Treatment with CPA completely removed the terminal tyrosine, as expected (Figure ). Subsequent treatment with TTL in the presence of tyrosine produced a restoration of the Cterminal tyrosine (Figure , peak at m/z 2861). Performing the same incubation in the presence of taurine rather than tyrosine did not produce a taurinated peptide (Figure ). Thus, under conditions where TTL can add tyrosine to a1CT-Y, it cannot add taurine to a1CT-Y, indicating that the enzyme responsible for this addition is different from TTL.