Detection by mass spectrometry of highly increased amount of S-sulfonated transthyretin in serum from a patient with molybdenum cofactor deficiency.

Serum transthyretin has several isoforms, most of which are caused by disulfide linkage with cysteine residue at position 10. We found an ion peak 80 D larger than unmodified transthyretin by electrospray ionization mass spectrometry and assigned it to S-sulfonated transthyretin. The peak height was <2% of total transthyretin in control sera from more than 200 individuals including infants. Transthyretin from a patient with molybdenum cofactor deficiency was analyzed, and the peak was prominent, higher than 85% of total transthyretin. In patients with this disease, the presence of elevated levels of sulfite leads to the formation of S-sulfonated cysteine. The peak can be used as a diagnostic marker for molybdenum cofactor deficiency, although more sera from patients with this disease should be tested.