Phosphorylation sites in riboflavin-binding protein characterized by fast atom bombardment mass spectrometry

The capability of fast atom bombardment mass spectrometry for characterization of phosphorylation sites in a tryptic peptide from chicken egg yolk riboflavin-binding protein has been evaluated. The quality of information about molecular weight, amino acid sequence, phosphorylation sites, and microheterogeneity is evaluated as a function of the sign of the ions analyzed, the nature of the counter ions associated with the phosphate substituents, sample matrix, and various instrumental parameters. The intact octaphosphorylated 23-residue peptide was found to be susceptible to mass spectral analysis. Information from the negative ion spectrum was used in conjunction with complete sequence information and experiments which showed that all phosphates were attached to serine residues. Phosphorylated and unphosphorylated serine residues were identified and the sample was shown to be homogeneously octaphosphorylated.