodium tetrathionate. The amino acid mixture is used to check the procedure and to establish the color factor for S-sulfocysteine on a Beckman-Spinco Model 120C amino acid analyzer. The color values obtained for both amino acids are identical to those obtained when the derivitization procedure is applied to the calibration mixture for the analyzer. The presence of a dip in the baseline just preceding the peak from S-sulfocysteine may make it desirable to integrate the peak by hand rather than with an electronic integrator. The aspartic acid is used as an internal standard. The conclusion is that under the experimental condition described, serine and threonine do not form substances that interfere in the analysis of S-sulfocysteine by this procedure. The cysteine determined quantitatively after hydrolysis as S-sulfocysteine using the procedure described reopens the possibility of determining the half-cysteine residues in modified proteins, for example, S-sulfoproteins, after acid hydrolysis.