High-performance liquid chromatographic determination of urinary cysteine and cystine

A method for quantitative determination of cystine and its monomer, cysteine, is presented using reversed phase HPLC with precolumn derivatization by o-phthaldialdehyde (OPA). The new application exhibits a two step procedure for overcoming the minimal fluorescence of the OPA-derivatives of cysteine and cystine: after reduction of cystine to cysteine by dithiothreitol the sulfhydryl-group of cysteine is blocked with iodoacetic acid. Optimum reaction conditions of this derivatization procedure are described. Special precautions relating to the rather fast oxidation of cysteine are necessary. Collecting urine directly into sulphosalicylic acid prevents oxidation of cysteine up to 5 days. Initial results of urinary concentration of cysteine and cystine of healthy control persons and patients suffering from cystinuria are presented.