Fig. 8.2. Conversion of two molecules of cysteine into one molecule of cystine.
Analysis of methionine and cysteine in plasma and serum is also sensitive to the methods used before the chromatographic analysis. Plasma and serum should be deproteinized as soon as possible after collection because deproteinizing before freezing helps to prevent loss of methionine and cysteine (Stein and Moore, 1954; DeWolfe et al., 1967).
Difficulties in analysis for the sulphur amino acids make it crucial that analytical methods are described in research papers. Unfortunately, much of the older research, and some more current research, failed to use adequate analysis methods and therefore the results of experiments can be difficult to interpret.
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