Effect of cysteine dosage on erythrocyte glutathione synthesis rate in a patient with cystathionine beta synthase deficiency

Cystathionine β-synthase (CBS)-deficient patients develop premature arteriosclerosis and thrombosis leading to a high risk of a vascular event before the age of 30 years. In CBS deficiency the transsulfuration pathway is impaired, leading to markedly elevated levels of homocysteine and methionine, and severely decreased levels of cystathionine and cysteine. Through autooxidation these elevated levels of homocysteine might induce excessive production of reactive oxygen species (ROS). ROS are involved in endothelial damage and are neutralized by antioxidants. In humans the main antioxidant is glutathione (GSH). Its production mainly depends on the amount of available cysteine. Since cysteine levels in CBS deficiency are decreased, GSH production is presumed to be low. Accordingly, all CBS-deficient patients receive cysteine supplements, which supposedly stimulate GSH synthesis. However, data on the effect of cysteine dosage on GSH synthesis in CBS-deficient patients are lacking. Therefore, in a CBS-deficient pyridoxine non-responsive female patient, concentration and fractional synthesis rate (FSR) of erythrocyte GSH were measured by infusion of l-[3,3-²H₂]cysteine tracer during prolonged cysteine supplementation with 88 and 40 mg/kg per day. Erythrocyte GSH concentration and its FSR at cysteine supplementation with 88 versus 40 mg/kg per day were 1.25 versus 1.30 mmol/L and 230 versus 254% per day, respectively. These data suggest that in a CBS-deficient patient exogenous supply of 40 mg cysteine/kg per day is sufficient to maintain GSH synthesis in erythrocytes. Further studies in larger patient groups should be initiated to measure the effects on GSH metabolism to further elucidate the correct dose of cysteine supplements in CBS-deficient patients.