Interference of carbohydrates in the quantitation of protein-bound citrulline by amino acid analysis

After cation-exchange chromatographic separation of the charge isomers of human myelin basic protein, the citrulline-containing component was purified from the unbound fraction by gel permeation chromatography. Sephadex G75 (superfine) resolved high- and low-molecular-weight contaminants from the 18.5K myelin basic protein. However, carbohydrates leached from the column material by the acidic eluant interfered with citrulline quantitation by amino acid analysis. It appears highly probable that during acid hydrolysis of the protein, glucose reacts with citrulline, the ureido group of the latter forming a condensation product with the sugar which subsequently undergoes further chemical degradation to products which are not easily identifiable. Methods for removing the interfering sugars prior to amino acid analysis are discussed.