Arginine-containing peptides from cells grown in <sup>13</sup>C<sub>6</sub>-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. E. BorekWeronika ZouJuan RappsilberJuri E. SawinKenneth 2015 <p>Mass spectra of tryptic peptides (SLAELCLGLVQEAIDASILSQQESSNSLDLVR, LNQVELQLSER, and IWHHTFYNELR, in (A), (B) and (C), respectively) from <i>S</i>. <i>pombe</i> (A, B) Mto2[17A]-GFP and (C) actin, isolated from <i>car2∆ arg1-230 lys3-37</i> cells grown in either (i) unlabeled arginine (Arg-0) and unlabeled lysine (Lys-0), or (ii) <sup>13</sup>C<sub>6</sub>-arginine (Arg-6) and <sup>13</sup>C<sub>6</sub><sup>15</sup>N<sub>2</sub>-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 <sup>13</sup>C<sub>5</sub>-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.</p>