We also performed a Tyr-deTyr cycle with ChET, as a source of taurinated tubulin. This shows that taurine is not removed by CPA (Figure , compare to Figure ). TTL restored tyrosine to a1CT-Y, but does not alter a1CT-Y+taurine (Figure ). This demonstrates that taurine does not participate in the Tyr-deTyr cycle. Thus, taurination is likely an enzymatic reaction involving a hitherto unknown enzyme. Finally, we investigated the enzymatic acitivity of VASH1/SVBP, an endogenous tubulin carboxypeptidase , on the a1CT peptides of ChET (taurinated tubulin) (Figure ). Prior to treatment, the detyrosinated, and the taurinated species are observed as well as the unmodified peptide (Figure ). Following treatment with VASH1/SVBP, the relative abundance of a1CT greatly diminishes due to detyrosination, with accompanying increase in a1CT-Y; however the taurinated species (m/z 2805) is left unchanged. Thus, neither CPA not the endogenous tubulin carboxypeptidase VASH1/SVBP, are able to remove taurine from a modified a-tubulin C-terminus, indicating that an as-yet unknown enzyme is involved.