S-Sulfocysteine - Investigation of cellular uptake in CHO cells.

For the generation of therapeutic proteins in cell culture, high producing clones are used. These clones have a high demand in amino acids to support cell growth and productivity. l-cysteine (Cys) is critical in highly concentrated feeds due to low stability of Cys and low solubility of the oxidation product cystine at neutral pH. S-sulfocysteine (SSC) was developed to substitute the Cys source and fed-batch experiments using SSC showed good cellular performance regarding viable cell density and titer, indicating uptake and metabolization of SSC by Chinese hamster ovary cells. However, the responsible transporter allowing cellular uptake remains unclear and was studied in this work. Due to the structure similarity of SSC with cystine and glutamate, it was proposed that the cystine/glutamate antiporter (xc-) allows cellular uptake of SSC. The uptake was assessed via transporter inhibition using sulfasalazine and transporter overexpression using either sulforaphane or sulforaphane-N-acetylcysteine during fed-batch experiments. Following daily addition of 50 µM and 100 µM sulfasalazine, the extracellular SSC concentration was increased by 65 % and 177 % respectively, suggesting a reduced uptake due to xc- inhibition. In contrast, enhanced transporter activity through 15 µM sulforaphane and sulforaphane-N-acetylcysteine treatment, induced a 60 % and 52 % reduced extracellular SSC concentration, respectively. These inverse uptake results strongly suggest that xc- is facilitating the transport of SSC.

The effect of dietary protein restriction in a case of molybdenum cofactor deficiency with MOCS1 mutation.

Molybdenum cofactor deficiency (MoCD) is an autosomal recessive inborn error of metabolism that results from mutations in genes involved in molybdenum cofactor (Moco) biosynthesis. MoCD is characterized clinically by intractable seizures and severe, rapidly progressing neurodegeneration leading to death in early childhood in the majority of known cases. We report on a patient with an unusual late disease onset and mild phenotype, characterized by delayed development and a decline triggered by a febrile illness and a subsequent dystonic movement disorder. Magnetic resonance imaging showed abnormal signal intensities of the bilateral basal ganglia. Blood and urine chemistry tests demonstrated remarkably low serum and urinary uric acid levels. A urine sulfite test was positive. Specific diagnostic workup showed elevated levels of xanthine and hypoxanthine in serum with increased urinary sulfocysteine (SSC) levels. Genetic analysis revealed a homozygous missense mutation at c.1510C > T (p.504R > W) in exon 10 of the MOCS1 in isoform 7 (rs1387934803). At age 1 year 4 months, the patient was placed on a low protein diet to reduce cysteine load and accumulation of sulfite and SCC in tissues. At 3 months after introduction of protein restriction, the urine sulfite test became negative and the urine SCC level was decreased. After starting the protein restriction diet, dystonic movement improved, and the patient's course progressed without regression and seizures. Electroencephalogram findings were remarkably improved. This finding demonstrates that the dietary protein restriction suppresses disease progression in mild cases of MoCD and suggests the effectiveness of dietary therapy in MoCD.