Specific monitoring of Syk protein kinase activity by peptide substrates including constrained analogs of tyrosine

The ability of Syk protein tyrosine kinase to phosphorylate peptides, where tyrosine had been replaced by conformationally constrained analogs, has been exploited to develop highly selective substrates suitable for the specific monitoring of Syk activity. In particular we have synthesized a peptidomimetic, RRRAAEDDE(L-Htc)EEV (syktide), with a 3(S)-7-hydroxy- 1,2,3,4-tetraidroisoquinoline-3-carboxyl acid residue (L-Htc) replaced for tyrosine, which is phosphorylated by Syk with remarkable efficiency while it is not affected to any appreciable extent by a number of protein tyrosine kinases tested so far. These properties make syktide the first choice substrate for the specific monitoring of Syk.