New method for determining cystine in leukocytes and fibroblasts

Area of Science:

• Biochemistry
• Medical Genetics
• Analytical Chemistry

Background:

• Cystinosis is a rare lysosomal storage disorder caused by defective cystine transport.
• Accumulated cystine in lysosomes leads to cellular damage and organ dysfunction.
• Accurate cystine measurement in leukocytes and fibroblasts is essential for diagnosis and treatment monitoring.

Purpose of the Study:

• To develop and validate a sensitive and reliable assay for quantifying cystine concentrations in human leukocytes and cultured fibroblasts.
• To establish a method suitable for routine clinical use in diagnosing cystinosis and monitoring cysteamine therapy.

Main Methods:

• Cells were treated with N-ethylmaleimide to prevent cysteine oxidation and disulfide exchange.
• Cystine was reduced to cysteine, derivatized with monobromobimane, and quantified using automated HPLC with fluorescence detection.
• Assay validation included linearity, imprecision, detection limit, and stability studies.

Main Results:

• The assay demonstrated linearity up to 200 micromol/L cysteine with within-run and day-to-day imprecision <5%.
• The detection limit was 0.3 micromol/L, and recovery of added cystine ranged from 69-86%.
• Cystine remained stable in frozen leukocytes for at least 2 months.

Conclusions:

• N-ethylmaleimide effectively prevents cysteine oxidation and disulfide exchange, ensuring accurate cystine measurement.
• The validated HPLC method provides an adequate detection limit for diagnosing cystinosis and monitoring cysteamine treatment in clinical settings.
• This assay facilitates precise cystine quantification in patient-derived cells for improved disease management.