Differentiating the Transferred Amine. To establish whether PRO-TRANS(248) transfers the α-or ε-amine of the donor, we prepared reactions with Nα-L-Lys and tracked the isotopic label by UPLC-MS (Figure ). Indeed, a +1 shift in the m/z value of 3 compared to that produced in reactions with unlabeled L-Lys was observed, confirming the transfer of the αamine in preference to the ε-amine.