New Method for Determining Cystine in

Background: Cystinosis is a rare inborn error of cystine transport, leading to accumulation of cystine in the lysosomes. To diagnose cystinosis and monitor treat-ment with cysteamine, adequate measurements of cys-tine concentrations in leukocytes and cultured fibro-blasts are required. Methods: Cells were sonicated in the presence of excess N-ethylmaleimide to prevent oxidation of cysteine to cystine and disulfide exchange reactions of cystine with available sulfhydryl moieties. Cystine was measured as cysteine after reduction with sodium borohydride and derivatization with monobromobimane, followed by separation with automated HPLC and fluorescence de-tection. Results: The assay was linear to 200 mmol/L cysteine