Figure 1. A representative GC–MS chromatogram of single ion monitoring of the HFAA derivative of agmatine.

(A) A freeze-clamped liver extract from an overnight fasted rat. The chromatogram corresponds to approx. 2 pmol of agmatine (m/z 284). (B) Same extract shown in (A) after spiking with [guanidino-¹⁵N₂]agmatine (2 nmol/g). (C) Mitochondrial extract after incubation for 7 min with 10 mM [guanidino-¹⁵N₂]arginine. Peak corresponds to approx. 4 pmol of [guanidino-¹⁵N₂]agmatine (m/z 286). (D) Same extract shown in (C) after the addition of unlabelled agmatine (∼1.2 nmol/3 mg of protein). The m/z ratio 286/284 was used to determine the isotopic enrichment as well as the concentration of agmatine by isotope dilution.