Intracellular distribution of cystine in cystine-loaded proximal tubules

Cellular cystine loading with cystine dimethyl ester has been shown to inhibit transport in proximal convoluted tubules perfused in vitro and decrease the rate of oxygen consumption in suspensions of proximal tubules. The present study was designed to examine the intracellular distribution of cystine in this model of the Fanconi syndrome of cystinosis and to determine whether cystine or its degradation product, cysteine, is the cytotoxic agent in cystine-loaded rabbit proximal tubules. Tubules were incubated with 2 mmol/L cystine dimethyl ester for 10 min at 37°C and subjected to cellular fractionation. The intralysosomal cystine content (272 ±125 nmol/mg protein) was significantly higher than that measured in the nucleus (8.7 ± 2.0 nmol/mg protein) and cytosol (9.8 ± 4.0 nmol/mg protein (p < 0.05). Electron micrographs of tubules loaded with cystine depicted large swollen lysosomes. To determine whether cystine or its breakdown product, cysteine, was the cytotoxic agent in tubules incubated with cystine dimethyl ester, the intracellular cystine and cysteine contents were measured and found to be 86.5 ± 14.8 and 5.7 ± 1.7 nmol/mg protein, respectively. These tubules had a 50% decrease in the rate of O₂ consumption. To examine whether the increased level of intracellular cysteine played a role in this decrease in O₂ consumption, we loaded tubules with 2 mmol/L cysteine methyl ester for 10 min. Despite an intracellular cysteine concentration of 59.6 ± 5.8 nmol/mg protein, cysteine-loaded tubules had a rate of O₂ consumption equal to that measured in control tubules. Thus, intracellular cystine loading significantly increases intralysosomal cystine content. The inhibition of tubular respiration with cystine dimethyl ester is due to cystine rather than its degradation product, cysteine.