Arginine-containing peptides from cells grown in ¹³C₆-arginine (Arg-6) exhibit a single lower-molecular-mass peak that indicates loss of a single “heavy” atom from a portion of the Arg-6 pool.

Mass spectra of tryptic peptides (SLAELCLGLVQEAIDASILSQQESSNSLDLVR, LNQVELQLSER, and IWHHTFYNELR, in (A), (B) and (C), respectively) from S. pombe (A, B) Mto2[17A]-GFP and (C) actin, isolated from car2∆ arg1-230 lys3-37 cells grown in either (i) unlabeled arginine (Arg-0) and unlabeled lysine (Lys-0), or (ii) ¹³C₆-arginine (Arg-6) and ¹³C₆¹⁵N₂-lysine (Lys-8), as indicated. In peptides from cells grown in Arg-6, the single “pre-peak” (indicated in blue) indicates conversion of a portion of Arg-6 to ¹³C₅-arginine (Arg-5). Masses of monoisotopic peaks for each growth condition (i.e., assuming no conversion of labeled arginine) and inferred peptide charge-states are indicated in magenta.