“Heavy”-isotope label from ¹³C₆¹⁵N₄-arginine (Arg-10) is converted into other amino acids in fission yeast.

<p>Mass spectra of tryptic peptides (TIFFKDDGNYK and GIDFKEDGNILGHK in (A) and (B), respectively) from <i>S</i>. <i>pombe</i> Mto2[17A]-GFP fusion protein isolated from <i>car2∆ arg1-230 lys3-37</i> cells grown in either (i) unlabeled arginine (Arg-0) and unlabeled lysine (Lys-0), (ii) ¹³C₆-arginine (Arg-6) and ¹³C¹⁵N₂-lysine (Lys-8), or (iii) ¹³C₆¹⁵N₄-arginine (Arg-10) and ¹³C₆¹⁵N₂-lysine (Lys-8), as indicated. In peptides from cells grown in Arg-10, additional higher-molecular-mass peaks are observed (iii), indicating conversion of “heavy”-isotope label into other amino acids. Such peaks are not observed from cells grown in Arg-6. To simplify comparison, peptides shown here do not contain arginine residues, so the masses of monoisotopic peaks of peptides from cells grown in Arg-6 and Arg-10 are identical (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0129548#pone.0129548.g002" target="_blank">Fig 2</a>). Mass-to-charge (m/z) ratios of monoisotopic peaks and inferred peptide charge-states are indicated in magenta.