Disulphide reduction in lysosomes. The role of cysteine

Published evidence indicates that cystine-containing proteins have their disulphide bonds reduced during proteolysis in lysosomes. However, the intralysosomal accumulation of cystine in the cells of patients with cystinosis has been seen as evidence that protein cystine residues are not reduced. The data are reconcilable and fully in harmony if it is postulated that cysteine from the cytoplasm is the physiological reducing agent.