Cystinotic fibroblasts accumulate cystine from intracellular protein degradation.

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Abstract
Fibroblasts derived from patients with cystinosis, an autosomal recessive condition, accumulated the disulfide amino acid cystine within lysosomes. The metabolic defect leading to the cystine accumulation and the source from which the cystine was derived are unknown. In this report data was presented showing that cystine in these cells accumulated from the degradation of endogenous protein. This conclusion was based upon: no demonstrable synthesis of cystine from serine; no difference in cystine reaccumulation between glutathione-depleted and non-glutathione-depleted cystinotic cells; recovery of labeled cystine only when the protein pool was labeled; reversible inhibition of cystine reaccumulation by known inhibitors of lysosomal protein degradation (chloroquine and NH4Cl).