Quantitative determination of phosphoserine by high-performance liquid chromatography as the phenylthiocarbamyl-S-ethylcysteine. Application to picomolar amounts of peptides and proteins.

A method is described that permits the phosphoserine content of proteins and peptides to be determined in picomolar amounts. A micro-batch reaction first converts phosphoserine into S-ethylcysteine. Hydrolysis with 6 M hydrochloric acid then yields the free amino acid, which is coupled with phenyl isothiocyanate to give the corresponding phenylthiocarbamylamino acid. This derivative is determined quantitatively in the range 10-20 pmol by reversed-phase high-performance liquid chromatography. The method works well with either small peptides or proteins in the low picomole range.