Figure 1.

Mechanism of action of cysteamine.

Notes: (A) In normal lysosomes, cystine and other cationic amino acids, such as lysine, freely traverse the membrane. (B) In cystinotic lysosomes, cationic amino acids can freely traverse the membrane, but cystine cannot due to the lack of the transporter cystinosin. Therefore, cystine accumulates inside the lysosome. (C) In cysteamine-treated lysosomes, cysteamine combines with cystine to form the mixed disulfide cysteine–cysteamine, which can use PQLC2, a cationic amino acid transporter, to exit the lysosome, hence reducing the cystine content of the cell.