To produce smaller peptides and achieve better S/N, the anionic fraction of CNBr fragments of ChET was digested with AspN, yielding an N-terminally truncated peptide from a1CT-Y+X (m/z 749.2520). Figure displays the high-resolution mass spectra of AspN cleaved a1CT-Y+X, where X was considered taurine. Figure displays the isotopic peak distribution of the measured data (black) and compared to a theoretical isotopic distribution of the same peptide sequence with taurine included (red). Notably, the measured and theoretical abundances are nearly identical. Conversely, Figure compares the measured data for the detyrosinated peptide (black) with the taurine prediction in which a significant difference is observed. Finally, residual analysis of the peak intensities between the measured detyrosinated and measured taurinated versus the theoretical indicate that high differences are shown if sulfur is not included (Figure ). The finding of C-terminal a1CT-Y taurination in avian erythrocytes prompts the question of whether taurination exists in other tissues. Since brain has high intracellular concentrations of taurine, 1,2 and brain tubulin has a complex complement of PTMs, we isolated tubulin from chicken, rat, and bovine brains and compared the CNBr peptides obtained from each. The a1-CT region of the three resulting spectra are shown in Figure , demonstrating that taurine PTM was not found in any brain sample. For each spectrum, an arrow is provided at m/z 2805, the expected (average mass) position of a1CT-Y+taurine. To investigate the potential role of enzymes that modify the tubulin CTT, tubulin was detyrosinated with carboxypeptidase A (CPA) and retyrosinated with tubulin tyrosine ligase (TTL) , under varying conditions (see Methods) , using bovine brain tubulin (BBT) as a source of nontaurinated tubulin. BBT was prepared in the same fashion as the avian erythrocyte tubulin, and the mass spectral range corresponding to a1CT of BBT is shown in Figure . The full length (a1CT) and detyrosinated peptides (a1CT-Y) are found, as well as a peak identified as the detyrosinated with an addition of glutamic acid (a1CT-Y+E), presumably as γ-glutamylation, e.g. at E406 (see Scheme1). The arrows show the positions where the taurinated peptides would occur if they were present.