“…These labeled substrates facilitated the development of activity assays [ 3 , 143 , 155 , 156 , 157 ]. The availability of substrate variants that serve as donor only, acceptor only, or that are not processed by the GT enzyme and thus function as inhibitors (see below) contributed greatly to the understanding of the specificity of the GT and TP catalyzed reactions [ 138 , 147 , 158 , 159 , 160 , 161 , 162 , 163 ]. Thanks to these substrates, lasting problems have been solved.…”