Cysteic acid ninhydrin reaction

Fig. 6 Typical ion-exchange separation of amino acids followed by postcolumn reaction with ninhydrin. Separation was achieved employing Beckman 6300 amino acid analyzer with cation-exchange column. Three sodium buffers were used with a stepwise elution scheme as supplied/recommended by the manufacturer. Detection was the sum of 440-nm and 570-nm absorbance. Standard three-letter abbreviations for amino acids are used also, CA = cysteic acid, tau = taurine, and nle = norleucine. Data was supplied by Stephen D. Smith, Ross Products Division of Abbott Laboratories, Columbus, OH.

Water-soluble a-DNP-Arg, a-DNP-His, s-DNP-Lys, bis-DNP-His, O-DNP-Tyr, DNP-cysteic acid (CySOjH), and DNP-cystine (Cys)2 have been identified on silica gel plates in the n-propanol-34% ammonia (7 3 v/v) system. Although separation of DNP-Arg and s-DNP-Lys is incomplete Rf values of 0.43 and 0.44, respectively), both of them can be detected because of the color difference produced in the ninhydrin reaction. 

In the case of amino acid analysis, the quantification of cysteine can be difficult because it is oxidized to cystine during acid hydrolysis. To circumvent this problem, cysteine can be oxidized to cysteic acid with performic acid prior to analysis. Alternatively, cysteine can be converted to the pyridyl ethyl derivative and subsequently detected by postcolumn reaction with ninhydrin. Still another method involves the production of carboxymethyl cysteine following alkylation with iodoacetic acid. All of these cysteine derivatives can be separated by either reversed-phase precolumn or ion-exchange postcoT umn methods. 

Investigations into urine and plasma samples must consider that certain compounds such as antibiotics are not metabolized after its administration. Antibiotics pass through the body without changing their structure and, finally, get into the urine or plasma and interfere with the amino acid analysis via reaction with ninhydrin. Perry et al. [52] reported this problem for the first time. In urine samples oxidized with hydrogen peroxide, they detected a signal in the retention range of cysteic acid and homocystine, which they attributed to o-penicillamine-sulfonic acid. This compound was formed by oxidation of the antibiotic D-peni-cillamine, which was administered to the patient whose urine was investigated. 

---