Cystine inhibits creatine kinase activity in pig retina

Cystinosis is an autosomal recessive disorder associated with lysosomal cystine accumulation caused by defective cystine efflux. Visual deficit is a possible consequence of cystine accumulation in cornea and retina. Fibroblasts from cystinotic patients present ATP deficit with intact mitochondrial energy-generating capacity by an unknown mechanism. Considering that creatine kinase is a thiol enzyme crucial for energy homeostasis in retina, and disulfides like cystine may alter thiol enzymes, the main objective of the present study was to investigate the effect of cystine and cysteamine, the drug used for treatment of cystinotic patients, on creatine kinase activity in cytosolic and mitochondrial fractions of the retina from adult pigs.

Retina was isolated from 6-month-old Landrace pigs, homogenized and mitochondrial and cytosolic fractions separated by centrifugation. Cytosolic and mitochondrial creatine kinase activities were determined in the presence of different concentrations of cystine and/or cysteamine.

Cystine inhibited the enzyme activity in a dose- and time-dependent manner and cysteamine prevented and reversed the inhibition caused by cystine, suggesting that cystine inhibits creatine kinase activity by oxidation of the sulfhydryl groups of the enzyme.

Considering that creatine kinase is a crucial enzyme for retina energy homeostasis, in case cystine leaves lysosome these results provide a possible mechanism for cystine toxicity and also another beneficial effect for the use of cysteamine in patients with cystinosis.