Table 1.

Activated macrophages catalyze the incorporation of the guanido nitrogen of l-arginine into protein-bound NTyr

m/z	Authentic NTyr	NTyr from 15N-l-Arg + Macrophage
236	91.4 ± 14.1	21.5 ± 0.4
237	16.9 ± 2.6	89.3 ± 3.9
238	5.3 ± 0.9	16.5 ± 1.0
352	100.0 ± 16.3	18.1 ± 4.6
353	28.1 ± 4.8	100.0 ± 10.4
354	11.9 ± 2.3	75.2 ± 5.2

Relative ion intensities of the major fragments, m/z 236–238 [M − C₇H₁₈O₃Si₂]⁻ and m/z 352–354 [M − C₃H₉OSi]⁻ derived from the corresponding tri-trimethylsilyl derivatives. Values represent the mean ± SD of three determinations.