Evaluation of enzymatic and magnetic properties of γ-glutamyl-[1-13C]glycine and its deuteration toward longer retention of the hyperpolarized state

Dynamic nuclear polarization (DNP) is an emerging cutting-edge method of acquiring metabolic and physiological information in vivo. We recently developed γ-glutamyl-[1-13C]glycine (γ-Glu-[1-13C]Gly) as a DNP nuclear magnetic resonance (NMR) molecular probe to detect γ-glutamyl transpeptidase (GGT) activity in vivo. However, the detailed enzymatic and magnetic properties of this probe remain unknown. Here, we evaluate a γ-Glu–Gly scaffold and develop a deuterated probe, γ-Glu-[1-13C]Gly-d2, that can realize a longer lifetime of the hyperpolarized signal. We initially evaluated the GGT-mediated enzymatic conversion of γ-Glu–Gly and the magnetic properties of 13C-enriched γ-Glu–Gly (γ-Glu-[1-13C]Gly and γ-[5-13C]Glu–Gly) to support the validity of γ-Glu-[1-13C]Gly as a DNP NMR molecular probe for GGT. We then examined the spin-lattice relaxation time (T1) of γ-Glu-[1-13C]Gly and γ-Glu-[1-13C]Gly-d2 under various conditions (D2O, PBS, and serum) and confirmed that the T1 of γ-Glu-[1-13C]Gly and γ-Glu-[1-13C]Gly-d2 was maintained for 30 s (9.4 T) and 41 s (9.4 T), respectively, even in serum. Relaxation analysis of γ-Glu-[1-13C]Gly revealed a significant contribution of the dipole–dipole interaction and the chemical shift anisotropy relaxation pathway (71% of the total relaxation rate at 9.4 T), indicating the potential of deuteration and the use of a lower magnetic field for realizing a longer T1. In fact, by using γ-Glu-[1-13C]Gly-d2 as a DNP probe, we achieved longer retention of the hyperpolarized signal at 1.4 T.